Core Exam 3

What is a kinase?

An enzyme that adds phosphate to proteins

It is NOT the same as phosphorylase/pyrophosphorylase

What does a kinase do?

Protein kinases participate in many cellular functions such as:

• Transcription

• Cell structure and motility

• Exocytosis

• Receptor desensitization

How are kinases classified, and what determines their substrate specificity?

Classified based on the amino acid they add the phosphate

The specificity for Ser/Thr or Tyr is determined by:

• Consensus sequences on substrate

• Size of the binding site (catalytic cleft) in kinases

What are the structural features of kinases?

– Kinases have regulatory and catalytic domains

– Some regulatory domains are separate polypeptides

– The catalytic domain binds ATP and the substrate

How are Ser/Thr kinases regulated?

– PKA – activated by cAMP binding to regulatory subunits

– PKB – translocation to membrane, activation by phosphorylation

– PKC – autophosphorylation, translocation to membrane

– CaMK – binding to CaM, autophosphorylation

How are Tyr kinases regulated?

– Autophosphorylation on Tyr residues (RTKs, JAKs)

– Both phosphorylation and dephosphorylation of Tyr residues (Src)

– Phosphorylation of both Thr and Tyr residues (MAPKs)

What is a phosphatase?

– Removes phosphate from proteins

– The catalytic site of Ser/Thr PPases has a dinuclear metal center

– The catalytic site of a Tyr PPase has a cysteine center

What does a phosphatase do?

Regulates cellular functions in conjunction with kinases

What are the different kinds of phosphatases?

– Ser/Thr specific (PP1, PP2A. PP2B (calcineurin))

– Tyr specific (PTP)

– Both Ser/Thr and Tyr (DuSP)

How are phosphatases regulated?

By phosphatases:

– PP1 (1 catalytic subunit, multiple regulatory subunits; RVxF motif in regulatory subunit binds to catalytic subunit)

– PP2 A (Catalytic subunit, regulatory subunit, scaffolding subunit)

– PP2B (Regulatory subunit is a calcium binding protein)

What is signal transduction?

Mechanism by which cells respond to the changes in the external environment

Why is signal transduction important?

Mechanism by which cells respond to the changes in the external environment

What are the different types of transduction processes?

Paracrine, Endocrine, Contact-dependent, Synaptic

What are first and second messengers?

First messengers are Lipids, peptides, small molecules, proteins

• Not all first messengers are soluble, small molecules

Second messengers are intracellular ions, chemicals, proteins

What is the difference between Convergence and Divergence?

Convergence is when multiple receptors work through one second messenger

Divergence is when one receptor works through multiple second messengers

Why do we need receptors?

Membranes are not permeable to most first messengers

What are the structural features of receptors?

Membrane-spanning

• External Ligand binding site

• Hydropathy plots define membrane-spanning domains

Intracellular

• Distinct Ligand binding and DNA binding domains

• Translocate to the nucleus and act as transcription factors

What are the different classes of receptors?

Second messenger (G-protein, ionotropic)

Function (death receptors)

Activity (tyrosine kinases, notch signaling)

What are the characteristics of G-protein coupled-receptors?

Association with G-proteins, multiple membrane-spanning domains

What are the characteristics of Ionotropic receptors?

Multiple subunits, ion channel

What are the characteristics of Death receptors?

Intracellular signaling complex leading to activation of proteins

What are the characteristics of Receptor tyrosine kinases?

Dimerization, autophosphorylation, multiple signaling intermediates

What are the characteristics of Notch protein?

Proteolysis to release the transcription factor domain

What are the different states/configurations of the membrane receptors?

Open and Closed

Active (R) and inactive (r) configurations

What are agonists and antagonists?

Agonists decrease entropy, bind to R state, transduce signal

Antagonists increase entropy, bind to r and R states, do not transduce signal

What is the relationship between receptor occupancy and activity?

All receptors do not have to be occupied for maximal response

What is the relationship between occupancy and signal transduction?

Not all receptors need to be occupied to get full activity

How is a receptor turned off?

Desensitization

Sequestration

Downregulation

Inactivation

• Signaling partner

Which of the following is FALSE about the three major cytoskeletal elements in eukaryotic cells?

A) Microtubules are hollow, and in vertebrate cells, a typical microtubule has an outside diameter of 24-25 nm.

B) Microfilaments and microtubules are polar, serve as substrates for molecular motor proteins, and can be quite dynamic. 

C) Antibodies to neurofilament proteins can be used for histology as reliable markers for oligodendrocytes.

D) During mitosis, microtubules are the spindle fibers and actin filaments are the contractile elements of the cleavage furrow.

E) Intermediate filaments are not polar.

C) Antibodies to neurofilament proteins can be used for histology as reliable markers for oligodendrocytes.

Which of the following is TRUE about microtubule dynamics, according to the dynamic  instability mechanism?

A) GTP-tubulin must be hydrolyzed to GDP-tubulin for microtubule assembly to occur.

B) A longer microtubule undergoing catastrophe is less likely to be “rescued” than a shorter microtubule undergoing catastrophe.

C) The presence of a GTP-tubulin cap on the plus end of the microtubule ensures that it will keep assembling and not undergo catastrophe.

D) Free tubulin dimer subunits consist of alpha and beta tubulin, each of which has a bound GDP. 

E) Individual microtubules in a population always undergo identical assembly/disassembly behaviors at the same time.

C) The presence of a GTP-tubulin cap on the plus end of the microtubule ensures that it will keep assembling and not undergo catastrophe.

In the test tube, if you were to try to assemble microtubules with a non-hydrolysable analogue of GTP, what would happen?

A) No microtubule assembly would occur.

B) Microtubule assembly would occur, but the microtubules would be very stable and not undergo catastrophe.

C) Microtubules would assemble, but they wouldn’t have any stored energy in their lattice to get any work done.

D) Microtubules would assemble, but they would not be polar.

E) Both B and C are correct

E) Both B and C are correct

Regarding “selective stabilization,” which is FALSE?

A) Microtubules in cells may undergo a variety of dynamic behaviors, as if trying out different configurations, and then a particular configuration is stabilized.

B) Selective stabilization could occur by the binding of stabilizing proteins along the microtubule’s length, such as doublecortin.

C) Selective stabilization could occur by the capture of the plus end by a structure or protein in the cell’s cortex.

D) In the test tube, if the only protein is the tubulin subunits, the microtubule would most definitely still undergo selective stabilization.

E) Selective stabilization is often important for cells that migrate to move toward a desired target.

D) In the test tube, if the only protein is the tubulin subunits, the microtubule would most definitely still undergo selective stabilization.

Which is FALSE of microtubule-severing proteins? 

A) They are enzymes.

B) Katanin and spastin are examples.

C) To sever a microtubule, they fit all the way around the microtubule and create torque to break it.

D) One thing they can do is cut microtubules free of the centrosome so that the microtubules can move around in a cell.

E) Microtubule severing-proteins are help organize microtubules by cutting them into short enough pieces that can be readily moved around by motor proteins.

C) To sever a microtubule, they fit all the way around the microtubule and create torque to break it.

Which is true of microtubule dynamic instability?

A) GTP hydrolysis is required for microtubule assembly.

B) All microtubules in a population are assembling or disassembling simultaneously.

C) The GTP cap must be hydrolyzed completely to GDP-tubulin in order for the microtubule to continue to assemble.

D) All free tubulin exists as GDP-tubulin.

E) Rescue is the term used for when a microtubule undergoing catastrophe is able to re-establish a GTP-cap and thereby resume assembling.

E) Rescue is the term used for when a microtubule undergoing catastrophe is able to re-establish a GTP-cap and thereby resume assembling.

Which is true of microtubule polarity?

A) The two ends of the microtubule are called positive and negative because they differ in charge.

B) Cytoplasmic dynein moves toward the plus end of the microtubule.

C) The plus end of the microtubule is the end that is embedded in the pericentriolar material in simple interphase cells.

D) In a simple interphase cells, Kinesin transports vesicles away from cell center toward cell periphery. 

E) EB1 and EB3 are proteins that associate with minus ends of microtubules.

D) In a simple interphase cells, Kinesin transports vesicles away from cell center toward cell periphery. 

Do actin filaments undergo dynamic instability similar to microtubules?

A) No, they are totally different.

B) Yes, identical in every way.

C) Pretty close, except actin filaments use ATP whereas microtubules use GTP for dynamic instability.

D) Pretty close, except actin filaments use GTP whereas microtubules use ATP for dynamic instability.

C) Pretty close, except actin filaments use ATP whereas microtubules use GTP for dynamic instability.

What is the difference between actin filaments configurations in filipodia verse lamellipodia?

A) No difference.

B) Actin filaments appear as straight parallel bundles in filopodia but mesh-like networks in lamellipodia.

C) There is only one actin filament per filopodia whereas there are multiple actin filaments in lamellipodia.

D) Myosin only acts upon the actin filaments in lamellipodia.

B) Actin filaments appear as straight parallel bundles in filopodia but mesh-like networks in lamellipodia.

Dephosphorylation is one method to turn “off” (inactivate) which of the following protein kinases?

A) PKC

B) CaM Kinase

C) PKB

D) MAP kinase

C) All of the above

All of the above

Which one of these kinases are NOT activated by phosphorylation?

A) Protein Kinase A

B) CaM kinase

C) Protein kinase C

D) Janus-activated kinase

Protein Kinase A

Metal ions in the active site are a feature of

A) Serine/Threonine phosphatases

B) PP1

C) Protein tyrosine phosphatases

D) Dual specificity phosphatases

E) All of the above

A) Serine/Threonine phosphatases

Signaling intermediates activated by active (phosphorylated) receptors is an example of

A) Signal Divergence (single receptor-ligand activating multiple pathways)

B) Signal Convergence

C) Neither

A) Signal Divergence (single receptor-ligand activating multiple pathways)

Identify the TRUE statement:

A)Partial agonists ONLY bind to the “R” state of the receptor (they can bind to both)

B) Antagonists bind to the “R” state of the receptor and can transduce the signal (the definition of antagonist is opposite to this statement)

C) Full Agonists bind to the “r” state of the receptor and can transduce the signal (the r state cannot transduce the signal)

D) Antagonists bind either the “r” or the “R” state of the receptor but cannot transduce the signal

E) Inverse agonists bind to the “R” state of the receptor (they bind to the “r” state)

D) Antagonists bind either the “r” or the “R” state of the receptor but cannot transduce the signal

Desensitization, Receptor sequestration and Down-regulation are processes that lead to a signal being turned “off”. Which one of the following statements is TRUE?

A) A specific receptor can either be desensitized or down-regulated (a receptor can either be one or the other, not both)

B) The response to down-regulation is synthesis of new receptor protein (need to replace a proteolyzed protein)

C) Sequestered receptors can remain on the cell membrane (“sequestered” means internalized)

D) Desensitization does not require post-translational modification (the state of desensitization – not responsive to stimulus – is due to a structural change which is a result of post-translational modification)

B) The response to down-regulation is synthesis of new receptor protein (need to replace a proteolyzed protein)

What does the cytoskeleton do, and what are its three main elements?

It handles transport and architecture.

Three elements

• Microtubules

• Intermediate filaments

• Microfilaments (actin filaments)

Compare microtubules, actin filaments, and intermediate filaments.

• Microtubules: tubulin subunits, 25 nm, hollow, polar, dynamic, highly conserved, railways for motors

• Actin filaments: actin subunits, 7 nm, non-hollow, polar, dynamic, highly conserved, railways for motors

• Intermediate filaments: various protein subunits, 10 nm, non-hollow, non-polar, non-dynamic, diverse, NOT railways for motors

Why do IF types matter, and how do the cytoskeletal elements work together?

IFs are useful histological markers and can be relevant to disease. The three elements coordinate, divide labor, and sometimes overlap in function

What is tubulin made of, and in what form is it found in cells?

Alpha- and beta-tubulin, each about 55 kDa, and are about 40% alike. They are “always” found as an alpha-beta heterodimer

Cells contain both forms; microtubules and free dimers, which exchange

Describe the structure of a microtubule

A hollow tube about 25 nm outer diameter (about 14 nm inner), typically about 13 protofilaments make up a microtubule in vertebrate cells

Describe microtubule polarity.

Polarity runs along the entire length

Alpha-tubulin is exposed at the minus end

Beta-tubulin at the plus end

The plus end is favored for both assembly and disassembly

Plus and minus have nothing to do with electrical charge (don’t say “positive and negative”)

How do GTP on alpha- vs. beta-tubulin differ?

Beta-tubulin’s GTP is hydrolyzable and is the important one for dynamics.

Alpha-tubulin’s GTP is not hydrolyzable, and its function is unknown (possibly an evolutionary vestige of a common ancestor protein)

What is needed for tubulin to assemble into microtubules in vitro, and what is critical concentration?

• 37°C

• Magnesium

• GTP

• Tubulin above the critical concentration.

Critical concentration is the subunit concentration above which polymer forms. It is like salt crystallizing out of solution, but microtubules are non-equilibrium polymers, so it is more complicated

How does a turbidity assay work, and what are the phases of the assembly curve?

A spectrophotometer measures cloudiness

Tubulin is clear and microtubule polymer is turbid.

The curve has three phases: nucleation (lag), elongation, and steady state

Is GTP hydrolysis required for assembly?

No. GTP is required, but a non-hydrolyzable analog is sufficient.

This was the “mystery” that dynamic instability resolved.

How was microtubule polarity discovered, and which end has the lower critical concentration?

Microtubules assembled onto a seed grew differently at the two ends because the ends have different critical concentrations

The plus end has the LOWER critical concentration than the minus end

What is treadmilling?

When Cc(T) < C < Cc(D), subunits add at the plus end and leave the minus end, creating the illusion of movement

How did the “old ideas” differ from dynamic instability (c. 1986)?

Old ideas: treadmilling is common in cells and does work, and all microtubules in a population behave roughly the same

Dynamic instability: individual microtubules in a population do very different things at the same time, using a GTP cap, catastrophe, and rescue

What is the GTP cap, and when does hydrolysis occur?

All free tubulin is GTP-tubulin

As it adds to the plus end it forms a GTP cap, and while the cap exists the microtubule keeps growing

Hydrolysis to GDP-tubulin happens only after assembly, and it takes time

What is the GDP-lattice and why is it useful?

The body of the microtubule behind the cap, made of GDP-tubulin. It stores energy (like a loaded spring), so disassembly can do work, such as helping separate chromosomes in mitosis.

Define catastrophe and rescue.

Catastrophe: stochastically, hydrolysis catches up to assembly, the GTP cap is lost, and the microtubule rapidly disassembles

Rescue: the cap is re-established before the microtubule is completely gone, so growth resumes.

How does free GTP-tubulin concentration affect stability?

High concentration favors a stable, growing microtubule

Low concentration favors an unstable one

How would a microtubule built with a non-hydrolyzable GTP analog differ from a normal one?

It would be abnormally stable, because the whole lattice acts like a GTP cap.

There would be no catastrophe and no stored energy to release.

What is selective stabilization of microtubules?

Stabilization of particular microtubules by other cell components, through plus-end capture (“search and capture”) or proteins that bind the lattice and prevent disassembly (MAPs, doublecortin, STOP/MAP6, plectins, others)

Why are motor proteins needed, and which way do they walk?

Lattice energy does only a little work relative to a complex cell’s needs, while motors do a lot

Cytoplasmic dynein walks toward the minus end (retrograde)

Most kinesins walk toward the plus end (anterograde)

What are the motor domain and cargo domain?

The motor domain is the globular ATPase part that interfaces with the microtubule. The cargo domain interfaces with the organelle being moved

What is processivity, and what are motors used for?

Processivity is the ability to take many steps along a microtubule without detaching. Motors distribute organelles (e.g., Golgi) and carry outward transport for exocytosis and inward transport for endocytosed material.

How are microtubules oriented in interphase vs. mitotic cells, and why does it matter?

Interphase: minus ends at a central focus (centrosome), plus ends outward

Mitosis: two foci at opposite ends, minus ends toward the foci, plus ends outward

Orientation matters because motors move in only one direction, so polarity pattern determines cargo transport direction

Which cells have microtubule organization that centralized foci cannot explain, and how might those arrays form?

Epithelia, neurons (axons and dendrites), plants, myotubes, and S. pombe

Possibilities: (1) multiple centrosomes, (2) gamma-TuRCs redistributed to non-centrosomal sites, (3) microtubules breaking off the centrosome and moving into new patterns

What is nucleation, and what is the lag?

Nucleation is getting new microtubules started.

The lag is slow de novo nucleation from free subunits. If a nucleation seed is present, there is no lag

What is the centrosome, and what does gamma-tubulin do?

A pair of centrioles surrounded by pericentriolar material (PCM).

The PCM contains gamma-tubulin as gamma-TuRCs (ring structures) that nucleate microtubules, anchoring the minus ends and leaving plus ends pointing outward

How can motors organize microtubules, not just move cargo?

Motors like cytoplasmic dynein can crosslink and transport microtubules themselves, moving them into new arrangements

What are microtubule-severing proteins and what do they do?

Hexameric ATPases that cut microtubules (katanin, spastin, katanin-like-2, fidgetin, fidgetin-like 2)

Severing frees microtubules from the centrosome so motors can move them into new arrangements

What are +TIPs?

Plus-end associated proteins (EB1, EB3, CLIPs, CLASPs) that associate with the plus end only during assembly

GFP-EB1 appears as fluorescent “comets” marking growing plus ends in live cells

Describe cilia and flagella.

Cilia are short and flagella are long. Both have a “9 + 2” arrangement (nine outer doublets, two central singlets) with very stable microtubules, and they beat through flagellar dynein

The basal body is a centriole (triplets) that contacts the cell membrane and anchors them

How are microtubules and related proteins relevant to disease?

Microtubule dynamics, stability, and transport are essential, so defects cause disease and microtubules are drug targets.

Examples:

Taxol - stabilizes microtubules, cancer therapy

IF proteins - histological markers, disease-relevant

What methods are used to study microtubules?

Immunostaining of fixed cells (e.g., tubulin monoclonal antibody with fluorescein secondary; actin polyclonal with rhodamine secondary), electron microscopy (e.g., 9 + 2 cross-sections), GFP-fusions in living cells (e.g., GFP-EB1), and turbidity assays in vitro

How do actin and microtubules work together in growth cones and cytokinesis?

Growth cone: actin (red) at the leading edge, microtubules (green) in the center, coordinated for turning

Cytokinesis: microtubules form the spindle/midzone while actin forms the contractile ring at the cortex

How does actin dynamic instability differ from that of microtubules?

Actin uses ATP instead of GTP (convergent evolution)

Subunits go from ATP-actin to ADP-Pi-actin to ADP-actin as the filament ages

The plus end is the barbed end, and the minus end is the pointed end

What configurations do actin filaments take?

Microvilli, adhesion belt, cell cortex, filopodia, lamellipodium, stress fibers, and contractile ring

Why do actin filaments form such varied configurations?

A vast variety of actin-regulatory proteins shape them

General classes: myosin motors, crosslinkers/bundlers, monomer-binding proteins, nucleators (e.g., Arp2/3 for branched networks), capping proteins, and severing/depolymerizing proteins

What is the structure of a cell membrane, and what can cross it by simple diffusion?

A phospholipid bilayer (hydrophilic heads out, hydrophobic tails in) with embedded integral proteins, peripheral proteins, cholesterol, and glycoproteins/glycolipids.

Only low molecular weight, non-polar, lipid-soluble molecules diffuse through directly.

What determines the direction of movement for uncharged molecules versus ions?

Uncharged molecules move down their concentration gradient.

Ions can’t cross the bilayer and are also affected by electrical forces, so their flux depends on the electrochemical gradient (concentration and membrane potential).

How do ion channels differ from transporters?

Both are selective facilitated diffusion proteins. When open, channels let ions flow freely.

Transporters bind a set number of molecules and change shape to move them, so they can saturate.

Glucose and amino acids use transporters because they are too polar for the bilayer and too large for channels.

Compare primary and secondary active transport.

Primary uses ATP hydrolysis directly (pumps, e.g., the Na⁺/K⁺ ATPase, which moves 3 Na⁺ out and 2 K⁺ in).

Secondary uses an ion’s electrochemical gradient to drive transport (e.g., Na⁺-glucose cotransport).

• Secondary depends on primary because the pump sets up the Na⁺ gradient.

How are the four key ions distributed, and what must a cell have to generate a membrane potential?

K⁺ is 20–40x higher inside. Na⁺ is 8–10x higher outside. Cl⁻ is 10–20x higher outside. Ca²⁺ is more than 10,000x higher outside.

A membrane potential requires this uneven distribution plus channels with selective permeabilities.

What are the types of ion channels?

Leak (open randomly, mostly open at rest), voltage-gated (open with changes in membrane potential), ligand-gated (open when a chemical binds), and light-gated (open with light, from the bonus section)

What is the Nernst equation, and what does it represent?

E_ion = (RT/zF) ln([ion]ₒ/[ion]ᵢ), or at room temperature (58 mV/z) log([ion]ₒ/[ion]ᵢ).

It gives the electrochemical equilibrium potential for an ion, where chemical and electrical forces are equal and opposite and there is no net flux.

What are the approximate equilibrium potentials for Na⁺, K⁺, Cl⁻, and Ca²⁺?

E_Na = +55 mV, E_K = −90 mV, E_Cl = −65 mV, E_Ca = +125 mV

What is driving force, and what is it for each ion at rest (Vm = −65 mV)

Driving force = Vm − E_ion. Na⁺ has a very large inward force (+55 vs −65).

K⁺ has a small outward force (−90 vs −65). Cl⁻ has none (−65 vs −65).

How do driving forces change with depolarization (−40 mV) and hyperpolarization (−90 mV)?

At −40 mV, the Na⁺ force decreases while the K⁺ and Cl⁻ forces increase. At −90 mV, the Na⁺ force increases, K⁺ has no net force (Vm = E_K), and the Cl⁻ force reverses.

Current is the flow of positive charge, so Cl⁻ current is opposite to Cl⁻ ion movement. At −40 mV that means outward current but inward Cl⁻ flux (hyperpolarizing).

At −90 mV it means inward current but outward Cl⁻ flux (depolarizing).

What determines net ion flux, and what is Ohm’s law?

Net flux depends on both driving force and permeability (channels must be open).

Ohm’s law is ΔV = IR, where R = 1/g (g is conductance).

Channels act as resistors, and resting R depends on resting channel conductance.

What did Bernstein’s K⁺-only hypothesis predict, and what did experiments show?

It predicted Vm = E_K, so a 10-fold change in [K⁺]ₒ should shift Vm by 58 mV

This held at high [K⁺]ₒ, but at low [K⁺]ₒ the relationship became non-linear and Vm never reached E_K, because the membrane also has some Na⁺ permeability and the Na⁺ driving force grows as the cell hyperpolarizes.

What does the Goldman-Hodgkin-Katz equation do, and why is resting Vm not equal to E_K?

It combines the Nernst equation with relative permeabilities to predict membrane potential with no synaptic input.

Resting permeability is P_K > P_Na >> P_Cl, so Vm sits between E_K and E_Na but close to E_K (about −65 mV).

Why is resting Vm stable, and what does the Na⁺/K⁺ pump do at rest?

Na⁺ flux (large driving force × low permeability) is equal and opposite to K⁺ flux (small driving force × high permeability)

The pump maintains the concentration gradients against the slow leak.

What is voltage clamp, and what did Hodgkin and Huxley learn with it?

The experimenter sets the voltage and records the current needed to hold it. In the squid giant axon, small steps gave only leak current.

Past threshold, large steps gave an early inward Na⁺ current then a delayed outward K⁺ current.

TTX blocks voltage-gated Na⁺ channels and TEA blocks voltage-gated K⁺ channels, which let them separate the currents.

How do voltage-gated K⁺ channels behave?

Depolarization opens them and hyperpolarization closes them. They are slow to activate and deactivate

K⁺ current grows with depolarization because both the number of open channels and the driving force increase.

How do voltage-gated Na⁺ channels behave?

The activation gate (m) opens quickly (about 0.1 ms) on depolarization.

The inactivation gate (h) closes within about 1 ms and stays closed until repolarization.

States: resting (closed, ready), activated (open), inactivated (closed, not ready).

What is threshold, and why is the action potential all-or-none?

Threshold is the Vm at which depolarization opens enough voltage-gated Na⁺ channels to start a positive feedback loop: depolarization opens more Na⁺ channels, which causes more Na⁺ influx and more depolarization

Once started, it runs to completion regardless of stimulus size.

What is the ionic basis of each phase of the action potential?

At rest P_K >> P_Na

In the rising phase, Na⁺ channels open and P_Na >> P_K, so Vm heads toward E_Na (about +55 mV, the theoretical maximum, though the peak falls short because Na⁺ channels inactivate and K+ permeability)

In the falling phase, Na⁺ channels inactivate and K⁺ channels open, so Vm returns toward E_K.

In the undershoot, K⁺ conductance stays elevated and Vm dips below rest

What causes the absolute and relative refractory periods, and why do they matter?

Absolute refractory (no action potential possible) is due to Na⁺ channel inactivation

Relative refractory (a larger depolarization is needed) is due to the K⁺-mediated undershoot

Together they set the neuron’s maximal firing rate

Why are action potentials unidirectional, regenerated, and fast in some axons?

Na⁺ channels behind the active region are inactivated, so the signal only moves forward. The action potential is regenerated along the axon, keeping the same height and duration

Large axon diameter (low axial resistance) and myelination (higher membrane resistance, lower capacitance) speed conduction, and myelin allows saltatory conduction between nodes of Ranvier