NPB101 Midterm 1

Study guide material

What is the plasma membrane?

• the phospholipid bilayer and all the associated proteins/molecules

• many are transmembrane proteins

• these grant selective permeability to ions, glucose and other molecules

Nucleus

• hosts the genome and is the site of transcription which produces mRNAs that are exported

Ribosomes

• these are the site of protein synthesis (translation) 

• Found on the ER(studded) or in the cytoplasm (free)

ER/Golgi complex

• uses a vesicle-based system (budding and fusion) to sort new proteins to either the PM, out of cell (soluble proteins releases via exocytosis) or lysosomes

• cytoplasm and other organelles get their proteins from free ribosomes

Mitochondria

• produces ATP from glucose or fatty acids (can use amino acids in a pinch)

ATP Production - Glycolysis

• occurs in cytosol

• does not require oxygen

• yields 2 ATP

ATP Production - Kreb Cycle

• occurs in mitochondira

• does not require oxygen 

• yields 2 ATP

ATP Production - ETC

• occurs in mitochondria

• requires oxygen

• yields 28-32 ATP

What do Lysosomes do?

• digest debris by fusing with intracellular vesicles often derived from endocytosis

Peroxisomes

Detoxify free radicals

What does the cytoplasm consist of?

• semi-liquid cytosol

• aqueous compartment (intermediate metabolism occurs)

• organelles

• cytoskeleton

What are Microtubles?

• dynamic polymers of tubulin

• form highways for movement of transport vesicles via kinesin and dynein motor proteins and cilia and flagella for generating movement

Cilia - motile, hair-like protrusions on cell surface

Flagella - motile appendage enabling cellular movement (sperm)

What are microfilaments?

• dynamic polymers of actin

• in association with myosin, motor proteins 

• produce cellular contractions (ex. muscle fibers)

What are intermediate filaments?

• longer proteins produced by an array of different genes

What is the cause of complex multicellular life and the requirement of various specialized cells?

Differential gene expression

Functions of a Cell

• obtain nutrients and oxygen

• exchange of materials

• intracellular transport

• metabolism

• synthesis

• reproduction

Tissue

• aggregate of cells and extracellular material

• 4 main types: muscles, nervous, epithelial and connective

Components of muscle tissues

skeletal, cardiac and smooth

• all needed for contraction

Components of Nervous tissue

• signaling

• central and peripheral nervous

Components of Connective tissue

• structural support

• tendons, bones and blood

Components of epithelial tissue

• exchange

• epithelial sheets (form boundaries)

• glands (secretion)

  • exocrine (external secretion)

  • endocrine (internal secretion) 

Organ

(two or more primary tissues organized to perform a function)

composed of all 4 tissue types

• Epithelial

  • epithelial sheet - barrier to digestive juices

  • exocrine gland - secretes digestive juices

  • endocrine gland - regulates exocrine secretion

• Muscle

  • smooth muscle - stomach wall

• Nervous

  • peripheral nerves - regulate contraction

• Connective

  • structural support

What is homeostasis and why is it important?

• Dynamic maintenance of a stable internal

(extracellular) environment within the organism

- essential to survival of each cell

- requires continual exchange of materials between the

intracellular and extracellular spaces

- each organ system contributes by counteracting changes of

internal environment

What are some factors of homeostasis that must be maintained

Concentration of nutrients

(circulatory, digestive, muscular, nervous, and endocrine systems)

- Concentration of O2 and CO2

(circulatory, respiratory, muscular, and nervous systems)

- Concentration of waste products

(circulatory, digestive, urinary, muscular, nervous, and endocrine systems)

- pH

(respiratory, urinary, and nervous systems)

- Concentration of water and electrolytes

(circulatory, digestive, urinary, skeletal, muscular, integumentary, nervous,

and endocrine systems)

- Temperature

(muscular, integumentary, and nervous systems)

- Volume and pressure

(circulatory, nervous, and endocrine systems)

- Defense against foreign invaders

(immune system)

Homeostasis Control Systems - Intrinsic

• local control systems built into an organ

  • e.g. increased CO 2 production leads to relaxation of smooth muscle and dilation of blood vessels

Homeostasis Control Systems - Extrinsic

• external control system outside of an organ permitting coordinated regulation of several organs

Negative feedback

change in a controlled variable triggers a response that opposes the change

What are the components of homeostasis and explain them?

• sensor - mechanism to detect the controlled

variable

• set point - the desired value of the variable

• integrator - compares the sensor’s input

with the set point

• effector - adjusts the value of the controlled

variable

Positive Feedback

reinforces the change in a controlled variable occurs relatively rarely

Feedforward control

response occurring in anticipation of a change in control variable

What are the types of intercellular communication?

• gap junctions

• transient direct contact

• extracellular chemical messengers

What are the 2 major control systems of the human body?

• endocrine system

• nervous system

What are the chemical communication types?

Endocrine (hormonal): long-range via bloodstream.

Paracrine: local diffusion to nearby cells.

Synaptic (neurotransmitter): very short range, fast, highly specific.

What are the basic features of the receptor classes?

• Nuclear receptors: intracellular, ligand-activated transcription factors (slow genomic effects).

• GPCRs (G protein-coupled receptors): 7-TM proteins coupling to G proteins → second messengers; widely used for hormones/neurotransmitters.

• Enzyme-linked receptors: transmembrane receptors with intrinsic or associated enzymatic activity (often tyrosine kinases) that trigger phosphorylation cascades.

• ionotropic receptors: ligand-gated ion channels that mediate fast electrical/ionic responses.

What does the brain rely on?

• neurons specialized for

  chemical and electrical signaling.

• The 86 billion neurons in the human brain and

  interconnected by 100+ trillion synapses where a chemical neurotransmitter is released by

  one neuron and detected by another (note: electrical synapses also exist).

What is the basis of electrical signaling in neurons?

• ion movement across the plasma membrane

What are the two types of transmembrane proteins for ion/molecule movement?

• carriers (transporter proteins) = are used to escort molecules across the membrane that can’t diffuse unassisted. One example is the glucose transporter which allows cells to take up glucose from the blood

• Ion channels (channel protein) = are different types of membrane proteins needed for electrical signaling in neurons, muscle and cardiac tissue. They are permeable to specific ions such as Na+ or K +.

Binding sites for carriers

• Facilitated diffusion uses a fixed affinity site and transports down the concentration gradient.

• Pumps have variable affinity sites and transport uphill, against the concentration gradient.

What does the Na and K+ ATPase do?

• it is a pump that transports 3 Na out and 2 K in with each turn of cycle ( 1 ATP)

Mnemonic: 3-2-1 NOKIA

What is the role of the NA/K ATPase?

establish and maintain concentration gradients

Do Ion channels have binding sites?

• No binding sites

• they have pores which allow for diffusion-like permeation

What are the two driving forces used to make ions move?

• Chemical Driving force: diffusion down a

concentration gradient

• Electrical Driving force: results from electrostatic

interactions at a distance

Note: At any moment, each driving force can be represented as a vector which has direction and magnitude. At any moment, ions experience a net driving force which is the vector sum.

What does the membrane potential in neurons results from?

• charge separation across the membrane

• by convention the polarity is reference inside relative to outside

e.g. at rest there is an excess of negative charges on the inside and excess of positive charges on the outside, for a resting potential of -70 mV.

Why do Na and K concentration gradients not run down during normal physiological operations?

• The amount of charge separation underlying biologically meaningful electrical signaling is

  extremely small compared to the total number of ions in bulk solution on both sides of the

  membrane

What does Ion equilibrium potential imply? and what equation is used.

• the equilibrium potential implies that this is the point in which membrane potential has no net charge movement for that ion

• calculated via the Nernst equation

What determines a cell’s resting membrane potential?

RMP depends on all permeant ions weighted by their relative permeabilities. K⁺ permeability dominates due to more K⁺ leak channels than Na⁺ channels. It is calculated using the Goldman–Hodgkin–Katz (GHK) equation.

What is a graded potential and how does it behave?

A small, local change in membrane potential that passively dissipates over distance and time. It decreases in amplitude away from the current source and does not propagate actively.

What is an action potential?

A rapid, “all-or-none” electrical signal initiated at the axon hillock that propagates along the axon to terminals, triggering neurotransmitter release.

• the brain is a synaptic network

Which channels generate action potentials and how?

Voltage-gated Na⁺ and K⁺ channels open and close in a voltage- and time-dependent sequence, producing transient changes in membrane permeability during the AP.

How do Na⁺ and K⁺ driving forces change during an AP?

At AP onset, Na⁺ driving force is strong (depolarization). At AP peak, Na⁺ driving force weakens while K⁺ driving force strengthens (repolarization).

What is the absolute refractory period?

~1 ms period when a neuron cannot fire another AP regardless of stimulus strength. Occurs while Na⁺ channels are open/inactivated and ends when inactivation is removed.

What is the relative refractory period?

A few milliseconds following the absolute period when another AP can occur only with a stronger stimulus. Occurs while K⁺ channels are still active and until resting potential is restored.

What determines the speed and reliability of AP propagation?

Axon diameter, membrane resistance, internal (axial) resistance, and myelination.

What is contiguous conduction?

Continuous AP propagation along unmyelinated axons via sequential activation of voltage-gated Na⁺ and K⁺ channels along the membrane (like a “stadium wave”).

What is saltatory conduction?

APs jump between Nodes of Ranvier in myelinated axons. Active conduction occurs at nodes, while passive current flows under myelin sheaths—making it faster and more efficient.

How do membrane and internal resistance affect current flow?

Low internal resistance (large axons) favors AP propagation; high internal resistance (small axons) causes current leak. Myelin increases membrane resistance, channeling current along the axon.

How does myelin affect capacitance and AP speed?

Myelin decreases capacitance, lowering the time constant so the membrane potential changes faster, speeding up AP propagation.

How does Na⁺ channel inactivation ensure unidirectional AP spread?

Once Na⁺ channels inactivate, the membrane behind the AP cannot depolarize immediately, forcing the signal to move forward only. Colliding APs cancel each other out.

What happens when myelin is lost (e.g., in multiple sclerosis)?

AP propagation becomes slow and unreliable. In MS, demyelination in the cerebellum leads to poor motor coordination and “action tremors.”

What are synapses?

Junctions between a neuron and its target cell that allow communication.

What are electrical synapses?

Synapses that use connexin protein tunnels (gap junctions) for direct electrical coupling; allow passive current flow between cells. Found in cardiac syncytium, rare in mature brain.

What are chemical synapses?

Synapses that use neurotransmitter release to communicate between neurons; dominate in the mature brain.

What is excitation-secretion coupling?

The Ca²⁺-dependent exocytosis of neurotransmitters during synaptic transmission.

What are ionotropic receptors?

Receptors that are ligand-gated ion channels; open directly to cause fast changes in membrane potential.

What are metabotropic receptors?

G-protein-coupled receptors that trigger second-messenger cascades which act indirectly on ion channels.

What is the difference between EPSP and IPSP?

EPSP (excitatory postsynaptic potential): depolarizes cell toward threshold.

IPSP (inhibitory postsynaptic potential): hyperpolarizes cell away from threshold.

What ions flow through ionotropic glutamate receptors?

Both Na⁺ and K⁺; Na⁺ dominates, causing depolarization (reversal potential ≈ 0 mV → excitatory).

What ions flow through ionotropic GABA receptors?

Cl⁻; reversal potential ≈ −70 mV → inhibitory.

What do metabotropic GABA receptors typically activate?

K⁺ channels (E_rev ≈ −90 mV), also inhibitory.

What is an equilibrium potential?

The membrane potential at which a specific ion has no net movement across the membrane.

What is a reversal potential?

The voltage at which the net current through a channel equals zero.

What is convergence?

Multiple presynaptic neurons synapsing onto a single postsynaptic neuron.

What is divergence?

A single neuron sending output to multiple postsynaptic targets.

What is spatial and temporal summation?

Spatial: PSPs from multiple synapses combine simultaneously.

Temporal: PSPs from one synapse occur in rapid succession and add together.

When is an action potential fired?

When summed PSPs reach threshold at the axon hillock.

Afferent vs Efferent pathways

Afferent: Ascending, carrying sensory input toward the brain (via dorsal roots).

Efferent: Descending, carrying motor output away from the brain (via ventral roots).

What is functional localization?

The concept that specific brain regions perform specific functions.

What is a topographic map in the brain?

The organized mapping of sensory or motor surfaces onto the cortex (e.g., retinotopic, somatotopic).

Why aren’t chemical messengers inherently excitatory or inhibitory?

The effect depends on receptor identity, not the transmitter itself.

What do receptor cells do?

Transduce environmental energy (light, sound, touch) into a receptor potential

• a change in membrane potential.

What is a sensory epithelium/surface?

A sheet of receptor cells specialized for a sensory modality (e.g., retina, cochlea, skin)

How is sensory information encoded?

By the rate and timing of action potentials in receptor cells or their targets.

What is the role of the thalamus in sensation?

Relay station for visual, auditory, and somatosensory information to primary cortices.

What is a receptive field?

The region on a sensory surface where stimulation changes a neuron’s firing.

What is lateral inhibition?

Neural mechanism where activated neurons suppress neighbors, sharpening receptive fields- side channel suppression

What determines sensory acuity?

Receptor density and receptive field size

more dense/smaller fields → higher acuity.

What are topographic maps?

Spatial organization of the sensory surface maintained through higher processing levels (labelled-line coding).

What does the pupil do?

Controls the amount of light entering the eye

What does the lens do?

Focuses light onto the retina, the 2-D sensory surface.

What are rods and cones?

Photoreceptors

• Rods: high sensitivity, low light, no color.

• Cones: color vision, bright light, high acuity.

How do photoreceptors transduce light?

Light → activates photopigment → G-protein cascade → cleaves cGMP → closes Na⁺ channels → hyperpolarization → stops glutamate release → disinhibits downstream neuron → RGC fires spikes.

What are retinal ganglion cells (RGCs)?

Output neurons of the retina; their axons form the optic nerve.

What happens at the optic chiasm?

Medial axons cross the midline → visual fields are split between hemispheres.

What is retinotopic mapping?

Spatial mapping of retinal input preserved in primary visual cortex.

What are the ‘what’ and ‘where’ pathways?

Cortical streams from primary visual cortex for object identification and spatial processing.

What is sound?

A mechanical wave of alternating compression and rarefaction in air or water.

How does sound reach the cochlea?

Reflected by pinna → ear canal → tympanic membrane → ossicles → oval window → cochlear fluid motion.

What is the basilar membrane?

Membrane in the cochlea that resonates at specific frequencies (high near base, low near apex)

What are hair cells?

Mechanoreceptors with stereocilia; bending opens mechanically-gated ion channels → K⁺ influx → depolarization → transmitter release.

What is a tonotopic map?

Spatial map of sound frequencies maintained from cochlea through auditory cortex.