A&P Module 1.docx

  1. Define common anatomical terms and body planes
    1. Superior (a.k.a., cephalic or cranial): toward the head; the upper part of a structure
    2. Inferior (a.k.a., caudal): away from the head; toward the lower part or tail of a structure.
    3. Anterior (a.k.a., ventral): nearer to/at the front of the body.
    4. Posterior (a.k.a., dorsal): nearer to/at the back of the body.
    5. Medial: Nearer to the midline
    6. Lateral: Farther from the midline.
    7. Ipsilateral: On the same side of the body as another structure.
    8. Contralateral: on the opposite side of the body from another structure.
    9. Proximal: Nearer to the attachment of a limb to the trunk; nearer to the origination of a structure.
    10. Distal: farther from the attachment point of a limb to the trunk or farther from the origination of a structure.
    11. Superficial (a.k.a., external): toward or on the surface of the body.
    12. Deep (a.k.a., Internal): away from the surface of the body.
    13. Sagittal: longitudinal plane that divides the body into left and right sides
      1. Median (midsagittal): longitudinal plane that divides the body into equal left and right sides
      2. Parasagittal or paramedian: longitudinal plane that divides the body into unequal left and right sides
    14. Frontal (a.k.a., coronal): longitudinal plane that divides the body into anterior and posterior portions
    15. Transverse: horizontal plane that divides the body into superior and inferior portions
    16. Oblique: plane that passes through the body at an oblique angle (other than 90◦)
  2. Explain homeostasis and the components and types of feedback systems.
    1. Homeostasis: the underlying principle of physiology (Patton and Thibodeau 2019) (Ch. 2)
      1. Homeostasis describes the body’s desire to maintain “status quo” or a stable internal environment despite perturbations from the external environment.
        1. Example of homeostasis from everyday life: HVAC seeks to maintain a constant temperature inside your house despite a drop in temperature outside the house
        2. Example of homeostasis in the body: The body seeks to do the same regarding temperature regulation. It seeks to maintain a temperature of 98.6◦ F whether the weather be hot or cold.
      2. Feedback loops are cycle of events that seek to maintain a constant internal environment. They involve three basic components that seek to regulate a controlled variable such as body temperature, blood glucose levels, muscle length, etc. These three components are:
        1. Sensor/ receptor: body structure that monitors changes in a controlled variable. The information it senses is sent to an integrator via an afferent pathway (typically a nerve).
        2. Integrator/ Control Center: This structure dictates the set point by which a controlled variable is maintained. It receives input from the sensors regarding the controlled variable and evaluates whether there is a deviation from the set point. If the controlled variable has deviated from the set point, it generates output commands that are sent to effectors along efferent pathways (again, typically a nerve).
        3. Effector: a body structure that responds to commands from the integrator/control center in order to change the controlled variable.
      3. Types of feedback loops
        1. Feedback systems can respond to perturbations in controlled variables by either reversing or enhancing the perturbation.
        2. Thus, there are two types of feedback loops:
        3. Negative feedback loops reverse a change in a controlled variable
        4. Positive feedback loops enhance a change in a controlled variable
      4. Many pathologies result from that behaviors or activities that interfere with the body’s ability to maintain homeostasis (e.g., smoking, excessive physical or psychological stress, overexposure to things such as alcohol or UV radiation, etc.)
  3. Explain the hierarchy of body structure and describe each level.
    1. Atomic
    2. Biomolecules (proteins, lipids, carbohydrates)
    3. Microscopic structures formed from biomolecules (biomembranes, cytoskeleton)
    4. Organelles: subcellular structures
    5. Cells: the basic structural and functional unit
    6. Tissue
      1. Structurally similar cells that group together to perform a common function
      2. Four primary types of tissue (and cells): epithelial, connective, muscle, nervous
    7. Organ: specialized structure composed of two or more different tissues
    8. Physiological system: two or more organs that function together
    9. Body
  4. Describe the compartmentation and subcompartmentation of body fluids, including the relative amounts of fluid in the major compartments and the nature of the water in the interstitial compartment.
    1. Classical physiological compartmentation system
      1. A healthy individual is comprised of about 60% H2O. This is referred to as total body water.
      2. The total body water is distributed between two main fluid compartments: the extracellular fluid (ECF) and intracellular fluid (ICF)
      3. Extracellular fluid (ECF)
        1. Contains ⅓ of the total body water
        2. Separated from the external environment by the skin and the epithelia of the respiratory, digestive, and urogenital systems
        3. Separated from the ICF by plasma membranes
        4. Cells take up O2 and nutrients from the ECF and discharge metabolic waste products into the ECF
        5. Subcompartments of the ECF
          1. Blood plasma

The fluid portion of the blood

Plasma makes up ¼ of the volume of the ECF

          1. Interstitial fluid (ISF)

The fluid surrounding the cells outside of blood vessels

Interstitial fluid makes up ¾ of the volume of the ECF

Separated from the plasma by the capillary walls

As blood flows through capillaries, solutes move between the plasma and interstitial fluid by diffusion

As a result, the composition of the interstitial fluid tends to resemble the composition of the incoming plasma

The nature of water in the interstitial fluid

In soft tissues, the interstitium (space between cells) contains a large amount of proteoglycan filaments, which are exceptionally narrow & closely packed molecules

99% of the water in the interstitial fluid of soft tissues is trapped between the proteoglycan filaments, forming a gel that isn’t free to move about easily

          1. Transcellular fluid: a third “minor” compartment of ECF

Defined as the fluid contained by epithelial-lined spaces

Components

Serous membrane-lined spaces/body cavity

Serous membranes produces serous fluid and include two layers

Visceral layer that lines an organ

Parietal layer that lines the space

Examples

Peritoneum

Pleural

Pericardium

Other epithelial lined spaces include the cerebrospinal fluid, GI fluids, urine in the bladder, synovial joint fluid, aqueous humor, etc.

      1. Intracellular fluid (also called cytosol)
        1. Fluid contained within cell membranes
        2. ⅔ of the total body water is inside cells
    1. Clinical perspective
      1. Intravascular space
        1. Plasma volume plus blood cells (total blood volume)
        2. Functions in maintenance of blood pressure and flow
    2. “Third spacing”
      1. Definitions vary among authors
        1. Accumulation of fluid in the interstitial space (edema), or…
        2. Accumulation of transcellular fluid (ascites, pleural effusion, or in the bowel)
        3. Interstitial fluid (or transcellular fluid) is considered by clinicians to be the third body fluid compartment or “third space”
      2. What both definitions agree on is that fluid accumulates in the extravascular spaces after having moved out of the intravascular space
      3. The important thing is that regardless of which space the fluid accumulates in, when a patient shows signs of hypovolemia without external fluid loss, they’re probably third-spacing
      4. What could cause “third-spacing”?
        1. Increased hydrostatic pressure in the capillaries (e.g., via right heart failure or deep vein thrombosis)
        2. Plasma protein loss (e.g., albumin) due to liver disease, renal disease, burns, etc.)
        3. Increased capillary permeability due to inflammation or anaphylactic shock
  1. Describe what biomembranes and epithelial tissues are and what they do.
    1. Biomembranes
      1. Microscopic structures assembled from lipid and protein biomolecules
        1. Plasma membranes separate the intracellular and extracellular fluids
        2. Biomembranes also separate the interiors of subcellular organelles from the cytosol
        3. These will be studied in more detail in the next lectures
    2. Epithelial tissues
      1. An epithelium is a sheet of cells that covers a body surface or lines the internal surface of a body cavity
      2. One of the four primary tissue types
        1. Epithelium
        2. Connective
        3. Muscle
        4. Nervous
      3. Classified according to their shape and # of cell layers
        1. Shape: squamous, cuboidal, columnar
        2. # of cell layers: Simple, stratified, Pseudostratified
      4. Specifically named epithelial tissues
        1. Endothelium: a sheet of squamous (flat) cells, one cell thick, that lines the internal surfaces of blood or lymph vessels
        2. Mesothelium: a sheet of squamous cells, one cell layer in thickness, that lines the internal surfaces of serous cavities (pleural, pericardial, peritoneal cavities)
      5. Functions
        1. Protection from mechanical trauma, abrasion, desiccation, microorganisms
        2. Separation of body cavities or compartments and transport of fluid and solutes between compartments
        3. A substance passing through an epithelial membrane must pass through a cell’s plasma membrane at least twice, plus the cytosol, so epithelial cells have control over what passes through the epithelium from one compartment to another
        4. Secretion of glandular products
  2. Describe the components and characteristics of fibrous connective tissue.
    1. Connective tissues bind together and support the various structures of the body. It also conveniently partitions various structures into compartments.
    2. Connective tissue is composed of:
      1. Extracellular matrix (ECM)
        1. Fibrous proteins (e.g., collagen & elastin)
        2. Proteoglycans (e.g., chondroitin sulfate, hyaluronic acid)
        3. The term “ground substance” is the ECM minus the fibrous proteins
      2. Cells (e.g., fibroblasts, chondrocytes, red blood cells, osteocytes)
    3. Types of connective tissue
      1. Fibrous
      2. Dense fibrous
      3. Bone
      4. Cartilage
      5. Blood
    4. Today we’ll focus mainly on dense fibrous connective tissue (ligaments, tendons & deep fascia)
    5. Proteins of fibrous connective tissue
      1. Collagen
        1. Bundles of protein filaments with high tensile strength; provides most of the tensile strength in tissues
        2. Low compliance (doesn’t alter its shape very easily)
        3. High elastance (snaps back to its original shape following deformation)
      2. Elastin
        1. Also strong, but less so compared to collagen
        2. Also higher compliance and lower elastance than collagen
  3. Explain what a fascia is, its characteristics, and what it does.
    1. Fascia
      1. A term that denotes a thin connective tissue membrane composed of closely packed collagen fibers that run irregularly in different directions within the membrane
      2. In Latin, it means “band” (Patton and Thibodeau 2019) (Box 9.2)
      3. Significance
        1. Fasciae (plural of fascia) form the walls of tissue compartments in the neck and limbs
        2. Dense enough to hold fluid
          1. injected fluids, such as interfascial blocks

Femoral nerve block

Pectoral plane block

        1. edema &/or blood, etc.
      1. Being composed mainly of collagen, fasciae are stiff and don’t “give” much; increased compartmental fluid volume can raise the pressure in a compartment
    1. Clinical example: the axillary sheath
      1. A cylindrical fascia that enclosed a compartment surrounding the brachial plexus (large nerves that supply the upper limb) and axillary artery as they pass from the neck, through the shoulder, and into the arm
      2. Clinical application: to perform an axillary block, you inject up to 50 cc of local anesthetic into this sheath; the anesthetic is contained by the connective tissue sheath so it stays concentrated around the nerves of the brachial plexus
  1. Describe the lipid portion of the plasma membrane: its constituents, characteristics, how it controls membrane permeability, etc.
      1. General function: compartmentation; membranes separate cells and subcellular compartments from their surrounding environment
        1. Each cell’s plasma membrane separates the intra- and extracellular fluid compartments
        2. Membranes also separate the contents of subcellular compartments (mitochondria, endoplasmic reticulum, Golgi complex, lysosomes, etc.) from the cytosol, the fluid inside the plasma membrane but outside the nucleus and organelles
      2. Structure: biological membranes are lipid bilayers with certain specific membrane proteins inserted into the bilayer
    1. Membrane lipids
      1. Biomembranes are formed mostly from phospholipids, plus some cholesterol and other types of lipids
      2. Phospholipids are amphiphilic (a.k.a. amphipathic)
        1. Amphiphilic molecules have both hydrophilic (i.e., “water-loving”; water-soluble) and hydrophobic (i.e., “water-fearing”; less water-soluble) regions
        2. Each phospholipid molecule consists of a polar (hydrophilic) head group and two nonpolar (hydrophobic) fatty acid chains
    2. The most stable configuration for phospholipids in an watery environment is a lipid bilayer, two layers of phospholipid molecules that mirror each other
      1. Hydrophobic fatty acid chains are oriented toward the middle of the bilayer
      2. Hydrophilic heads cover the two opposite surfaces of the bilayer
      3. The presence of water on both sides of the bilayer stabilizes the plasma membrane and prevents the phospholipids from moving out of position
    3. The lipid bilayer is fluid, about the same consistency as vegetable oils that are liquid at room temperature
    4. Phospholipid molecules are free to move about laterally in the membrane; this is just another way of saying that the lipid bilayer is fluid
      1. Lateral diffusion occurs within a monolayer of the cell membrane. It occurs millions of times per second
      2. Flip-flop from one layer of the cell membrane to the other is extremely rare
  2. Define and explain the relationships among (i.e., which terms go together) the terms, polar, nonpolar, hydrophilic, hydrophobic, lipophilic, lipophobic, water-soluble and lipid-soluble.
    1. The term polar describes a molecule that has an overall neutral charge, but the electrons are asymmetrically distributed so one end is slightly positive while the other end is slightly negative; for example, water, urea, glycerol
    2. The term nonpolar describes molecules or a regions of a molecule that have a preponderance of nonpolar bonds; for example, C-H bonds
    3. Polar chemical groups are hydrophilic because they can form hydrogen bonds with water molecules; hydrophilic molecules are relatively water-soluble and lipid-insoluble (lipophobic)
    4. Nonpolar chemical groups are hydrophobic; all of their atoms are uncharged and nonpolar and can’t interact with water molecules; these groups are relatively lipid-soluble (lipophilic) and water-insoluble
    5. Hydrophilic and hydrophobic are relative terms
      1. A molecule or region of a molecule can be both hydrophilic and hydrophobic; i.e., they are amphipathic.
      2. Most solutes fall somewhere in the middle of these two ends of a spectrum (i.e., hydrophobic and hydrophilic).
      3. For a given solute, these two characteristics can be quantified by a measure called the oil-water partition coefficient (βx)
      4. The partition coefficient (bx) is an index of a solute’s lipid solubility:
        1. bx = [x]oil

[x]water

        1. bx = 1, the solute is equally soluble in water and oil
        2. bx > 1, the solute is more soluble in oil than water
        3. bx < 1, the solute is more soluble in water than oil
        4. Significance of bx
          1. for a solute to pass through the lipid bilayer portion of the plasma membrane, it must be able to dissolve in the lipid
          2. in other words, it must be somewhat lipid-soluble; the more polar a molecule is, the less readily it can pass through the lipid bilayer portion of the plasma membrane
      1. Other takeaways
        1. Hydrophilic is synonymous with lipophobic
        2. Lipophilic is synonymous with hydrophobi
  1. Explain protein structure: conformation and conformational change, ligand binding, the four levels of protein structure, etc.
    1. In a typical biomembrane, about 50% of the mass is protein
    2. General introduction to protein structure
      1. Conformation
        1. Every protein is a chain (or multiple chains) of amino acids in a specific sequence. The sequence of amino acids in a protein specifies is shape.
        2. A protein’s three-dimensional shape is called its conformation
          1. A protein’s function is determined by its conformation. Changes in a protein’s conformation normally cause it to change or lose function.
          2. A protein’s conformation results from the folding of its amino acid constituents
        3. Proteins have four structure levels that determine their final conformation
          1. Primary structure: the sequence of the amino acid chain (a.k.a., polypeptide chain)
          2. Secondary structure: Amino acid’s side chains (i.e., its R groups) in one polypeptide chain can weakly interact/bind with one another to yield one of two structures:

Alpha helix

Beta sheet

        1. Tertiary structure: interaction of R groups in a single polypeptide chain can again yield an even larger three-dimensional structure/ conformation
        2. Quaternary structure: interaction of more than one polypeptide chain
      1. A protein’s final folded conformation is either its tertiary structure (for proteins that consist of only one polypeptide chain) or its quaternary structure (for proteins that consist of more than one polypeptide chain)
    1. All proteins bind to other molecules
      1. Binding may be tight or short-lived, but is always very specific
      2. The term “binding affinity” refers to the amount of intermolecular force between a ligand and protein; high-affinity binding implies a tighter, longer-lasting bond
      3. A substance, no matter what it is, that’s bound by a protein is called a ligand
      4. The region of a protein that associates with a ligand is a binding site, usually a cavity in the protein surface formed by a particular arrangement of amino acids
      5. A protein can have multiple binding sites for different ligands
    2. Association-dissociation of ligands
      1. Think of the binding of a ligand (L) to a protein (P) for which the ligand has a certain affinity as a reversible chemical reaction that produces a ligand-protein complex (LP):
        1. L + P ↔ LP
      2. There is constant association (reaction proceeding rightward) and dissociation (reaction proceeding leftward) at a rate that’s determined by the relative concentrations of L, P and LP
      3. According to the law of mass action, the rate of the association reaction (forming LP) is proportional to the concentrations of L and P; for example, if you double the concentration of L, the rate of association will double (assuming that there is ample P for the reaction)
      4. When the concentration of ligand is high, the rate of the association reaction will be greater than the rate of the dissociation reaction (assuming we keep have ample P for the reaction and that the concentration of P stays constant)
      5. When the concentration of ligand is low, the rate of the dissociation reaction will be higher than the rate of the association reaction (assuming that the concentration of P stays constant)
      6. The law of mass action can be used to describe the interaction of a neurotransmitter (L) to a cell membrane receptor (P) or a solute (L) to a membrane transporter (P)
        1. At low solute (L) levels, the rate of transport across the cell membrane increases as solute binds to the receptor membrane transporter proteins (P)
        2. At high solute (L) levels, however, all of the membrane transporter proteins (P) may be bound to solute. If this is the case, the transport rate will remain constant even if we increase the solute (L) levels further. Thus the finite number of membrane transporter proteins (P) limits the rate of (LP) formation.
          1. A physiological example is seen in patients with diabetes mellitus (i.e., hyperglycemia). Here glucose (L) that has been filtered in the glomerulus is not 100% reabsorbed in the proximal tubule due to saturation of its glucose transporter proteins (P). Thus, these patients pass glucose in the urine.
          2. A pharmacology example is seen in elimination of a drug (L), which is dependent on the concentration of the enzymes (P) that eliminate/ biotransform it.
    3. Conformational change
      1. Nearly every protein has at least two different conformations
      2. Proteins change conformation in response to local environmental changes, such as a change in the electrical field, temperature, or tension (stretch) around the protein
      3. Also, the binding of a ligand to a protein is an extremely important factor that can cause the conformation to change
      4. Conformational changes can expose or hide an enzyme’s active site (where a chemical reaction occurs following biding of a substrate)
      5. A conformational change caused by the binding of one ligand can affect the ability of a different ligand to bind. Allosteric binding occurs at a site that is different from the active site; however, it changes an enzyme’s conformation such that it either activates or inhibits the enzyme.
    4. Membrane proteins may be grouped into two broad structural classes:
      1. Intrinsic (also called integral) membrane proteins
        1. Amphiphilic: hydrophobic transmembrane regions interact with the hydrophobic tails of the phospholipid molecules; hydrophilic regions are exposed to water on either side of the membrane, often span the full thickness of the lipid bilayer
        2. Often these proteins are folded and have regions that pass back and forth through the membrane multiple times
          1. Cell signaling receptor proteins must relay information from outside the cell (i.e., the interstitial fluid) to inside the cell; thus, they must have an odd number of membrane-spanning regions
          2. Many signaling receptor proteins span the cell seven times and are referred to as heptameric proteins. An extracellular ligand binds to this protein which then passes a signal to a G-protein located inside the cell. We’ll be focusing on this heptameric protein receptors and G proteins in an upcoming lecture.
      2. Extrinsic (a.k.a. peripheral) membrane proteins: bound by weak molecular interactions to the hydrophilic regions of intrinsic membrane proteins on either the internal or external surface of the plasma membrane
    5. Movement of proteins within the membrane
      1. Many proteins are free to diffuse laterally within the membrane (like “protein icebergs floating in an ocean of lipid”) therefore some specific membrane proteins are able to interact with each other
      2. However, certain membrane proteins are anchored intracellularly to specific regions in the plasma membrane; for example, dystrophin in the skeletal muscle
  1. Describe what channel and carrier proteins are, how they work and what they do.
    1. Channel proteins
      1. Contain a narrow, highly selective, pore that connects the cytosol with the extracellular fluid
      2. An understanding of ion channels is going to be essential to your clinical understanding of pharmacology and physiology
      3. Key points
        1. Only very small particles are able to pass through channels (for example, ions or water; we’re mainly concerned with ions)
        2. Ionic substances are water-soluble (hydrophilic, lipophobic) because water molecules are attracted to their positive or negative charges
        3. Most channels are highly selective; for example, allowing potassium ions (K+) to pass through but hardly anything else
        4. Some channels are only relatively selective; for example, allowing different positively charged ions (cations) to pass but excluding negatively charged ions (anions)
        5. Channels merely provide a route for the simple diffusion of ions through the plasma membrane; because they are charged, ions can’t pass through the lipid bilayer
        6. Channels are gated – they can be open or closed, depending on local conditions
          1. Channels have at least two conformations, open (activated) and closed (inactivated or deactivated)
          2. Mechanosensitive (a.k.a. mechanically gated) channels are opened by mechanical forces (pressure, stretch, etc.)
          3. Voltage-sensitive (a.k.a. voltage-gated) channels are opened by changes in the electrical potential across the plasma membrane
          4. Chemosensitive (a.k.a. chemically gated or ligand-gated) channels are opened by the binding of a specific chemical
        7. Some channels, called “leak” channels, are open when most channels are closed. Essentially, they are always open.
          1. Leak channels conduct mostly K+, but they also conduct small amount of Na+
          2. These channels, along with the inward rectifier K+ channel, are responsible for the resting membrane potential (we will discuss this concept in an upcoming lecture)
        8. When an ion channel opens, it’s open to both sides of the membrane at the same time. This is in contrast to carrier proteins, which open only to open side of the membrane at a time (next topic).
        9. The opening and closing of gated ion channels changes the rate of diffusion of specific ions through the plasma membrane; importantly, this changes the electrical activity associated with the plasma membrane
    2. Carrier proteins (also called carriers or transporters)
      1. These intrinsic membrane proteins differ importantly from channels in that the solute being transported actually binds with the carrier protein
      2. A conformational change in the carrier protein moves the bound particle across the membrane and releases it on the other side
      3. Unlike a channel, a carrier or transporter is never open to both sides of the membrane at the same time
      4. Carrier protein activity may be passive or active
        1. Many carrier proteins only allow solutes to cross a membrane passively (“downhill”, down a concentration gradient, from high to low concentration)
        2. Some carrier proteins are coupled to an energy source and can transport solutes across a membrane actively (“uphill”, up a concentration gradient, from low to high concentration)
    3. Plasma membrane receptors: intrinsic membrane proteins that bind a specific chemical messenger (a ligand) on the exterior of the cell, then, using various mechanisms, convey a message into the cell’s interior to trigger some action (discussed further in an upcoming lecture)
    4. Plasma membrane enzymes: some membrane proteins are enzymes that are responsible for producing intracellular second messengers (also discussed further in an upcoming lecture)
  2. Explain what endocytosis (pinocytosis and phagocytosis) and exocytosis are and what they do.
    1. Vesicle-mediated transport
      1. Endocytosis and exocytosis are important mechanisms for moving materials across a plasma membrane; you need to be aware of what they are and what they do, but the exact mechanism of these processes is complex and isn’t important for you to know
      2. Endocytosis: a small region of the plasma membrane is drawn inward and pinches off to form a small, intracellular, membrane-bound vesicle containing something that used to be outside the cell
        1. Pinocytosis
          1. Ingestion of fluid and solutes via small pinocytic vesicles
          2. Performed continuously by most cells
        2. Phagocytosis
          1. Ingestion of large particles (microorganisms, dead cells, etc.)
          2. The ingested particle ends up inside the cell and surrounded by a vesicle made of a portion of the plasma membrane
          3. Only performed by specialized cells associated with the immune system
      3. Exocytosis
        1. Vesicle-mediated export of molecules from a cell
        2. Mechanism
          1. Exocytosis is nearly always triggered by an event which causes an increase in the intracellular Ca2+ concentration
          2. An intracellular vesicle containing specific molecules fuses with the cytoplasmic side of the plasma membrane
          3. The vesicle membrane merges into the plasma membrane and the contents of the vesicle are released into the extracellular fluid
          4. This is the mechanism by which neurotransmitters are released from neurons for synaptic transmission
  3. Describe simple diffusion, explain each variable in Fick’s first law of diffusion and predict how a change in each variable would affect the rate of diffusion. Be familiar with and explain how variables in the Stokes-Einstein equation affect diffusion of solutes through the membrane. Summarize how different types of solutes diffuse through biomembranes.
    1. Simple diffusion through a plasma membrane
      1. Net movement of solute from one side of the plasma membrane to the other due to random thermal motion (Brownian motion)
      2. The factors influencing the diffusion of an uncharged solute (x) across a membrane from one compartment to another are summarized in Fick's first law of diffusion:
        1. Jx = - Dx * A * DC

T

Where: Jx = flux (i.e., rate of diffusion) of solute x (mol/sec)

Dx = diffusion coefficient of the solute (see below)

A = area of the membrane

DC = concentration difference (of solute x)

DT = distance over which diffusion occurs (i.e., thickness of the partition [membrane])

        1. Dx in this equation is defined by the Stokes-Einstein equation:
          1. Dx = k * T

6 p r h

Where: k = Boltzmann’s constant (relates the average kinetic energy of a gas per to its temperature)

T = temperature (in degrees Kelvin)

r = radius of the solute

h = viscosity of the solvent

        1. Analysis of the equation
          1. Since most of the parameters in this equation are constant in the body (e.g., body temp is normally 310 K and the solvent is normally water (thus, h is constant), the main variable is the radius of the solute
          2. If any of the terms in the numerator increase in value, the rate of diffusion will increase; if the thickness of the membrane increases, the rate of diffusion will decrease
      1. Diffusion through the lipid bilayer of a plasma membrane
        1. In biological systems, we also must account for the properties of the membrane (i.e., it favors diffusion of hydrophobic substances since it is lipophilic)
          1. The partition coefficient (bx) is an index of a solute’s lipid solubility:

bx = [x]oil

[x]water

          1. Thus, Fick’s first law of diffusion becomes

Jx = - bx * D * A * DC

DT

        1. Since most parameters in the above equation remain constant in a biological system, Fick’s law can be simplified to:
          1. Jx = - P * DC
          2. Here, P (the permeability coefficient) incorporates D, DT, A and bx
          3. The larger the DC or P, the more rapid the diffusion of solute will be
          4. P will be largest for solutes with higher lipid solubility (large value for bx) and smaller size (lower radius)
      1. Simple diffusion through channel proteins
        1. As previously discussed, small ionic solutes can diffuse through membrane channels
        2. Fick’s first law of diffusion cannot fully explain the diffusion of ionic solutes because electrostatic forces are also involved; this will be covered in an upcoming lecture
      2. Summary of diffusion through biological membranes
        1. A lipid bilayer is impermeable to diffusion of hydrophilic, ionic solutes (like H+, Na+, HCO3–, K+, Ca2+)
        2. Large, polar molecules (like glucose) can diffuse through a lipid bilayer to a very limited extend (too slowly to be of any biological significance); carrier proteins are necessary to transport a significant amount of these solutes
        3. Small polar solutes (such as H2O, urea, glycerol) can slowly diffuse through a lipid bilayer; channel proteins are necessary if diffusion must be rapid
        4. Hydrophobic molecules such as steroid hormones, O2, CO2 and anesthetic gases, rapidly cross lipid bilayers
  1. Define osmosis and osmotic pressure, explain the mechanism of osmosis, and qualitatively predict how increases or decreases in the terms of van’t Hoff’s equation affect osmosis.
    1. Definition: movement/diffusion of water across a semipermeable membrane from a region of high water concentration (a dilute solute solution) to a region of low water concentration (a solute solution of higher concentration)
    2. Semipermeable membrane: permeable to water but impermeable to at least one solute
    3. Osmotic pressure
      1. Osmotic pressure, along with hydrostatic pressure, is a force that leads to net movement of water across membranes; for example, the capillary wall
      2. Quantification of osmotic pressure
        1. Equal to the hydrostatic pressure that would have to be applied to a fluid compartment to prevent osmosis or…
        2. The height of a column of fluid that results from osmosis
      3. The factors affecting osmotic pressure are expressed in van't Hoff's law:

p = s (nCRT)

where:

p = osmotic pressure

s = reflection coefficient

n = # of particles yielded by the dissociation of the solute

C = molar concentration of the solute

R = gas constant

T = temperature in Kelvin

        1. Reflection coefficient
          1. Takes into account the properties of the membrane (i.e., can the solute cross the membrane?)
          2. Not all membranes are the same

Some have fenestrations: large holes in which solutes can easily diffuse

The size of the spaces between the cells (i.e., intercellular clefts) are not uniform (some are narrower, some are wider)

Paracellular ion permeability at tight junctions is largely determined by their claudin composition.

Some tight junctions have a lot of claudins, some have fewer

The blood brain barrier has a lot of claudins therefore, paracellular ion permeability is nearly zero

        1. Significance of this equation: important parameters that determine the osmotic pressure
          1. the total number of individual solute particles in the solution, which is determined by

n, # of particles yielded by the dissociation of the solute

C, molar concentration of the solute

          1. the chemical characteristics of the particles (i.e., is the membrane permeable to them?)
        1. Two factors affect the number of solute particles: chemical concentration (c) and, for a molecule that dissociates, the number of particles yielded by the dissociation (i)
      1. Oncotic pressure (colloid osmotic pressure): osmotic pressure resulting specifically from protein molecules (colloids)
  1. Summarize the properties of carrier-mediated transport.
    1. Properties of protein-mediated transport
      1. Displays saturation kinetics
        1. An increase in the concentration of a solute on one side of the membrane relative to the concentration on the other side will increase the rate of transport up to the point when the transport system is saturated
        2. When all the transporters are being used, transport will be at its maximal rate
      2. Specificity
        1. Only molecules with a particular chemical structure are transported by the carrier protein
        2. Specificity is not absolute—similar molecules may be transported by the same carrier
      3. Carrier proteins (transporters) compared with channel proteins
        1. Channels are simply proteins that selectively permit small particles to pass through by simple diffusion; the diffusing solute doesn’t affect the channel protein conformation
        2. Carrier proteins (or transporters) are membrane proteins that interact with the solute being transported
          1. The solute to be transported occupies a binding site on the transporter
          2. The transporter undergoes a reversible conformational change that relocates the binding site and bound solute to the opposite side of the plasma membrane
          3. Net transport may be passive (“downhill” from higher solute concentration to lower solute concentration)
          4. If coupled to an energy source, some transporters can also transport a solute “uphill” against a concentration gradient
  2. Explain what facilitated transport, primary and secondary active transport are, what they can do and how they work.
    1. Facilitated transport (also called facilitated diffusion)
      1. A membrane carrier protein is required because the solute being transported is either too large, too charged, or too polar to passively diffuse rapidly enough through the lipid bilayer
      2. There must be a concentration gradient for the transported solute and the solute always moves down its concentration gradient (“downhill” from higher to lower concentration)
      3. Example: glucose transport through the blood-brain barrier
        1. There are five different human glucose transporters; the one that transports glucose across the blood-brain barrier is called GLUT-1
        2. This transporter spontaneously changes conformation so that its glucose binding site is alternately exposed either to the extracellular fluid or intracellular fluid
        3. If, as is usually the case, the glucose concentration is higher in the plasma than in an endothelial cell in a capillary in the brain, there will be a higher probability of glucose binding when the binding site exposed to the extracellular side than when it’s exposed to the intracellular side (review “association-dissociation of ligands” above)
        4. Glucose that’s bound to the transporter has a greater probability of dissociating on the intracellular side where the glucose concentration is lower
        5. Some glucose will be transported from the side of the membrane where the glucose concentration is lower to the side where it is higher, but the net transport will always be from higher to lower concentration
  3. Explain the function of the Na+-K+ ATPase.
    1. Active Transport
      1. Properties
        1. A membrane carrier protein is required
        2. The transporter must be coupled with one of two energy sources:
          1. Adenosine triphosphate (ATP) hydrolysis
          2. An ionic concentration gradient
          3. Can move a solute “uphill” against a gradient
        3. Primary active transport
          1. The carrier mechanism responsible for the movement of a solute is directly coupled to ATP hydrolysis
          2. Example: the Na+-K+ ATP’ase (also called the Na+-K+ pump)

An integral membrane protein made up of 2 protein subunits; present in all cells

During each cycle, the pump hydrolyzes one ATP molecule to adenosine diphosphate (ADP) and inorganic phosphate (Pi) while transporting 3 Na+ ions out of the cell and 2 K+ ions into the cell

First one ATP molecule, then 3 Na+ ions bind to the intracellular side of the protein

Next, ATP is hydrolyzed to ADP + Pi. Pi binds to the pump (i.e., it is phosphorylated) while ADP is released into the ICF. Phosphorylation of the pump changes its conformation

The conformational change exposes the Na+ binding site to the extracellular fluid and greatly reduces the affinity of the protein for Na+, thus the 3 Na+ are able to dissociate even though the extracellular concentration of Na+ is high

Simultaneously, a high-affinity site that binds 2 K+ is exposed on the extracellular side of the pump

The binding of the 2 K+ causes a second conformational change

The Pi is released, the K+ binding site opens to the intracellular fluid, and the affinity of the protein for K+ decreases dramatically, thus the 2 K+ are able to dissociate even though the intracellular concentration of K+ is high

      1. Secondary active transport (also called coupled transport)
        1. Two solutes are transported simultaneously
        2. One solute moves with its concentration gradient (i.e., downhill) while the other moves against its concentration gradient (i.e., uphill)
          1. The concentration gradient for downhill movement is maintained by the active transporter.

The ion that moves downhill is normally Na+

The active transporter is normally the Na+-K+ ATP’ase

The solute moving uphill uses the kinetic energy from downhill movement of the other ion to power its movement (see slide for further explanation)

          1. Cotransport example: Na+-glucose cotransporter (SGLT)

Na+ binds to the extracellular side of the transport protein (also called a symporter since both solutes will be moving in the same direction); this causes a conformational change that increases the affinity of the transporter for glucose so glucose also binds to a separate binding site

The binding of both solutes causes a second conformational change that relocates the Na+ and glucose binding sites to the intracellular side of the cotransporter

Because of the low intracellular Na+ concentration, Na+ readily dissociates; this decreases the affinity of the transporter for glucose so glucose also dissociates although the glucose concentration is higher than outside the cell

Cotransport is also called symport

          1. Countertransport (also called antiport)

In the previous example, both solutes were transported in the same direction, however some transporters transfer one solute in one direction and the other solute in the opposite direction

Example: cardiac muscle Na+-Ca2+ exchanger

3 Na+ attach to the extracellular side of the exchanger

1 Ca2+ attaches to the intracellular side

The binding of both Na+ and Ca2+ cause a conformational change that relocates both binding sites so that Na+ dissociates into the cardiac myocyte while Ca2+ dissociates into the extracellular fluid

  1. Apply knowledge of cell structure and function to anesthesia practice.
    1. Cytoplasm
      1. The contents of the space contained within the cell membrane but outside the nuclear envelope
      2. Consists of the organelles and the surrounding fluid, the cytosol
    2. Cytoplasmic Organelles
      1. Ribosomes
        1. Large cytoplasmic particles formed inside the nucleus from ribosomal RNA and proteins
        2. Composed of two subunits (one large and the other smaller) that are both produced in the nucleolus
        3. Ribosomes are involved in the process of translation: sequencing of amino acids into polypeptides using information coded on messenger RNA
      2. Rough (or granular) endoplasmic reticulum (RER)
        1. A network of membrane-bound cavities with ribosomes attached to the external membrane surface
        2. Function
          1. Translation of proteins and polypeptides

Folding of protein into tertiary and/or quaternary conformation

post-translational modification of proteins (e.g., glycosylation)

          1. Packaging these proteins into carrier vesicles
        1. These carrier vesicles then head to the Golgi complex
    1. Golgi complex (a.k.a. Golgi apparatus)
      1. Structure
        1. A stack of flattened membrane-bound sacs, located near the RER, that continually turns over
        2. One face of the Golgi complex (i.e., “cis” region) is continually formed from carrier vesicles from the RER; these vesicles fuse together forming a sac
        3. Medial regions are also formed via the fusion of carrier vesicles that pinch off (“bud”) from the cis region
        4. The far side of the Golgi complex (i.e., “trans” region) is formed from carrier vesicles that “bud” from the medial Golgi
          1. Secretory vesicles “bud” off from this region. These vesicles contain proteins that are ready to be utilized
          2. Secretory vesicles are relegated to either the constitutive or regulated pathway

Constitutive: secretory vesicles are delivered continuously and in an unregulated fashion; occurs in most cells

Regulated pathway: secretory vesicles are delivered in a highly regulated fashion that requires activation by a hormone or neural signal; occurs in specialized cells such as endocrine or exocrine cells

      1. Function
        1. Polypeptides made in the RER are modified by attaching or removing certain chemical groups, especially sugar moieties
        2. Different classes of proteins are segregated into separate regions of the Golgi complex and ultimately are packaged into different vesicles with different destinations. They are three possible destinations for proteins
          1. Lysosomal and peroxisomal enzymes
          2. Secreted proteins and peptides
          3. Membrane proteins
    1. Lysosomes
      1. A product of the rough endoplasmic reticulum and Golgi complex
      2. Structure: membrane-bound vesicles containing numerous acid hydrolase enzymes
        1. These organelles contain numerous H+ pumps that import H+ ions
        2. Their hydrolase enzymes work best in an acidic environment (pH~ 5)
      3. Function: intracellular digestion of endocytosed material (extracellular particles or bacteria) or retired/worn-out organelles (Patton and Thibodeau 2019) (p. 85)
        1. Autophagy is the process of digesting and removing work-out organelles from the cell
        2. The process of autophagy is as follows:
          1. Worn-out organelles are enveloped by membrane from either the endoplasmic reticulum or Golgi apparatus to form a capsule.
          2. The capsule then merges with a lysosome that then begins to degrade and then recycle the component of the worn- out organelles.
          3. Undigested components form a residual body.
        3. Digestion of extraocular material is similar; however, in his case, the materials to be digested by the lysosome are enveloped by the plasma membrane (i.e., endocytosis)
    2. Tay-Sachs disease results from a lack of a lysosomal enzyme (HexA) (Tortora and Dickerson 2021) (p. 87)
      1. As a result, a membrane glycolipid accumulates that is prevalent in neurons
      2. The defective neurons can lead to seizures, muscle rigidity, lack of coordination, blindness and/or dementia. Children with Tay- Sachs disease often die before age 5.
    3. Proteasomes (Patton and Thibodeau 2019) (p. 85-6)
      1. Another protein-destroying organelle that resembles a hollow drum
      2. Although similar to the lysosome in function, it destroys one protein at a time instead of entire organelles or bacteria
        1. One of its primary functions is to remove improperly folded proteins (such that their proper conformation is altered; thus, the protein’s function is affected)
        2. When the proteasome isn’t functioning properly, it can cause pathologies such as Parkinson’s disease0F1 or Alzheimer’s disease2(Patton and Thibodeau 2019) (p. 86 & 93)
    4. Smooth endoplasmic reticulum (SER)
      1. Structure: another network of membrane-bound cavities without attached ribosomes
      2. Functions
        1. SER membrane contains enzymes for
          1. Synthesis of lipids & steroids
          2. Assembling the lipid portion of lipoproteins
          3. Detoxification of drugs and harmful metabolites (liver and kidneys)
        2. In muscle cells, the SER is specialized for concentrating and storing calcium ions (Ca2+)
    5. Mitochondria
      1. Structure
        1. Spherical or cylindrical in shape
        2. Bounded by outer and inner membranes that create two separate compartments
          1. Intermembrane space: between the inner and outer mitochondrial membranes
          2. Mitochondrial matrix: contained within the inner mitochondrial membrane
      2. Function: oxidative ATP production (this process is covered in more detail in biochemistry)
        1. The Krebs cycle takes place in the mitochondrial matrix
        2. Oxidative phosphorylation
          1. Electron transport chain

Series of proteins embedded in the inner mitochondrial membrane.

These proteins take hydrogens liberated in the Krebs cycle and sequentially pass the electrons to oxygen as the protons accumulate in the intermembrane space

          1. ATP synthase

Proton channel/enzyme complex where most ATP is made

Embedded in the inner mitochondrial membrane and is powered by a flow of protons from the intermembrane space into the mitochondrial matrix

    1. The cytoskeleton
      1. Structure
        1. A three-dimensional protein structure distributed throughout the cytosol
        2. Composed of three types of filamentous proteins
          1. Microfilaments

Structure: composed of numerous globular actin (G-actin) monomers that form fibrous actin (F- actin); thinnest of the filamentous proteins (5 – 8 nm)

Requires ATP to polymerize, which occurs at its plus end

Disassembly occurs at the minus end

Treadmilling: refers to concomitant and continuous processes of assembly and disassembly of microfilaments and microtubules

          1. Function: muscle contraction, IC trafficking, cytokinesis, microvilli
        1. Intermediate filaments (e.g., keratin, vimentin)
          1. Structure: intermediate in size
          2. Function: provides structural support to the cell at (cytoskeleton) at desmosomes
        2. Microtubules (polymers of a- and β-tubulin)
          1. Structure: composed of polymers of a- and β-tubulin heterodimers

Requires GTP

Growth occurs at the plus end

Thickest of the filamentous proteins

          1. Function: intracellular transport (with help of molecular motor such as dynein and kinesin), flagella & cilia, mitotic spindle
        1. Functions: strengthens cells, maintains cellular shape, anchors organelles, moves cells, moves organelles
  1. Summarize the structure and function of the nucleus.
    1. Nucleus
      1. The nuclear envelope is a semipermeable membrane that segregates the nuclear contents from the cytoplasm
      2. Contents: the nucleus contains the chromosomes and also proteins whose purpose is to “pack” the genetic material
        1. DNA
          1. Sugar & phosphate backbone
          2. 4 nucleotides

Cysteine

Guanadine

Adenine

Thymine

          1. Protein (histones)

DNA is wound around these proteins to pack them tightly into a small space

Heterochromatin

DNA is tightly packed around histones

Transcription here is difficult

Euchromatin

DNA is loosely packed around histones

Transcription can easily occur here

Epigenetics is a rapidly evolving field in biology that is focusing on transcription regulation via DNA/histone interactions; that is, what signaling pathways cause heterochromatin to become euchromatin so that transcription can occur

          1. RNA

Ribosomal RNA (rRNA) is produced in the nucleolus

Messenger RNA (mRNA) synthesis occurs on DNA (euchromatin)

Transfer RNA (tRNA) is also made in the nucleus

      1. Transcription
        1. Most important day-to-day activity that takes place in the nucleus
        2. Process of transferring the genetic code from deoxyribonucleic acid (DNA) to one of three types of ribonucleic acid (RNA)
          1. MRNA
          2. rRNA
          3. tRNA
  1. Explain the nomenclature (autosomes, sex chromosomes) and number of human chromosomes (haploid, diploid) in somatic and reproductive cells.
    1. Chromosomes
      1. A chromosome is one single, continuous DNA double helix
      2. Humans have 24 structurally distinct chromosomes identified by numbers or letters
        1. Chromosomes 1-22, the autosomes, are equal in number in males and females
        2. The other two chromosomes, X and Y, are sex chromosomes (i.e., allosomes) and differ in males and females (XX in females, XY in males)
      3. Useful DNA terms
        1. A gene is a sequence of DNA bases that contains the information required for the production of one of the three types of RNA
        2. A chromosome is an array of specific genes, with each gene being found at a specific position or locus on the chromosome
          1. Genes are typically found at the same locus in all people. That is, we can find the gene for the β–globin peptide of hemoglobin on chromosome 11 (short arm position 15.4) in any person we are karyotyping.
          2. Although each individual has the same genes found in the same loci, human genes are not all exactly identical. This is due to errors in DNA replication (i.e., mutations) that occur over multiple generations. Thus, the version of a gene that you inherit from your mother is mostly but not 100% identical to the version of a gene you inherit from your father. These nearly identical but still alternate versions of a gene are referred to as alleles.
        3. Homologous chromosomes (a.k.a., homologs):
          1. Definition: chromosomes that have the same genes in the same loci but not necessarily identical having identical DNA sequences (i.e., alleles)
          2. One chromosome is maternal, one is paternal
        4. Ploidy: number of chromosome sets in a cell
          1. Diploid: two sets of chromosomes

One set of chromosomes is maternal and the other set is paternal.

Seen in human somatic cells.

          1. Haploid: contains only one set of each chromosome

Designated by N, where N = # of chromosomes (in humans N = 23)

Seen in human gametes.

        1. Chromatid: one-half of a replicated chromosome (seen during DNA replication in mitosis or meiosis)
          1. Chromatids are held together by a centromere
          2. Sister chromatids are identical – they are replications of 1 of your 46 chromosomes
          3. Non-sister chromatids are homologous

One is from mom and its homologous counterpart is from dad.

These cross-over during meiosis to form recombinant chromatids

        1. Karyotype: an individual’s complete set of chromosomes that are arranged in numerical order
      1. Cellular chromosome numbers
        1. Somatic cells
          1. Somatic cells include all of the body cells except reproductive cells—eggs or sperm
          2. Each autosome is paired for a total of 44, plus one pair of sex chromosomes for a grand total of 46
          3. In humans, the term diploid describes a cell with 46 chromosomes; somatic cells are diploid
          4. One of the autosomes in each pair is a maternal chromosome, the other is a paternal chromosome; because they have the same genes on them, they are called homologous chromosomes
          5. Also, one of the sex chromosomes is maternal and the other is paternal (because females have two X chromosomes, an offspring can only inherit an X chromosome from its mother; males have one X and one Y chromosome, so an offspring can inherit either sex chromosome from its father)
        2. Reproductive cells
          1. Each sperm or egg contains only a single copy of each autosome for a total of 22, plus a single sex chromosome for a grand total of 23
          2. In humans, the term haploid describes a cell with 23 chromosomes; reproductive cells are haploid
  1. Define gene, genetic locus, and gene transcription.
    1. Genes
      1. A gene is a sequence of DNA bases that contains the information required for the production of one of the three types of RNA
      2. A chromosome is an array of specific genes, with each gene being found at a specific position or locus on the chromosome
        1. Genes are typically found at the same locus in all people. That is, we can find the gene for the β–globin peptide of hemoglobin on chromosome 11 (short arm position 15.4) in any person we are karyotyping.
        2. Although each individual has the same genes found in the same loci, each individual’s genes are not identical to his/her neighbor. Due to missteps in DNA replication (i.e., mutations) over multiple generations, identical genes are referred to as alleles. This is due to the fact any two individual’s genes are mostly but not 100% identical.
      3. Since you inherit one chromosome in each chromosome pair from each of your parents, you have two copies of nearly every gene—a maternal and a paternal copy
      4. The estimated number of human genes is 20,000 to 25,000
      5. Gene transcription: the process of transferring the genetic code from DNA to a messenger RNA (mRNA), ribosomal RNA (rRNA), or transfer (tRNA) molecule
        1. For now, just know the definition
        2. More details of the transcription process will come later in biochemistry
  2. Compare and contrast mitosis and meiosis in terms of which cells participate in each type of cell division and the products of each.
    1. Cell Division: Mitosis and Meiosis
      1. Cell cycle: the life cycle of a somatic cell
        1. Different somatic cells undergo cell mitosis at different intervals
          1. Epithelial cells, such as in the epidermis of the skin or lining of the gastrointestinal tract, divide almost continuously producing new cells at a rate equal to the rate of cell death
          2. Adult nerve and muscle cells virtually never divide
          3. Neoplastic cells have lost control over the frequency of cell division and divide rapidly and continuously
        2. Interphase
          1. The period between successive cell divisions
          2. Varies in length depending on the type of cell and its frequency of mitosis
          3. During interphase, the chromosomes replicate themselves so there will be enough DNA for two daughter cells.
      2. Mitosis
        1. Begins with a single diploid somatic cell
        2. Results in the production of two identical diploid daughter cells
      3. Meiosis
        1. Begins with a single diploid cell in the ovaries or testes
        2. Meiosis consists of two rounds of cell division that result in four unique haploid daughter cells
          1. “Unique” means that each chromosome in the daughter cells consists of a blend of genes from the maternal and paternal chromosomes
          2. This blending of maternal and paternal genes occurs via crossing over (a.k.a., genetic recombination) between non-sister chromatids. The result of this process is offspring that contain a higher degree of genetic variation.
        3. End product is reproductive cells—eggs or sperm
  3. Define terms used to describe single-gene inheritance. Given parental genotypes and a pattern of autosomal dominant or recessive inheritance, detail the possible genotypes of offspring using a Punnett square and state the likelihood of an offspring inheriting or carrying a genetic trait.
    1. Basic Genetics of Single-Gene Inheritance
      1. Terminology
        1. Allele: an alternative version of a gene that may occupy a given locus on a chromosome
          1. Wild-type allele: the one that occurs in the majority of the population (normal allele)
          2. Mutant allele: a gene that differs from the wild-type because of a mutation (a permanent change in DNA structure)
        2. Single-gene disorder: one that is determined by the alleles at a single locus; a mutant allele replaces the wild-type on one or both chromosomes
        3. Genotype: the set of alleles that make up a person’s genetic constitution
        4. Phenotype: the observable expression of a genotype—a detectable anatomical, biochemical, or molecular trait
        5. Heterozygous: the condition when the alleles at a given locus are different
        6. Homozygous: the condition when the alleles at a given locus are identical
      2. Genetic disorders with a single-gene inheritance pattern
        1. Classification of single-gene disorders
          1. Autosomal: the locus of the alleles is on an autosome
          2. X-linked: the locus of the alleles is on the X chromosome
          3. Dominant phenotype: the condition in which a disorder is expressed when the mutant allele is present on only one chromosome of a pair
          4. Recessive phenotype: the condition in which a disorder is only expressed when the mutant allele is present on both chromosomes of a pair
        2. Autosomal recessive inheritance
          1. Less common than autosomal dominant
          2. Only homozygotes with two mutant alleles and no normal allele are affected
          3. An individual can be unaffected but a carrier
          4. Enzyme deficiency disorders are usually recessive

When a person has only one mutant allele, the wild-type allele can often code the production of enough functional enzyme to prevent clinical symptoms

When a person has two mutant alleles, no functional enzyme is produced and clinical symptoms are present

          1. Common autosomal recessive disorders

Pseudocholinesterase deficiency: people who have this abnormality may be sensitive to certain drugs, including the muscle relaxants succinylcholine and mivacurium as well as other ester local anesthetics; these drugs are normally metabolized by the pseudocholinesterase enzyme

Sickle cell anemia

Cystic fibrosis

Phenylketonuria

          1. Autosomal recessive Punnett squares
        1. Autosomal dominant inheritance
          1. More than half of the known single-gene phenotypes are autosomal dominant traits
          2. Trait is expressed when only one mutant allele is present
          3. Each child of one affected parent has a 50% chance of inheriting the trait (the other parent is phenotypically normal)
          4. No unaffected carriers—if you carry the mutant gene, you are affected; if you are unaffected, you cannot transmit the gene
          5. Autosomal dominant phenotypes are associated with defective structural proteins (as opposed to enzymes)

Defective structural proteins are expressed by the mutant allele

Cannot be compensated for by the normal proteins expressed by the wild-type allele

          1. Common autosomal dominant traits

Malignant hyperthermia

People who have this abnormality are sensitive to certain triggering agents: volatile, gaseous inhaled anesthetics (halothane, enflurane, etc.) and depolarizing neuromuscular blockers (e.g. succinylcholine)

Triggering agents cause uncontrolled muscle contractions that generate heat and can cause potentially fatal hyperthermia

Familial hypercholesterolemia

Huntington’s disease

Neurofibromatosis

Autosomal dominant Punnett squares

        1. X-linked recessive inheritance
          1. The X and Y chromosomes are distributed unequally to males and females

Females receive an X chromosome from each parent

Males receive a Y chromosome from their father and an X chromosome from their mother

A mutant allele on a male’s X chromosome cannot have a wild-type “partner” since he lacks a second X chromosome

          1. X-linked phenotypes are generally restricted to males (typically expressed in all males who receive the mutant allele but only in homozygous females)
          2. Common X-linked recessive traits

Duchenne muscular dystrophy

Hemophilias A and B

Color-blindness

          1. Inheritance patterns
        1. X-linked dominant inheritance: extremely rare
  1. Explain the components of the transmembrane signaling mechanism used by extracellular ligands.
      1. Cell-to-cell communications
        1. The functions of organ systems require that the activities of individual cells are precisely coordinated
        2. Chemical and sometimes electrical signals are used by cells to communicate with each other
        3. Chemical signals may be metabolites, derivatives of amino acids or fatty acids, peptides, proteins or steroids
        4. The messages communicated between cells regulate the activities associated with maintenance of the internal environment (homeostasis) and so compose the substance of physiology
      2. Modes of cellular communication
        1. Paracrine: chemical signals released from a cell reach nearby cells by diffusion through the interstitial fluid
        2. Neurocrine: paracrine secretions (neurotransmitters) released from neurons that reach target cells by diffusion across a synaptic cleft
        3. Endocrine: chemical signals (hormones) travel through the blood to reach distant target cells
      3. Transmembrane signaling
        1. Every signaling molecule in the ECF must have a mechanism for conveying its signal to the interior of the cell
        2. Mechanisms
          1. Lipid-soluble hormones (e.g. cortisol or thyroid hormones) pass directly through cell membranes to interact with intracellular receptors and influence gene transcription
          2. Transmembrane enzymes have a hormone-binding receptor on the extracellular side and an enzyme on the cytoplasmic side; hormone binding changes the conformation of the receptor which activates the enzyme which then activates intracellular signaling proteins
          3. Some neurotransmitters interact with ligand-gated (a.k.a. chemically gated or directly gated) ion channels which act as both the receptor and effector
          4. This lesson will consider neurotransmitters and hormones which bind with receptors that are coupled with G proteins
  2. Explain the G protein activation-inactivation cycle.
      1. Overview of pathway components
        1. A signaling molecule (generically referred to a an agonist or ligand) that reaches a target cell via one of the modes of cellular communication through the ECF
          1. Agonist: a substance that has affinity for and stimulates physiologic activity at a cell receptor, thus triggering a biochemical response
          2. Ligand: any molecule that binds to a protein (to a receptor, for example)
        2. Membrane proteins
          1. These components are separate membrane proteins that are free to move about laterally in the cell membrane
          2. Receptor: a specialized molecule or group of molecules on the cell membrane that recognizes a specific agonist and triggers a response
          3. G protein: an extrinsic membrane protein that is activated by a receptor-agonist complex and, in turn, regulates the activity of an effector protein
          4. Effector protein: either a membrane enzyme complex that changes the intracellular concentration of a second messenger or an ion channel that alters membrane permeability
          5. Phosphorylated protein: typically one of the last components of the signaling pathway. The phosphorylated protein can either be:

A structural protein (e.g., ion channel or cytoskeletal component)

Enzyme (catalyzes a biological reaction)

          1. Conformational change is a common theme

Protein molecules have at least two different conformations (i.e., configuration, shape)

The conformation changes, for example, whenever something binds with or dissociates from the protein

When the structure of a protein changes, its function also changes

        1. Second messengers
          1. The agonists discussed in this lesson interact with receptors on the outer surface of the cell membrane
          2. These agonists (“first messengers”) rely on intermediary molecules to carry signals to organelles or intracellular proteins that produce a cellular response
          3. These intermediary molecules are known as second messengers
        2. Protein kinases and phosphatases are frequently involved in signal transduction
          1. The activity of many proteins (enzymes, for example) is influenced by whether or not the protein is phosphorylated, meaning that a phosphate group is covalently attached to the protein
          2. Phosphorylation and dephosphorylation change the conformation of a protein
          3. The activity of a given protein may be amplified or subdued by phosphorylation
          4. Protein kinases are enzymes responsible for phosphorylating specific proteins
          5. Protein phosphatases dephosphorylate specific proteins, thereby undoing the action of a kinase
  1. Describe how a receptor coupled to a G protein is altered by the binding of a ligand.
        1. Agonists reach target cells via the ECF and therefore have access to many cells
        2. Only cells with a receptor specific for a particular agonist can respond
        3. A G protein-coupled receptor is a receptor that is dependent on an interaction with a G protein to accomplish signal transduction (“G protein-coupled receptor” doesn’t just mean a receptor that happens to be bound to a G-protein at a particular time)
        4. All G protein-coupled receptors studied to date are intrinsic membrane glycoproteins. These proteins have seven membrane-spanning regions and are thus heptameric proteins.
        5. Receptor specificity: receptors have a limited tolerance for structural variation in agonists therefore cells only respond to certain signals
        6. Receptor affinity: a receptor can recognize and bind a specific agonist even at very low agonist concentration (< 10-7 M)
        7. When an agonist binds with a receptor molecule, the receptor’s conformation is temporarily changed so it can interact with a G protein
        8. Examples: a-adrenergic and b-adrenergic receptors, muscarinic cholinergic receptors, serotonin receptors, etc.
      1. Guanosine nucleotide-binding proteins (a.k.a. GTP-binding or G proteins)
        1. There are two classes of G proteins
          1. Monomeric G proteins

Composed of a single protein

e.g., Ras, Rho & Rab plus others

          1. Heterotrimeric G proteins

Composed of three distinct protein subunits: α-, β- and g-

We will focus on this group only in this handout

        1. Can exist in either of two states: activated (“on”) or inactivated (“off”)
          1. Activated state: G protein binds guanosine triphosphate (GTP) and can interact with an effector
          2. Inactivated state: G protein binds guanosine diphosphate (GDP) and cannot interact with an effector
        2. How does it work?
          1. An agonist binds to a G protein-coupled receptor, changing its conformation to an activated state
          2. The agonist-activated receptor complex binds with an inactive G protein (a G protein with GDP bound to it)
          3. The G protein releases GDP, allowing a GTP to bind and the G protein becomes activated
          4. The activated G protein can now interact with an effector molecule and alter the effector’s function
          5. An activated G protein has GTPase enzyme activity

After a few seconds (up to tens of seconds), the GTP is enzymatically hydrolyzed to GDP, splitting off an inorganic phosphate (Pi)

The G protein therefore “automatically” reverts back to the inactive state (with GDP bound) after a period of time

          1. G proteins consist of three subunits: alpha (α), beta (β), and gamma (γ)

When a G protein is activated, the α subunit splits from the βγ subunit

Both the α subunit and βγ-subunit can interact with membrane effector proteins

          1. Different G proteins may be excitatory or inhibitory

Gs stimulates a membrane enzyme (see below)

Gi protein family is inhibitory to certain membrane enzymes and channels (see below); BUT “inhibitory” only identifies the effect the Gi protein will have on the enzyme or channel; the ultimate cellular effect could still be excitatory—stay tuned

Technically, Gi belongs to a superfamily of heterotrimeric G protein dubbed “Gi/Go”.

Go is a separate G protein that is often found alongside Gi. We will see its effects in module 3 when we study the physiology of the autonomic nervous system.

An ill-defined G protein that activates Phospholipase A2 also belongs to this group (Boron and Boulpaep 2017) (pp. 62 - 63)

Gq stimulates another membrane enzyme (see below)

  1. Describe the mechanisms that lead to increased intracellular levels of cAMP or Ca2+.
  2. Summarize the significance of protein phosphorylation and dephosphorylation and the roles of protein kinases and phosphorylases.
  3. Know the components and common receptors of pathways that are associated with either Gs, Gi, Gq or Phospholipase A2.
        1. Adenylyl cyclase (a.k.a. adenylate cyclase or AC)
          1. AC is regulated when particular agonists bind to different G protein-coupled receptors
          2. AC is a membrane enzyme complex that converts adenosine triphosphate (ATP) to cyclic adenosine monophosphate (cAMP); cAMP is a well-characterized second messenger
          3. Most of the effects of cAMP are exerted through its activation of cAMP-dependent protein kinases
          4. AC is regulated by activated G proteins

Gs stimulates AC

Gi inhibits AC

Each G protein is regulated by different agonists and receptors

          1. cAMP isn’t an immortal molecule; an enzyme called cAMP phosphodiesterase inactives cAMP after a time
        1. Phospholipase C (PLC)[1]
          1. PLC is activated by certain agonists and receptors and Gq proteins
          2. PLC is a membrane enzyme that converts a membrane phospholipid (phosphatidylinositol bisphosphate) into inositol-1,4,5-trisphosphate (IP3) and diglyceride (a.k.a. diacylglycerol or DAG)
          3. IP3 and DAG are also second messengers; DAG remains in the cell membrane, IP3 is released into the cytoplasm
          4. IP3 causes the release of Ca2+ from the endoplasmic reticulum; the Ca2+ often works with DAG to activate a protein kinase
        2. Phospholipase A2
          1. PLA2 is activated by numerous pathways including one that includes a G protein[2] that directly stimulates PLA2
          2. PLA2 is an effector enzyme that converts a certain membrane phospholipid (FYI only: phosphatidylinositol bisphosphate) into arachidonic acid
          3. Arachidonic acid is converted by intracellular enzymes into other active metabolites

Prostaglandins (e.g., PGI2, PGE2)

Thromboxanes

Leukotrienes (regulators of the inflammatory response)

        1. Ion channels
          1. Some effectors are ion channels that are regulated directly by G proteins
          2. Direct modulation of ion channels by G proteins

An agonist attaches to a G protein-coupled receptor that then activates a particular class of G protein

The activated β/γ subunit of the G protein directly interacts with a special type of chemically gated ion channel

          1. Example: the sinoatrial (SA) node of the heart

Signaling molecule: Acetylcholine (ACh), released from the vagus nerve binds to … (go to ii)

G-protein-coupled receptor (GPCR): M2 receptor, which activates the (go to iii)

G protein: the β/γ subunit of the Gi protein, which binds to … (go to iv)

Effector: a ligand-gated potassium channel (i.e., KACh channel), which … (go to v)

Net effect: ↓ heart rate

KACh channel opens

K+ ion diffuses out of the SA node cell

SA node cell hyperpolarizes → ↓ its firing cell

↓ heart rate

          1. NOTE: ion channels can also be indirect modulated by:

Protein kinases

e.g., Protein Kinase A can modulate the SA node’s voltage-gated Ca2+ channel to increase heart rate

Signaling pathway

Signaling molecule: Epinephrine or norepinephrine binds to … (go to ii)

G-protein-coupled receptor (GPCR): the β1-adrenergic receptor, which activates … (go to iii)

G-protein: the a-subunit of Gs, which increases (go to iv)

Effector: adenylyl cyclase activity, which modulates (here it increases) (go to v)

Second messenger: cAMP concentration that modulates (here it increases) … (go to vi)

Kinase: PKA activity, which modulates (here it opens) … (go to vii)

Phosphorylated protein: in this scenario,voltage-gated Ca2+ channels

Net effect: Ca2+ enters the cell and increases the force of cardiac contraction

Second messengers

e.g., the Hyperpolarization-activated, cyclic nucleotide-gated channel in the sinoatrial node

More on this example next semester

In both of these cases, the ion channels are not acting as effectors in the G protein signaling pathway, they are acting as the phosphorylated protein

        1. Example of an ion channel being indirectly modulated by a protein kinase
    1. More examples of G-protein-mediated cell signaling pathways
      1. Vasoconstriction
        1. Angiotensin II\
          1. Angiotensin II (AT II) is a potent vasoconstrictor that is activated under certain conditions
          2. AT II binds to its receptor on vascular smooth muscle cells
          3. The AT II receptor activates a Gq protein
          4. Activated Gq protein stimulates phospholipase C which leads to increased levels of IP3 and DAG
          5. IP3 causes Ca2+ release from the sarcoplasmic reticulum
          6. Increased intracellular [Ca2+] triggers smooth muscle contraction
        2. See if you can pick out all of the signaling pathway molecules in this example (signaling molecule, G-protein-coupled receptor … phosphorylated protein)
        3. Note that ion channels are not involved in this example – either direct or indirect regulation
      2. Regulation of myocardial contractility
        1. Sympathetic nervous stimulation leads to norepinephrine release in the myocardium
        2. Binding of norepinephrine to a b1 adrenergic receptor on a contractile cardiac muscle fiber leads to the activation of a G protein (Gs type)
        3. The activated G protein stimulates adenylyl cyclase, which raises the intracellular concentration of cAMP
        4. cAMP activates PKA, which phosphorylates the L-type Ca2+ channel. The net effect is to increase the amount of intracellular Ca2+ in order to increase the force of the heart’s contraction (more in 2nd semester).
        5. See if you can pick out all of the signaling pathway molecules in this example (signaling molecule, G-protein-coupled receptor … phosphorylated protein)
      3. Pain sensation
        1. Bradykinin is a peptide that is activated in injured tissues
        2. Bradykinin activates a nociceptor (pain receptor) which first activates Gi/Go (Boron and Boulpaep 2017) (p. 479) and then it stimulates PLA2 via an indirect route
        3. PLA2 increases the intracellular level of arachidonic acid, which is metabolized into prostaglandin E2, a potent sensitizer of pain receptors (aspirin and NSAIDs have analgesic activity because they block the synthesis of prostaglandins)
        4. See if you can pick out all of the signaling pathway molecules in this example (signaling molecule, G-protein-coupled receptor, etc.)
  1. Describe the common terms and units of bioelectricity (including, but not limited to charge, voltage/electrical potential, current, resistance and conductance).
  2. Explain the source of charge carriers in biological systems
      1. Charge (Q) is a physical property of matter that causes it to experience a force when placed in an electromagnetic field.
      2. There are two types of charges
        1. Positive charges (e.g.., protons and cations)
        2. Negative charges (e.g., electrons and anions)
      3. Charges exert an electric force on one another
        1. Positive charges attract negative charges and vice versa
        2. Positive charges repel other positive charges and negative charges repel other negative charges
        3. Charge in biological systems (i.e., human body)
          1. In electrical circuits, electrons are the carriers of charge. Their movement causes electrical current.
          2. In biological systems, ions are the carriers of charge. Ions result from the dissociation of electrolytes
          3. Electrolytes dissociate in solution (e.g., water) to form ions

Cations (positively charged ions)

Anions (negatively charged ions)

        1. Charge is measured in units of Coulombs (C[i])
      1. Types of electrical energy
        1. Kinetic energy – energy of ions in motion
          1. If two like charges/ions are close to one another, electrostatic force will cause them to accelerate away from each other (i.e., they acquire kinetic energy)
          2. Likewise, if two unlike charges/ions are close to one another, electrostatic force will cause them to accelerate towards each other (again, they acquire kinetic energy)
        2. Potential energy – stored energy that depends on position
          1. If these charges/ions are somehow restrained from moving, they still have the ability to move towards or away from one another if the restraint is removed. This potential to accelerate towards or away from one another is called potential energy.
          2. In biology, the restraint between two charges in often times the cell membrane
      2. Voltage (V)
        1. Definition: Voltage is a measure of the electrical potential energy that exists between two charged objects[ii].
        2. A volt (V) is the unit of voltage
          1. Household batteries typically have voltages of 1.5 V (AAA to D cell) or 9V. AC circuits in the US have voltages of 120 V.
          2. In the body, electromotive forces are relatively small, so the most commonly used unit is the millivolt (mV)
        3. Voltage is also referred to as:
          1. Electromotive force (E instead of V). Thus, the higher this value is, the faster two charges will be driven towards or away from one another once the restraint is removed.
          2. Alternatively, voltage is referred to as electrical pressure. Again, the higher the value for voltage, the higher the “electrical pressure” that pushes the charges towards/away from one another.
      3. Current (I)
        1. If an electrical potential (voltage) exists and the ions are allowed to move, there will be a current
        2. Definition: the flow rate of charges as they pass a point or region.
          1. Flow is normally defined as the volume of a fluid moving past a given point per unit time (e.g., liters/minute).
          2. Current is defined as the volume/amount of charge flowing past a point per unit of time.
        3. The unit of current is the ampere or amp (A).
          1. Most currents in the body are very small
          2. For example, currents within a single cell are often in the nanoamp (nA) range or even picoamp (pA) range.
        4. Biological currents are the basis of a cell’s excitability (ability to signal to other cells), which allows nerves and muscles to respond to their environment.
        5. Biological currents are typically categorized by the ion that is carrying the charge. Common biological currents are:
          1. Na+ current (i.e., INa)
          2. K+ current (i.e., IK)
          3. Ca2+ current (i.e., ICa)
        6. In neurophysiology we most often talk about currents that flow:
          1. Across or the cell membrane
          2. Parallel to the cell membrane (i.e., down the axon)
      4. Resistance (R)
        1. Defined as opposition to current
        2. The unit of resistance is the ohm (Ω)
        3. Materials can be classified according to their resistive properties:
          1. Conductors have low resistance because they permit the free flow of electrons (e.g., most metals)
          2. Insulators have high resistance because they DO NOT permit the free flow of electrons (e.g., rubber, plastic, wood)
          3. In the body, the most important insulators are fatty membranes (e.g., cell membrane or myelin sheath)
      5. Conductance (G)
        1. Defined as the inverse of resistance G = 1

R

        1. The unit of conductance is the Siemens (S)
        2. Conductance of biological currents is most often described as being conducted through ion-specific channels
          1. gx (lower case g) refers to the conductance of ‘x’ ions through a single ‘x’-specific channel. ‘x’ refers to a particular ion species (e.g., Na+ or K+ or Cl-). Examples are”

gNa (the conductance of Na+ ions through a single Na+ channel)

gK (the conductance of K+ ions through a single K+ channel)

gx values are typically very low (e.g., 0.1 - 100 pS)

          1. Gx (upper case G) refers to the conductance of ‘x’ ions through all the ‘x’-specific channels within a cell.

NOTE: A single cell may have hundreds of Na+ channels. Gx refers to the conductance of ‘x’ ion through all the channels within a single cell.

Examples: GNa or GK

Gx µ gx * N * Po [1]

N = # of ‘x’ channels (e.g., Na+ channels) in the cell

Po = open probability of a ‘x’ channel

Po describes the probability that a channel will be open as a function of its activation stimulus (i.e., voltage for voltage-gated channels or mechanical activation for mechanosensitive channels)

Po is necessary parameter when describing conductance because, unfortunately, the gates of ion channels do not open and close in a very predictable fashion (more on this in module 2).

SIGNICANCE OF Po: Po be modulated via phosphorylation of a channel. We will expound on this quite a bit when we discuss the cardiovascular system in 2nd semester.

  1. State Ohm’s law. Given two of the three values in Ohm’s law, apply the law to calculate I, V, or R.
        1. Describes the relationship between voltage (V), current (I) and resistance (R)
          1. In electrical circuits, it describes the movement of electrons (I) from point A to point B across a resistance
          2. In neurophysiology, it describes the movement of ions, such as Na+, K+ & Ca2+, from point A to point B across a resistance

Point A to point B can be from inside the cell membrane to outside the cell membrane or vice-versa. The cell membrane is the resistance in this case.

Point A to point B can also represent movement from a point within an axon to another point within the axon. The contents of the cytoplasm provides the resistance in this case

        1. Ohm’s Law can be rearranged in various forms to express current or resistance/conductance
          1. I = V or I = V * G

R

          1. R = V or G = I

I V

        1. In electrophysiology, current describes the movement of one ion species at a time through their ion-specific channels.
          1. Because of this fact, we must use DV in our equation instead of just V.
          2. We will explain the details of why we use DV instead of V later in this lecture
  1. Explain the Principle of Macroscopic Electroneutrality and its significance.
      1. Principle of Macroscopic Electroneutrality
        1. The body seeks to maintain an electrically neutral environment in both its intracellular and extracellular environments
        2. The Principle of Macroscopic Electroneutrality states that the number of anions must equal the number of cations in any of the body’s bulk solutions (i.e., the ICF and the ECF).
        3. Although the Principle of Macroscopic Electroneutrality exists, a miniscule number of cations or anions can move across the cell membrane and create microscopic disparities in the number of anions versus cations in either the ICF or the ECF[i].
          1. The result of this microscopic charge disparity is the creation of a potential difference (i.e., a voltage) across the cell’s membrane.
          2. This is the subject of today’s lecture: the membrane potential (Vm)
      2. The basis of the membrane potential (Vm)
        1. The movement of ions across the cell membrane either into or out of the cell is the basis for the membrane potential (Vm) and a cell’s ability to be ”excitable”
          1. In an excitable cell at rest, ions move into or out of the cell through leak channels thereby creating the resting membrane potential (Vr)

Unlike most channels that must be activated by a stimulus to open (e.g., voltage, stretch, ligands, etc.), leak channels appear to be open all the time (i.e., they seem to have no gates).

Leak channels are most conductive to K+ ions, but they also conduct Na+ ions.

          1. In an activated excitable cell, ions move into or out of the cell through gated channels and thus alter the membrane potential (Vm).

The gates of gated channels are activated/opened via a specific stimulus (e.g., voltage for voltage-gated channels, stretch for mechanosensitive channels, various molecules for ligand-gated channels, etc.).

These channels are not open all the time, and they are rather specific for certain ions

Examples: a voltage-gated K+ channel, a ligand-gated cation channel.

        1. In excitable tissues (muscle and nerve), altering the Vm is the basis for cell signaling
          1. Excitable cells can alter the Vm and transmit these alterations across a distance. This allows for communication in the nervous system and it allows for muscle contraction.
          2. These alterations in Vm are accomplished largely via action potentials.
          3. We shall see that action potentials are formed when ions move across the cell membrane. They are then propagated sideways (i.e., laterally) within the cell’s cytoplasm.
          4. An appreciation of the ionic basis of the Vm is fundamental to an understanding of excitable cells and anesthesia
      1. Vm can be measured in the laboratory
        1. To measure Vm/Vr, a glass microelectrode (called the recording electrode) is inserted through the plasma membrane into the cell’s cytoplasm (i.e., the ICF)
        2. A second electrode (called the reference electrode or ground) is placed in the extracellular fluid
        3. If the electrodes are connected to amplifiers and a voltmeter, the electrical potential (Vm or Vr) across the plasma membrane can be measured
      2. There is an accepted convention for reporting Vm
        1. NOTE: I am using Vm here but Vr could also be used. Vr is simply Vm in a cell at rest.
        2. A voltmeter doesn’t measure absolute electrical potential (Vm) but rather the difference in the electrical potential between the interior of the cell (ICF) and its exterior (ECF)
        3. Vm is defined as the electrical potential inside a cell measured relative to its electrical potential outside
        4. Because the extracellular electrical potential is a reference level against which the intracellular potential is measured, we can define the extracellular potential as zero (0 mV)
        5. You must remember that anytime you see value a for Vm, it’s the amount of charge on the interior of the plasma membrane compared with 0 mV on the outside of the membrane
      3. Resting membrane potentials (Vr) vary from one type of excitable cell to another

Cell Type

Average Vr (in mV)

Neuron

–50 to –70

Skeletal muscle

–90

Cardiac muscle (atrial and ventricular)

–80

Cardiac Purkinje fiber

–90

Atrioventricular nodal cell

–65

Sinoatrial nodal cell

–55

Smooth muscle cell

–55

  1. Describe the convention used when reporting the membrane potential. Also, define membrane potential.
      1. Biologically important ions are asymmetrically distributed across the plasma membrane
        1. This fact is essential to establish Vm/Vr
        2. Ions diffuse across the cell membrane down their concentration gradients. As the ions move, they create a voltage within the cell’s interior.
      2. K+, Na+ & Cl– and are the major ions that affect Vm
      3. The most abundant intracellular anions (A–) are large, impermeant ones such as proteins and polyphosphate compounds; these can never diffuse across an intact plasma membrane since they are so large
      4. The asymmetrical distribution of ions across a cell membrane result can also be described as “ion concentration gradients” that exist across the plasma membrane
        1. THEORETICAL SCENARIO: If a cell had no ion channels, ions would not flow across the ion-impermeable cell membrane even though there are concentration gradients that favor diffusion.
          1. The result is the cell would NOT be polarized (polarized refers to the fact that cells normally have a negatively charged interior and an exterior of 0 mV).
          2. Question: If ions cannot cross the membrane, both the inside and the outside will be 0 mV. Why?
          3. Answer: The Principle of Macroscopic Electroneutrality states that there are equal number of cations and anions in any bulk solution (i.e., ICF or ECF); thus, the net potential of the bulk solution is 0 mV.
        2. REAL LIFE SCENARIO: a cell at rest conducts ions through leak channels. Leak channels conduct K+ and Na+ and Cl- [1], but they have the highest conductance for K+.
          1. As K+ ions diffuse down their concentration gradients, they go from the cytoplasm (i.e., ICF) to the ECF (i.e., interstitial fluid or ISF). This means there are slightly more K+ ions in the ISF than in the cytoplasm.
          2. The net result of this K+ movement from the cell’s interior to its exterior is that the cell’s interior has lost positive charge making it negative compared to the cell’s exterior. This is the crux of today’s discussion!
      5. Although K+ is the most permeable ion at rest, other ions (i.e., Na+ and even Cl-) can also move across the resting cell via leak channels.
        1. To begin, let’s see how the movement of just one ion at a time affects Vm (Section IV)
        2. Afterwards (Section V), we’ll see how the simultaneous movement of all three major ions affect Vr
  2. Explain the electrical consequences of selective permeability and diffusion of an ion.
      1. The discussion in this section (IV) is theoretical since most cells conduct several ions at rest.
        1. However, we’ll focus on one ion at a time since it allows us to calculate an individual ion’s Nernst potential (Ex), which is necessary to calculate the Vr of a cell
        2. Ex can also helps us deduce the changes in Vm as an excitable cell conducts an action potential
      2. K+ diffusion
        1. THEORETICAL SCENARIO: Consider a cell at rest with an intracellular [K+] equal to 140 mM and an extracellular [K+] equal to 5 mM. In addition, let’s make the cell membrane permeable (i.e., conductive) only to K+
          1. Remember that this is a theoretical example since most cells will also be permeable to other ions (i.e., Na+ & Cl-) at the same time
          2. In this scenario, let’s assume that both sides of the cell membrane start off with an equal number of positive and negative charges in both the ICF and the ECF (according to the Principle of Macroscopic Electroneutrality) although the concentrations of K+ in the ECF and ECF are not equal.
          3. Thus, initially (i.e., before any K+ ions start to move across the membrane), the voltage on both sides of the membrane is 0 mV and thus Vr = 0 mV
        2. Since the membrane in this example is conductive only to K+ ions, K+ will diffuse down their concentration gradient (i.e., out of the cell)
        3. As K+ ions diffuse out of the cell, positive charges in the ECF now outnumber the negative charges in the ECF. Also, negative charges in the ICF outnumber the positive charges.
          1. K+ diffusion results in the creation of an electrical potential difference across the resting cell’s membrane (Vr).

The inside of the cell is more negative compared to the outside.

NOTE that this negatively charged cell interior will begin to attract K+ ions thus preventing more K+ ions from leaving

          1. Has the Principle of Macroscopic Electroneutrality been violated?

Not really. In this case, a miniscule number of ions (1:50,000) have diffused across the cell and created a negatively-charged cell interior.

This negatively-charged cell interior prevents the diffusion of more K+ ions from the ICF to the ECF.

Since only 1:50,000 K+ ions have left the cell before K+ diffusion stops, the concentration of K+ in both the ICF and ECF has changed a negligible amount. Practically speaking, we can state their concentrations have not changed.

Thus, the [K+] in both the ECF and ICF are 99.998% of the original concentrations. This is not a significant change and thus we can state that the Principle of Macroscopic Electroneutrality has not been violated.

          1. Remember that by convention Vr or Vm always represents the voltage on the inside of the cell relative to the outside, which by convention is always 0 mV.

In this example, Vr is now more negative following K+ diffusion across the cell membrane

Even though logic would ascribe the absolute potential measured just outside the plasma membrane as a positive value, remember that Vr is a simply a relative value (voltage inside the membrane relative to the voltage outside the membrane). Thus, by convention, the voltage outside the membrane has an electrical potential of 0 mV, not a positive value.

      1. Electrochemical equilibrium
        1. At this point, we must keep in mind that the negative charge resulting from the outward diffusion of K+ ions will attract the remaining intracellular K+ ions and prevent their continued outward diffusion; likewise, the positive charge that has accumulated in the exterior of the cell will repulse further K+ diffusion
        2. In other words, the electrical potential that develops as a result of K+ diffusion down its concentration gradient opposes the further diffusion of K+ ions
        3. Thus, two forces are simultaneously determining the movement of K+
          1. Chemical concentration gradient (DC)
          2. Electrical potential (DV)
        4. To restate the consequences of K+ diffusion, diffusion of K+ down its concentration gradient cannot continue indefinitely because it eventually creates and is counteracted by an electrical potential inside the cell
        5. When the electrical potential (measured in millivolts, mV) exactly balances the driving force of the chemical concentration gradient, we say that an electrochemical equilibrium has been reached
          1. In this condition there is no net movement of K+ across the membrane
          2. The value of the electrical potential that prevents the net diffusion of an ion down its concentration gradient is called the equilibrium potential or Nernst potential (Ex)[1]; for K+ it is abbreviated as EK
      2. Calculating an ion’s equilibrium potential (Nernst potential)
        1. Let’s start by recognizing that the Nernst potential represents the Vm in which a single ion has reached chemical equilibrium (i.e., there is no net diffusion of the ion down its concentration gradient). Thus, the cell is at electrochemical equilibrium.
        2. The electrochemical gradient for any molecule (Dmx) can be calculated using the following equation:

x = TR*ln [X]i + zxFVm

[X]o

          1. It is not important to understand the values for each of these parameters or to memorize this equation. This is just a starting point to derive the Nernst equation.

However, the left half of the right side of the equation (i.e., TR*ln [X]i/[X]o) represents the potential energy created by an ion’s concentration gradient

The right half of the right side of the equation (i.e., zxFVm) represents the potential energy needed to move ions when an electrical potential exists

          1. When these two opposing gradients are in electrochemical equilibrium (i.e., they are equal in magnitude but opposite in sign [positive vs. negative]), Dmx = 0
        1. Some fancy mathwork can rederive this equation when Dmx = 0. It becomes

Ex = -61.5 log [Xin]

z [Xout]

where:

Ex = the equilibrium potential (Nernst potential) for an ion “x”; it represents Vm when Dmx = 0

z = the valence of ion x (+1 for Na+ or K+; –1 for Cl–)

[Xin] = the concentration of ion ‘x’ outside the cell

[Xout] = the concentration of ion ‘x’ inside the cel

IMPORTANT CAVEAT: When considering the Nernst potential, one must assume that the membrane is permeable to that one ion only. If the membrane is permeable to more than one ion at a time, the Vm will never reach Ex.

        1. Calculated Nernst potentials:
          1. ENa = +71 mV based on [Na]o = 145 mM and [Na]i = 10 mM
          2. ECl = –77 mV based on [Cl]o = 106 mM and [Cl]i = 6 mM
          3. EK = –89 mV based on [K]o = 140 mM and [K]i = 5 mM
          4. Remember the convention when reporting electrical potentials: it’s always the electrical potential measured inside the cell relative to the electrical potential outside (which by convention is 0 mV)
        2. Significance of the Nernst potential
          1. The Nernst equation tells you the electrical potential resulting from the diffusion of a single ion. (Try to envision it as the electrical potential that results if the cell starts with a Vr of 0 mV and then we let only that ion diffuse down its concentration gradient)
          2. CAVEAT: Remember that the resting cell membrane can conduct more than one ion simultaneously, namely Na+, K+ & Cl-. That will be the subject the next section (i.e., section V).
          3. At this point the greatest value to knowing the Nernst potential of an ion is this: it tells you what will happen to the Vm if we make the plasma membrane very conductive to that particular ion only

For example, if we have an excitable cell that’s mostly conductive to Na+ (as happens in an action potential), the Vm will approach a value close to ENa

If we suddenly make the membrane mostly conductive to K+, the Vm will rapidly shift toward the Nernst potential for K+ (EK)

          1. The magnitude of the Nernst Potential (Ex) is determined by the size of the ion’s concentration gradient (i.e., [ion x]IC – [ion x]EC). The larger the ion’s concentration gradient is, the faster diffusion will be and the larger the magnitude of Ex has to be to stop the ion’s diffusion.

You will need to understand this concept to understand how pathologies such as hyper- or hypokalemia affect Vr.

For the first test (i.e., module 1 exam), you will also have to determine how an ion’s concentration gradient affect Vr (see the handout entitled “Chord conductance variables and Vr”)

  1. State and be able to calculate the equilibrium potentials of Na+, K+, and Cl- given various concentrations of these ions. Also, explain the significance of the equilibrium potential.
      1. Unlike the Nernst potential of an ion (Ex) that results from the movement of just one ion across the membrane, Vr results from the simultaneous movement of three ions (K+, Na+ & Cl-) across the membrane
        1. Each of these three ions will move down their concentration gradients towards their Ex
        2. Recall that K+ is leaving the cell but Na+ enters the cell as they move down their respective ion concentration gradients.
          1. K+ movement makes the cell interior negative while Na+ movement makes the cell interior positive. As a result, neither ion ever reaches their Ex.
          2. However, the resulting Vr becomes stable at a point midway between EK (-89 mV) and ENa (+71 mV).
          3. The new, stable Vr would be exactly halfway between these points (i.e., -9 mV) IF these were only two ions that flowed through the membrane AND if they were conducted equally.

NOTE that we have neglected to consider Cl- movement across the membrane in this example.

Also note that all ions are NOT conducted equally in a cell at rest. STAY TUNED!

      1. The resting membrane potential (Vr) is defined as the membrane potential of a cell at rest (for an excitable cell, resting means between action potentials).
        1. Earlier we saw the Vr for a neuron is ~ -70 mV
        2. At Vr, ions are still flowing in or out of the cell as they attempt to reach their equilibrium potential, Ex. However, all ion currents sum to 0 A (i.e., IK + INa + ICl = 0)
          1. To restate, individual ion currents can exist at Vr (e.g., INa can be -20 pA, IK = +19 pA & ICl = +1 pA), but the sum of these individual currents is always 0 A.

Note that movement of positively-charged ions (e.g., Na+) into the cell represents a negative current. It may seem counterintuitive, but Na+ currents are negative by convention.

Similarly, movement of positively-charged ions (e.g., K+) out of the cell represents a positive current.

Movement of negatively-charged ions (e.g., Cl-) into the cell represents a positive current.

Don’t worry about whether a current is positive or negative in magnitude for now; just know that the three currents (IK, INa & ICl ) sum to 0 A at Vr.

However, DO note that individual currents (IK, INa …) still exist at Vr.

          1. We can calculate Vr (or Vm) using a modification of Ohm’s Law. Specifically, Ix = DVx * Gx (emphasis on the D in DVx)

Why the change?

We can only measure Ix for a single type of ion (e.g., Na+ or K+) since most channels are ion-specific.

DVx incorporates Ex, which is the voltage that results following diffusion of a single ion. This is subtracted from Vm, which represents the voltage resulting from the simultaneous diffusion of all three major ions (Na+, K+ or Cl-).

Here, DVx = Vm - Ex

This is the “driving force” (i.e., potential energy) behind an ion’s current (kinetic energy)

The driving force will be different for each ion since each ion has a different Ex

Gx in this equation represents an ion’s conductance

Each ion has a different conductance at Vr

An ion’s conductance is based upon the number of channels in a cell membrane that conduct that particular ion.

A leak channel has the greatest single channel conductance, gx, for K+ followed by gNa. gCl has the lowest conductance at rest[1].

Recall that cellular conductance (Gx) = gx * Nx * Po

In this case, it is the single channel conductance, gx , that is different changing and not the overall number of channels within the entire cell (N) or the channel’s open probability (Po)

In a resting cell (i.e., Vr): GK >> GNa > GCl

GNa is lower than GK at rest since leak channels conduct K+ well (thus their gK is high), but they don’t conduct Na+ very well (thus their gNa is low).

GCl is even lower than GNa at rest. since there aren’t many of Cl- channels that are open at rest.

          1. We can quantify each ion’s conductance at Vr via fractional conductance[2] (Silbernagl and Despopoulos 2015) (p. 32):

Fractional conductance in a resting cell = Gx

GT

where:

Gx = conductance of ion x

GT = total conductance of all three ions

GK = 0.85

GT

GNa = 0.12

GT

GCl = 0.03

GT

The sum of all fractional conductances must equal 1.0; here, 0.85 + 0.12 + 0.03 = 1

        1. Now we can solve for Vr where IK + INa + ICl = 0
          1. Relacing Ix with DVx * Gx yields:

(Vr – EK) * GK + (Vr – ENa) * GNa + (Vr – ECl) * GCl = 0

          1. Then rearranging and solving for Vr yields:

Vr = (EK*GK) + (ENa*GNa) + (ECl*GCl).

GT GT GT

This is called the chord conductance equation.

          1. Solving yields Vr = (-89 mV * 0.85) + (+71 mV * 0.12) + (-77 mV * 0.03)

= -69.4 mV

        1. Meaning of the Chord conductance equation
          1. This equation allows you to calculate Vr (or even Vm during an action potential) once you know the Nernst potentials and their relative conductances
          2. Significance of the equation

Because a resting cell has a high conductance to K+ and a much lower conductance to Na+ and Cl-, Vr in most cells is closer to EK than ENa or ECl

Vm approaches ENa and then EK during the course of an action potential as the relative conductances of these ions change (more on this in the neuromuscular module)

      1. The Na+-K+ ATPase also plays a role in Vr
        1. Concentration gradients, EK & ENa
          1. The primary role of the Na+-K+ ATPase is to maintain the concentration gradients of K+ and Na+ following an action potential.
          2. To do so, it pumps Na+ out of the cell and K+ back into cell. Thus, it prevents the intracellular and extracellular [K+] and [Na+] from changing significantly over long periods of time following millions of action potentials.
          3. By maintaining concentration gradients, it has an effect on the Nernst potentials for both K+ and Na+ (i.e., EK + ENa).
        2. It also has a small electrogenic effect on Vr
          1. Because it pumps 3 Na+ out of the cell for every 2 K+ it pumps into the cell, it makes the interior of the cell slightly more negatively charged
          2. This only accounts for about –4 mV of Vr, a small amount
          3. For example, a cell whose Na+-K+ ATPase is inhibited might have a Vr of –66 mV instead of –70 mV
        3. The contributions of the Na+-K+ ATPase to Vr can be seen best when a cell has a low ATP content (perhaps due to hypoxia).
          1. In this case, the Na+-K+ ATPase would slow or stop working, Na+ concentration inside the cell would rise and K+ concentration inside the cell would fall.
          2. The end result is that the cell would slightly depolarize.
  1. Apply understanding of the Chord Conductance equation to predict how changes in ion conductance or ionic gradients will qualitatively affect the membrane potential (either Vr or Vm).
  2. Explain the mechanism and the role of the Na+-K+ ATPase in the maintenance of the resting membrane potential.
  3. Explain how the membrane potential can be changed for the purpose of cell signaling in the nervous system.
      1. Thus far we’ve talked mostly about Vr, but excitable cells must change Vm for anything interesting to happen (i.e., for neural information to be conveyed or muscular contraction)
      2. The chord conductance equation tells us that Vr and Vm vary as a function of two main physiological factors:
        1. Ex, which is dependent on the chemical concentration gradients of the permeant ions
          1. Chemical concentration gradients are maintained by the kidneys (ECF concentrations) and the Na+-K+ ATPase (ICF concentrations)
          2. Chemical gradients change slowly (seconds to weeks) - too slowly to be useful for signaling. For this reason, they have more of an effect on Vr vs Vm, the latter must change rapidly during cell signaling.
          3. What causes them to change?

Fluid or electrolyte imbalances due to overhydration or underhydration

Diuretic drugs

Extreme dietary changes (e.g., eating tons of bananas)

Kidney disease

Endocrine diseases (such as hyperaldosterism)

Intravenous infusions

Prolonged vomiting or diarrhea

          1. Bottom line: there are a number of medical conditions that can change the Vr and therefore alter the function of the nervous and muscular systems. However, these changes occur MUCH too slowly for signaling purposes
          2. What happens when ion concentrations change?

Vr can change. It can become more positive (i.e., depolarize) or become more negative (i.e., hyperpolarize).

As Vr changes in response to changing ion concentrations, an action potential (and thus cell signaling) may be easier to generate (if Vr is depolarized) or more difficult to generate (if Vr is hyperpolarized).

Extracellular ion concentrations can change over time due to kidney disease or to endocrine disorders that lead to kidney abnormalities (e.g., Cushing’s or Addison’s disease)

Intracellular ion concentrations are less likely to change

Changes can occur as transporters and channels become dysfunctional or mutated.

Different cell types can have different intracellular ion concentrations. For example,

[Cl-]IC in neurons is typically 6 mM. This yields an ECl of -77 mV as calculated earlier.

[Cl-]IC in many other cells are closer to 20 mM yielding an ECl of around -45 mV. Since this ECl is more depolarized than a neuron’s ECl, this would cause Vr to become more depolarized according to the chord conductance equation[1].

K+ has the greatest effect on Vr because it has the highest fractional conductance at rest (0.85 as discussed above)

        1. Gx, which is the conductance of each ion
          1. The cell can change its membrane potential (Vm) by selectively changing the conductance/permeability to certain ions
          2. Conductance can change rapidly (within milliseconds); this is the physiological mechanism for changing Vm!
          3. Conductance is physiologically regulated by gated membrane channels or pharmacologically using local anesthetics, Ca2+ channel blockers, etc.
          4. Gated channels can be opened or closed depending on conditions around the channel

Voltage-gated channels

Open or close depending on a change in Vm

Principally located on the axon hillock, axon and axon terminals of a neuron

Ligand-gated channels (a.k.a. chemically gated or neurotransmitter-gated)

Open or close depending on the binding of a ligand (ligand: generally, any molecule that binds to another; in this context, a neurotransmitter that binds to a receptor)

Principally located in synapses on the dendritic spines of postsynaptic membranes

Mechanosensitive (a.k.a. mechanically gated) channels

Open or close depending on the amount of tension applied to the membrane [tension: a force that stretches something]

Principally located at the distal ends of peripheral processes of certain afferent (sensory) neurons

Purpose of gated channels

Gated channels permit us to selectively change the conductance (G) of the plasma membrane under certain conditions

As you’ll see, this enables us to produce meaningful neuronal signals

          1. What happens when conductance changes?

Conductance changes cause momentary changes in a cell’s Vm. These conductance changes occur due to the opening of channels that are not open when a cell is at rest. Thus, these conductance changes cause a change in Vm instead of Vr.

Recall that in a resting membrane, more K+ ions are passing through the membrane than any other ion. In addition, a small number of Na+ ions and Cl- are moving through the membrane at the same time

If the membrane is made significantly more conductive to Na+, Na+ momentarily diffuses into the cell faster than K+ and positive charge accumulates inside the plasma membrane.

Vm becomes more positive; thus, it depolarizes

In essence, Vm shifts towards ENa

Conversely, if the membrane is made significantly more conductive to K+, K+ quickly diffuses out of the cell and negative charge momentarily accumulates inside the plasma membrane

Vm becomes more negative; thus, it hyperpolarizes

In essence, Vm shifts towards EK

        1. How to predict a qualitative change in Vm when one of the variables in the chord conductance equation changes in a resting cell
          1. Ask yourself how the change (Ex or Gx) would affect the diffusion of the ion: would diffusion increase or decrease?
          2. Ask yourself in what direction the net diffusion will take place
          3. Based on your answers to the above, ask yourself if positive or negative charge has accumulated inside the cell

If more positive charge has diffused into the cell, Vm or Vr will shift to a more positive value.

If more negative charge has diffused into the cell, Vm or Vr will shift to a more negative value.

          1. Please see the file entitled “Chord conductance variables and Vm” on Canvas for more practice. An answer key is also available.