A& P Test 3 : Excitable Cells

Thin Filaments

• actin proteins

• passive “molecular rope” for the contractors tug of war

Tropomyosin

• regulatory protein

• thin, thread like, protein that overs binding sites (active sites) on the actin thin filament

Troponin

• regulatory protein

• Ca2+-sensitive protein that positions the tropomyosin

Thick Filaments

• Made mainly of myosin-II proteins, which use ATP to pull actin thin filaments inwardand produce musclecontraction.

Exactation

• events between a motor neuron and skeletal muscle fiber —> generate an action potential

Excitation

• A motor neuron releases ACh at the neuromuscular junction. ACh opens ligand-gated Na⁺ channelson themotor end plate, causing Na⁺ influx and depolarization (motor end-plate potential). If thresholdis reached,an action potential spreads across the sarcolemma and down the T-tubules

Contraction (and Relaxation) Cycle

• sliding filament theory

Steps of Excitation

• Nerve Signal Arrives

• Neurotransmitter Release

• Sarcolemma Depolarization

• Action Potential Propagation

• Voltage Sensing

• Calcium Release

• Unshielding Actin Sites

Excitation : Nerve Signal Arrives

• A somatic motor neuron action potential reaches the axon terminal at the Neuromuscular Junction

Excitation : Neurotransmitter Release

• the Arrival of the impulse triggers the exocytosis of acetylcholine (ACh) into the synaptic cleft

Excitation : Sarcolemma Depolarization

• ACh binds to ligand-gated sodium (Na+) channels on the motor end plate of Sarcolemma, causing Na+ to rush in and create and end-plate potential

Excitation : Action Potential Propagation

• This electrical wave sweeps across the sarcolemma and plunges deep into the muscle fiber via specialized invaginations called T-tubules

Excitation : Voltage Sensing

• The action Potential activates dihydropyridine receptors (DHPR) (voltage-gated Ca²+ channels) located in the T-tubule membranes

Excitation : Calcium Release

• DHPR is physically or allosterically linked to ryanodine receptors (RyR) on the terminal cistern of the sarcoplasmic reticulum (SR). This open RyR channels, allowing stored Ca²+ to flood out into the sarcoplasm

Excitation : Unshielding Actin Sites

• Calcium ions bind to troponin C on the thin filaments. This shifts the tropomyosin complex away, exposing the active binding sites on actin so myosin heads can attach and form cross—bridges

Diffusion :

• the tendency of molecules or ions to move from an area where they are in higher concentration to an area where they are in lower concentration, that is, down or along their concentration gradient

• Passive

• Mode of Force : Gradient

Selective Permeability

• allows some substances to pass while excluding others

• characteristic of healthy, intact cells

A Molecule will diffuse through a membrane if it is :

• Lipid Soluble

• Small enough to pass though membrane channels

• Assisted by a carrier molecule

Simple DIffusion

• kinetic energy

• Net movement of molecules from an area of their higher concentration to an area of their lower concentration, that is, down their concentration gradient

• Fats, oxygen, and carbon dioxide move through the lipid bilayer of the membrane

Facilitated Diffusion

• kinetic energy

• Same as simple diffusion, but the diffusing substance is attached to a lipid-soluble membrane carrier protein (carrier-mediated facilitated diffusion) or moves through a membrane channel (channel-mediated facilitated diffusion)

• Glucose and some ions move into cells

Carriers

• are transmembrane integral 3 proteins that are specific for transporting certain polar molecules or classes of molecules, such as sugars and amino acids, that are too large to pass through membrane channels

Channels

• are transmembrane proteins that transport substances, usually ions or water, through aqueous channels from one side of the membrane to the other

• selective due to pore size and the charges of the amino acids lining the channel

Leakage Channels

• are always open and simply allow ions or water to move according to concentration gradients

Gated Channels

• are controlled (opened or closed), usually by chemical or electrical signals

Osmosis

• kinetic energy

• Diffusion of water through a selectively permeable membrane

• Movement of water into and out of cells directly through the lipid bilayer of the membrane or via membrane channels (aquaporins)

Aquaporins

• Water also moves freely and reversibly through water-specific channels con structed by transmembrane proteins

• particularly abundant in red blood cells and in cells involved in water balance such as kidney tubule cells

Osmolarity

• the total concentration of all solute particles in a solution

Hydrostatic Pressure

• the back pressure exerted by water against the membrane

Osmotic Pressure

• the tendency of water to move into the cell by osmosis

Do major changes in hydrostatic and osmotic pressure occur in living animal cells?

No

If there is an imbalance of (the tendency of water to move into the cell by osmosis?

• 1. the solute concentration is the same on both sides of the plasma membrane

• 2. the membrane stretches to its breaking point

Tonicity

• the ability of a solution to change the shape or tone of cells by altering the cells’ internal water volume

Isotonic Solutions

• Cells retain their normal size and shape in isotonic solutions (same solute/water concentration as inside cells; water moves in and out)

Hypertonic Solutions

• Cells lose water by osmosis and shrink in a hypertonic solution (contains a higher concentration of nonpenetrating solutes than are present inside the cells)

Hypotonic Solutions

• Cells take on water by osmosis until they become bloated and burst (lyse) in a hypotonic solution (contains a lower concentration of nonpenetrating solutes than are present inside cells).

Active Process

• occurs whenever a cell uses energy to move solutes across the membrane

Primary Active Transport

• ATP

• Transport of substances against a concentration (or electrochemical) gradient. Performed across the plasma membrane by a solute pump, directly using energy of ATP hydrolysis.

• Ions (Na+, K+, H+, Ca2+, and others)

Symport System

• the two transported substances move in the same direction

Antiport System

• a membrane transport mechanism that moves two or more different molecules or ions across a phospholipid membrane in opposite directions

Electrochemical Gradients

• refers to the combined influence of two forces that dictate the movement of ions across a cell membrane

• Concentration and Electrical Gradient

Secondary Transport

• Uses an ATP-created ion gradient to move two solutes across a membrane. Symporters movethem in thesame direction; antiporters move them in opposite directions.

Vesicular Transport

• fluids containing large particles and macromolecules are transported across cellular membranes inside bubble-like, membranous sacs called vesicles

Transcytosis

• moving substances into, across, and then out of the cell

Vesicular Trafficking

• moving substances from one area (or membranous organelle) in the cell to another

• - energized by ATP

Steps in Endocytosis and Transcytosis

1. Coated pit forms: Clathrin helps select substances and fold the membrane inward.

2. Vesicle detaches from the membrane.

3. Coat proteins are recycled back to the plasma membrane.

4. Vesicle fuses with an endosome to sort its contents.

5. Membrane components and receptors are recycled back to the plasma membrane.

6. Contents are processed: They either fuse with a lysosome to be broken down or cross the cellandexit the opposite side (transcytosis).

Phagocytosis

• ATP

• Endocytosis

• A large external particle (proteins, bacteria, dead cell debris) is surrounded by a pseudopod (“false foot”) and becomes enclosed in a vesicle (phagosome).

• In the human body, occurs primarily in protective phagocytes (some white blood cells and macrophages)

Pinocytosis

• ATP

• Endocytosis

• Plasma membrane sinks beneath an external fluid droplet containing small solutes. Membrane edges fuse, forming a fluid-filled vesicle.

• Occurs in most cells; important for taking in dissolved solutes by absorptive cells of the kidney and intestine

Receptor-mediated endocytosis

• ATP

• Endocytosis

• Selective endocytosis and transcytosis. External substance binds to membrane receptors.

• Means of intake of some hormones, cholesterol, iron, and most macromolecules

Coat protein play an important role in endocytosis

• True

Caveolae

• Small tube- or flask-shaped membrane pockets that capture specific molecules forreceptor-mediatedendocytosis and help transport substances across cells (transcytosis).

Vesicular Trafficking

• ATP

• Vesicles pinch off from organelles and travel to other organelles to deliver their cargo.

• Intracellular trafficking between certain organelles, e.g., endoplasmic reticulum and Golgi apparatus
  
  

Exocytosis

• ATP

• Secretion or ejection of substances from a cell. The substance is enclosed in a membranous vesicle, which fuses with the plasma membrane and ruptures, releasing the substance to the exterior.

• Secretion of neurotransmitters, hormones, mucus, etc.; ejection of cell wastes

Excitable Cells

• any cell w/ a membrane that can hold a charge and propagate and electric signal

• has a non-zero resting membrane potential

2 Types of Excitable Cells

• Neurons - transmit electrical impulses

• Muscle Cells - communicate the stimulus to contract to intracellular machinery

Netflux (Jnet)

• occur from high to low until concentration gradient disappears

Ficks First Law of Diffusion : Jnet formula

P x A x (Co - Ci) / d

• Jnet = net flux, net rate of diffusion

• P = permeability

• A = surface area

• Co - Ci = concentration gradient

• d = diffusive distance

Jnet proportionality

• Jnet proportional to Permeability

• Jnet proportional to Surface Area

• Jnet proportional to Concentration Gradient

• Jnet inversely proportional to diffusive distance

Na+/K+ ATPase

• 3 Na+ out

• 2 K+ in

• Hydrolyzes 1 ATP

Neuron Cell Body

• consists of a spherical nucleus with a con spicuous nucleolus surrounded by cytoplasm

Neurofibrils

• are bundles of intermediate filaments (neurofilaments), are important in maintaining cell shape and integrity

Clusters of Cell bodies in CNS

• called nuclei

Clusters of Cell Bodies in PNS

• called ganglia

Dendrites

• short, tapering, diffusely branching extensions

• the main receptive or input regions, provide an enormous surface area for receiving signals from other neurons

• convey incoming messages toward the cell body

Multipolar

• Many processes extend from the cell body. All are dendrites except for a single axon.

• most common type in humans

• major type of Neuron in CNS

Bipolar

• Two processes extend from the cell body. One is a fused dendrite, the other is an axon.

• rare, found in special sense organs

Unipolar

• One process extends from the cell body and forms central and peripheral processes, which together comprise an axon. (like a T-shape)

• originate in bipolar neurons

Long axons are called

• nerve fibers

Each nerve has how many axons

• a single axon

The initial region of the axon arises from a cone-shaped area of the cell body called the

• axon hillock

Axon Collaterals

• Each neuron has only one axon, but axons may have occa sional branches along their length

Axon terminals

• The knoblike distal endings of the terminal branches

Axons act as the

• conducting region of neurons

• it generates nerves impulses and transmits them

Neurotransmitters

• are chemical messengers that play a vital role in how your brain and body communicate

Anterograde

• Moves substances away from the cell body, including mitochondria, cytoskeletalelements, membranecomponents, and neurotransmitter-making enzymes.

Retrograde

• toward the cell body

• mostly transports organelles returning to the cell body to be degraded or recycled

Myelin Sheath

• protects and electrically insulates fibers, and it increases the transmission speed of nerve impulses

• formed by Schwann cells

Myelinated Fibers

• conduct nerve impulses rapidly

• assocated with axons

Non-Myelinated Fibers

• conduct impulses more slowly

• dendrites are alway non-myelinated

Schwann Cells

• are specialized glial cells in the peripheral nervous system that play a crucial role in supporting nerve fibers. They are responsible for forming the myelin sheath around myelinated axons, which enhances the speed of electrical signal transmission

Nodes of Renvier

• are small gaps in the myelin sheath of myelinated nerve fibers

White Matter

• Regions of the brain and spinal cord containing dense collections of myelinated fibers

Gray Matter

• mostly neuron cell bodies and nonmyelinated fibers

Sensory or Afferent Neurons

• transmit impulses from sensory receptors in the skin or internal organs toward or into the central nervous system

• virtually all unipolar

Motor, or efferent, neurons

• carry impulses away from the CNS to the effector organs (muscles and glands) of the body\

• multipolar

Interneurons

• lie between motor and sensory neurons in neural pathways and shuttle signals through CNS pathways where integration occurs

What structure of the neuron receive a signal?

• Dendrite

What structure of the neuron transmits information along the length of the neuron?

• axon

What structure of the neuron communicates with the target cell

• axon terminals

What structure/regions of the neuron exhibit a resting membrane potential?

• primarily exhibited in the neuron's cell membrane

Where does the action potential occur?

• primarily occur in the axon

Where do graded potentials occurs?

• primarily occur in the dendrites and cell body of neurons

Where is neurotransmitter released?

• released at the axon terminals

Soma

• neuron cell body

Glial Cells

• Support cells in neurons

Which of the following is true about concentration gradients?

• energy is required to establish and maintain concentration gradients

Which of the following is true about diffusion?

• It is a passive process where molecules move from an area of high concentration to an area of low concentration.

Which of the following is true about Fick's law of diffusion?

• J_net is inversely proportional to distance

• J_net is proportional to permeability

• J_net is proportional to concentration gradient

• J_net is proportional to surface area

Which of the following describes the concentrations of ions in excitable cells?

• Na⁺ is in high concentration outside of the cell and low concentration inside of the cell. K⁺ is in low concentration outside of the cell and high concentration inside of the cell.

Which of the following is true about the resting membrane potential?

• There are high concentration gradients for both Na⁺ and K⁺, but the resting membrane potential results from a high permeability to K⁺ and a low permeability to Na⁺.

Which of the following are excitable cells?

• neurons

• skeletal muscles

• cardiac muscles

• smooth muscles