MCAT Biological and Biochemical Foundations of Living Systems Quiz: 3a Neuron Structure Signal Propagation

What this quiz covers

This quiz focuses on 3a Neuron Structure Signal Propagation, giving you a quick way to practice the rules, question types, and explanations that matter most for MCAT Biological and Biochemical Foundations of Living Systems.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. Higher density of ligand-gated Na+ channels on dendrites, which increases the amplitude of graded potentials and therefore conduction velocity
2. Myelination, which increases membrane resistance and decreases capacitance, allowing depolarization to spread farther between nodes before regeneration (correct answer)
3. Increased neurotransmitter packaging into synaptic vesicles at the axon terminal, which speeds action potential travel along the axon
4. A reversal of the normal direction of current flow such that action potentials move from axon terminals to soma more efficiently

Explanation: This question tests the understanding of neuron structure and signal propagation, focusing on factors influencing conduction velocity in myelinated versus unmyelinated axons. Myelination insulates axons, increasing membrane resistance and decreasing capacitance, which allows passive current spread over longer distances. In myelinated axons, this enables saltatory conduction where action potentials jump between nodes of Ranvier, speeding propagation. Choice B is consistent because myelination reduces current leak and capacitance, allowing faster depolarization spread in the myelinated axon. A distractor like choice A fails based on the misconception that ligand-gated channels directly affect axonal conduction velocity, whereas they primarily influence synaptic potentials. To apply to similar concepts, compare membrane properties like resistance and capacitance. Always confirm that myelination enhances speed without altering ion gradients directly.

Question 2

1. Increased membrane capacitance and decreased membrane resistance in the demyelinated region increase current leak and slow nodal depolarization, promoting conduction failure. (correct answer)
2. Increased dendritic spine density in the demyelinated region increases synaptic input, which compensates for lost myelin and ensures reliable saltatory conduction.
3. Decreased extracellular Na$^+$ concentration in the demyelinated region increases the driving force for Na$^+$ entry, speeding propagation through the lesion.
4. Upregulation of ligand-gated receptors along the demyelinated axolemma replaces voltage-gated Na$^+$ channels, restoring AP propagation without affecting speed.

Explanation: This question tests understanding of how myelin affects passive membrane properties and action potential propagation. Myelin decreases membrane capacitance and increases membrane resistance, allowing depolarizing current to spread efficiently between nodes. Demyelination reverses these changes: increased capacitance requires more charge to depolarize the membrane, while decreased resistance allows more current to leak out. These changes slow the spread of depolarization and can cause conduction failure if the arriving current is insufficient to trigger the next node. The correct answer identifies these passive property changes as the key factor. Choice B incorrectly invokes dendritic spines on axons, while choice C wrongly suggests decreased extracellular Na+ (which would reduce, not increase, driving force). A critical principle is that myelin's effect on passive membrane properties, not active channel changes, determines conduction reliability in demyelinating conditions.

Question 3

1. Neurotransmitter release is reduced because AMPA receptors normally repolarize the presynaptic terminal to terminate Ca$^{2+}$ influx.
2. Blocking AMPA receptors prevents presynaptic vesicle docking because AMPA receptors serve as Ca$^{2+}$ channels on the presynaptic terminal.
3. Blocking AMPA receptors prevents AP propagation down the presynaptic axon because glutamate binding is required to open voltage-gated Na$^+$ channels.
4. Neurotransmitter release occurs normally, but blocking postsynaptic ligand-gated cation channels prevents rapid Na$^+$ influx that would generate a fast EPSP. (correct answer)

Explanation: This question tests understanding of postsynaptic receptor function in synaptic transmission. AMPA receptors are ionotropic glutamate receptors that mediate fast excitatory postsynaptic potentials by allowing rapid Na+ (and K+) flux upon glutamate binding. Blocking these receptors prevents the fast depolarizing response while leaving presynaptic function intact, as evidenced by preserved metabotropic responses. The correct answer accurately describes normal neurotransmitter release with blocked postsynaptic reception. Choice B incorrectly assigns AMPA receptors to the presynaptic terminal, while choice C wrongly makes them necessary for axonal propagation. A key principle is that fast synaptic responses require functional postsynaptic ionotropic receptors, distinct from presynaptic release machinery or axonal conduction mechanisms.

Question 4

1. Higher extracellular K+ reverses axonal propagation so spikes travel from dendrites to soma, decreasing measured amplitude at the axon
2. Depolarized resting potential increases neurotransmitter release by opening postsynaptic ligand-gated Ca2+ channels, reducing spike amplitude
3. Higher extracellular K+ increases the Na+ electrochemical gradient, causing a larger Na+ influx and therefore smaller action potentials
4. Depolarized resting potential increases steady-state inactivation of voltage-gated Na+ channels, reducing available Na+ current during the upstroke (correct answer)

Explanation: This question tests the understanding of neuron structure and signal propagation, focusing on resting potential and Na+ channel availability. Elevated extracellular K+ depolarizes rest, increasing Na+ channel inactivation via voltage dependence. This reduces available channels for upstroke, lowering spike amplitude. Choice D is consistent because inactivation decreases Na+ current, explaining reduced amplitude. A distractor like choice C fails based on the misconception that higher K+ boosts Na+ gradient, whereas it affects resting voltage. To check, plot inactivation curves. Recall that steady-state inactivation rises with depolarization.

Question 5

1. Increased presynaptic Cl− conductance reduces terminal depolarization and/or increases shunting, decreasing activation of voltage-gated Ca2+ channels needed for vesicle fusion (correct answer)
2. Increased presynaptic Cl− conductance directly blocks postsynaptic ligand-gated receptors, preventing neurotransmitter binding
3. Increased presynaptic Cl− conductance reverses action potential direction so spikes no longer reach the terminal from the axon hillock
4. Increased presynaptic Cl− conductance increases intracellular Na+ at rest, strengthening depolarization and decreasing Ca2+ influx

Explanation: This question tests the understanding of neuron structure and signal propagation, particularly presynaptic inhibition mechanisms. Increased Cl− conductance hyperpolarizes or shunts the terminal, reducing Ca2+ influx and release. This inhibits without blocking spike invasion. Choice A is consistent because shunting impairs Ca2+ activation. A distractor like choice B fails due to the misconception that presynaptic inputs directly block postsynaptic receptors. For inhibition types, differentiate sites. Note axo-axonic synapses modulate release.

Question 6

1. Lower temperature increases neurotransmitter receptor affinity at dendrites, which directly increases axonal conduction velocity
2. Lower temperature increases extracellular Na+ concentration, which reduces the driving force for Na+ entry and speeds conduction
3. Lower temperature slows voltage-gated channel kinetics, delaying activation/inactivation and thereby slowing conduction (correct answer)
4. Lower temperature reverses the direction of propagation so spikes move from axon terminal to soma, increasing transit time

Explanation: This question tests the understanding of neuron structure and signal propagation, emphasizing temperature effects on channel kinetics. Lower temperature slows gating, delaying activation and conduction. This prolongs transit time. Choice C is consistent because kinetics slow overall propagation. A distractor like choice B fails due to the misconception that temperature alters concentrations, whereas it affects rates. For conditions, factor Q10 values. Note enzymes and channels are temperature-sensitive.

Question 7

1. Longer internodes (fewer nodes) generally increase conduction velocity by reducing the number of times the action potential must be regenerated, up to the point where internodes become too long for reliable depolarization (correct answer)
2. Shorter internodes always increase conduction velocity because more nodes provide more Na+ channels to push current forward
3. Internode distance primarily changes neurotransmitter diffusion across the synaptic cleft, which determines axonal conduction velocity
4. Internode distance reverses propagation direction by shifting spike initiation from the axon hillock to the axon terminal

Explanation: This question tests the understanding of neuron structure and signal propagation, focusing on internode length in saltatory conduction. Optimal internode length balances passive spread and regeneration frequency for maximal velocity. Longer internodes reduce regenerations but risk failure if too extended. Choice A is consistent because fewer nodes speed conduction up to a limit. A distractor like choice B fails due to the misconception that more nodes always accelerate, ignoring delay at each. For myelinated axons, optimize length constants. Confirm velocity peaks at intermediate internodes.

Question 8

1. Reduced myelin causes action potentials to initiate in dendrites rather than the axon initial segment, increasing conduction distance.
2. Reduced myelin increases neurotransmitter release probability at the neuromuscular junction, slowing axonal conduction.
3. Reduced myelin shifts the Na+ equilibrium potential to more negative values, slowing depolarization at the axon hillock.
4. Reduced myelin increases membrane capacitance and current leak across internodes, slowing the rate at which downstream nodes reach threshold. (correct answer)

Explanation: This question tests understanding of how myelin thickness affects conduction velocity in saltatory conduction. Myelin acts as an insulator that reduces membrane capacitance and prevents current leak across internodes, allowing depolarizing current to travel efficiently between nodes. When myelin is thinner, the membrane capacitance increases and more current leaks out across the internode, reducing the amount of depolarizing current that reaches the next node and slowing the rate at which it reaches threshold. The correct answer D accurately describes this mechanism of reduced conduction velocity. Answer B incorrectly introduces neurotransmitter release, which occurs at synapses, not along the axon during conduction. The fundamental principle is that myelin thickness directly affects the passive electrical properties of the axon, with thicker myelin providing better insulation and faster conduction.

Question 9

1. The EPSP is smaller because voltage-gated Na+ channels at the presynaptic terminal require postsynaptic depolarization to open and release vesicles.
2. Clamping near the Na+ equilibrium potential increases Na+ influx, which hyperpolarizes the postsynaptic neuron and reduces transmitter binding.
3. The EPSP is smaller because action potentials normally propagate from dendrites to soma only when Na+ is higher inside than outside.
4. Driving force for Na+ entry is reduced near the Na+ equilibrium potential, decreasing net inward current through the receptor and thus reducing the EPSP. (correct answer)

Explanation: This question tests understanding of driving force and synaptic current generation. Ionotropic receptors permeable to Na+ and K+ generate EPSPs through net inward current, primarily carried by Na+ influx down its electrochemical gradient. When the postsynaptic membrane is clamped near the Na+ equilibrium potential (~+60 mV), the driving force for Na+ entry (Vm - ENa) approaches zero, dramatically reducing Na+ influx through open receptors. The correct answer D correctly identifies that reduced driving force for Na+ decreases the net inward current and thus the EPSP amplitude. Answer B incorrectly claims that being near ENa increases Na+ influx, when the opposite is true. A critical principle for synaptic physiology is that current through an ion channel depends on both conductance (number of open channels) and driving force (difference between membrane potential and equilibrium potential).

Question 10

1. Opening of postsynaptic ligand-gated cation channels due to decreased postsynaptic depolarization at the axon hillock.
2. Vesicle fusion with the presynaptic membrane due to reduced Ca2+-triggered exocytosis following terminal depolarization. (correct answer)
3. Propagation of the presynaptic action potential due to reduced saltatory conduction across the presynaptic dendrites.
4. Resting membrane potential of the postsynaptic neuron due to decreased Na+ concentration outside the cell.

Explanation: This question tests understanding of calcium's role in synaptic transmission. When an action potential reaches the presynaptic terminal, voltage-gated Ca2+ channels open, allowing calcium influx that triggers vesicle fusion and neurotransmitter release. Reducing extracellular Ca2+ directly reduces this calcium influx, leading to less vesicle fusion and smaller postsynaptic responses. The correct answer B accurately identifies that Ca2+-triggered exocytosis (vesicle fusion) is the process most directly affected by low extracellular calcium. Answer A incorrectly focuses on postsynaptic events, but the question states the presynaptic action potential is normal, indicating the effect is presynaptic. The key principle is that calcium couples electrical signaling (action potential arrival) to chemical signaling (neurotransmitter release) at synapses.

Question 11

1. It can lower the local threshold for regenerative depolarization, making propagation more reliable across that segment when upstream current arrives. (correct answer)
2. It prevents propagation because Na+ channel activation must occur only after repolarization is complete at the upstream segment.
3. It primarily increases neurotransmitter release by directly opening Ca2+ channels in the postsynaptic membrane.
4. It causes action potentials to propagate from the axon terminal toward the soma because earlier Na+ activation reverses current direction.

Explanation: This question tests understanding of how sodium channel activation threshold affects action potential propagation. When Na+ channels at a downstream segment activate at more negative voltages, they can open in response to smaller depolarizations from upstream current. This effectively lowers the threshold for regenerative depolarization at that segment, making it easier for the propagating signal to trigger an action potential. The correct answer A recognizes that easier Na+ channel activation enhances propagation reliability by reducing the threshold for regeneration. Answer D incorrectly suggests reversal of propagation direction, but action potentials propagate unidirectionally due to Na+ channel inactivation behind the wavefront. The fundamental principle is that anything that facilitates Na+ channel opening at downstream sites will enhance propagation reliability.

Question 12

1. Blocking voltage-gated K+ channels slows repolarization, prolonging the action potential and often increasing the refractory period despite normal Na+ channel activation at nodes (correct answer)
2. Blocking voltage-gated K+ channels prevents neurotransmitter release by directly inhibiting ligand-gated Ca2+ receptors on the postsynaptic membrane
3. Blocking voltage-gated K+ channels stops depolarization because Na+ is normally higher inside the neuron than outside at rest
4. Blocking voltage-gated K+ channels causes action potentials to propagate from axon terminals back toward the soma because dendrites become the primary spike initiation site

Explanation: This question tests the understanding of neuron structure and signal propagation, specifically the role of voltage-gated ion channels in action potential dynamics during saltatory conduction. Voltage-gated K+ channels facilitate repolarization by allowing K+ efflux after Na+ influx during an action potential. In myelinated neurons, these channels at nodes of Ranvier ensure efficient repolarization to maintain rapid propagation. Choice A is consistent because blocking K+ channels with 4-AP slows repolarization, prolonging the action potential and refractory period while Na+ channels support depolarization. A distractor like choice C fails due to the misconception that Na+ is higher inside at rest, whereas it is actually higher outside, driving depolarization. To check similar concepts, verify if the blocker targets repolarization or depolarization phases. Recall that K+ channels primarily affect the falling phase of the action potential, not initiation.

Question 13

1. Greater axon diameter in Neuron X, which lowers internal (axial) resistance and increases conduction velocity (correct answer)
2. Shorter axon length in Neuron Y, which increases conduction velocity by reducing the number of voltage-gated Na+ channels required for depolarization
3. More neurotransmitter receptors on Neuron X's soma, which increases the speed of action potential propagation along its axon
4. Action potentials in Neuron Y travel from axon terminal to dendrite, which is faster because dendrites have lower capacitance

Explanation: This question tests the understanding of neuron structure and signal propagation, specifically how axon diameter affects conduction velocity in unmyelinated neurons. Larger axon diameter reduces axial resistance, allowing faster passive current spread and quicker depolarization. In unmyelinated axons, conduction relies on continuous regeneration along the membrane, influenced by internal resistance. Choice A is consistent because Neuron X's greater diameter lowers resistance, increasing velocity despite longer length. A distractor like choice B fails due to the misconception that shorter length inherently speeds conduction, ignoring resistance's role. For similar concepts, calculate velocity using cable theory parameters like diameter. Recall that diameter impacts speed more than length in unmyelinated fibers.

Question 14

1. Slower Na+ channel inactivation prolongs Na+ influx during the spike, broadening the action potential and potentially increasing Ca2+ entry at terminals (correct answer)
2. Slower Na+ channel inactivation prevents initial depolarization because K+ efflux must occur before Na+ influx can begin
3. Slower Na+ channel inactivation primarily reduces neurotransmitter binding to postsynaptic receptors by lowering vesicle content
4. Slower Na+ channel inactivation causes action potentials to propagate from axon terminal to soma because refractory periods are eliminated

Explanation: This question tests the understanding of neuron structure and signal propagation, specifically Na+ channel inactivation in action potential waveform. Na+ channels inactivate after activation, limiting influx duration and shaping the spike's rising phase. Slowed inactivation prolongs Na+ current, broadening the action potential. Choice A is consistent because extended Na+ influx widens spikes and may enhance terminal Ca2+ entry. A distractor like choice B fails due to the misconception that K+ efflux precedes Na+ influx, whereas Na+ drives depolarization first. For similar pharmacology, analyze phase-specific effects. Note that inactivation kinetics control spike duration independently of threshold.

Question 15

1. Lower presynaptic Ca2+ influx reduces the likelihood of synaptic vesicle fusion, decreasing quantal neurotransmitter release (correct answer)
2. Lower presynaptic Ca2+ influx prevents postsynaptic ligand-gated channels from binding neurotransmitter, even if neurotransmitter is released normally
3. Lower presynaptic Ca2+ influx increases Na+ entry at the axon hillock, raising spike amplitude and thereby reducing postsynaptic currents
4. Lower presynaptic Ca2+ influx reverses the Na+ gradient so Na+ becomes higher inside than outside, preventing depolarization

Explanation: This question tests the understanding of neuron structure and signal propagation, emphasizing Ca2+ in presynaptic release probability. Reduced Ca2+ channel opening lowers influx, decreasing vesicle fusion likelihood. This reduces quantal content and postsynaptic response. Choice A is consistent because lower probability impairs release. A distractor like choice B fails due to the misconception that presynaptic Ca2+ affects postsynaptic binding, which is independent. For drugs, assess release vs. reception. Note Ca2+ cooperativity in exocytosis.

Question 16

1. Opening of postsynaptic Cl$^-$ channels increases membrane conductance and shunts excitatory depolarizing currents, reducing the chance of reaching threshold at the AIS. (correct answer)
2. Opening of presynaptic Cl$^-$ channels depolarizes the axon terminal, increasing Ca$^{2+}$ influx and thereby suppressing neurotransmitter release.
3. Opening of postsynaptic voltage-gated Na$^+$ channels hyperpolarizes the membrane, preventing forward propagation from dendrites to the axon terminal.
4. Opening of postsynaptic ligand-gated Ca$^{2+}$ channels raises intracellular Ca$^{2+}$, which directly inactivates Na$^+$ channels along the axon to stop propagation.

Explanation: This question tests understanding of synaptic inhibition through shunting mechanisms. When inhibitory synapses open Cl- channels, they increase membrane conductance (decrease resistance) even if the reversal potential is near resting potential. This increased conductance 'shunts' or diverts depolarizing currents from excitatory inputs, making them less effective at depolarizing the membrane toward threshold. The correct answer identifies this shunting inhibition mechanism at the postsynaptic membrane. Choice B incorrectly places Cl- channels presynaptically and suggests depolarization, while choices C and D invoke incorrect channel types and mechanisms. A key principle is that inhibition can work through conductance increases that shunt excitation, not just through hyperpolarization.

Question 17

1. Increased K+ leak causes depolarization to occur after repolarization, reversing the action potential sequence
2. Increased K+ leak depolarizes the membrane because K+ is higher outside than inside at rest, promoting K+ influx
3. Increased K+ leak directly prevents neurotransmitter binding to postsynaptic receptors by lowering vesicle fusion probability
4. Increased K+ leak hyperpolarizes the membrane, making it harder for synaptic depolarizations to reach threshold at the axon initial segment (correct answer)

Explanation: This question tests the understanding of neuron structure and signal propagation, particularly resting K+ conductance and membrane potential. Increased K+ leak hyperpolarizes by enhancing K+ efflux toward equilibrium. This makes reaching threshold harder. Choice D is consistent because hyperpolarization reduces excitability. A distractor like choice B fails based on the misconception that K+ influx depolarizes, whereas efflux does. To apply, use Nernst equation. Recall leaks set resting potential.

Question 18

1. Reduced repolarization of the presynaptic membrane because AMPA receptors normally act as voltage-gated K+ channels
2. Reduced opening of presynaptic voltage-gated Na+ channels, preventing action potentials from reaching the terminal
3. Reduced postsynaptic Na+ (and some K+) conductance through ligand-gated AMPA channels, decreasing the excitatory postsynaptic potential (correct answer)
4. Increased retrograde propagation of action potentials from the axon terminal to dendrites due to loss of postsynaptic receptor binding

Explanation: This question tests the understanding of neuron structure and signal propagation, emphasizing postsynaptic receptor function in transmission. AMPA receptors are ligand-gated channels permitting Na+ (and K+) influx, generating excitatory postsynaptic potentials. Antagonism reduces this conductance, diminishing depolarization. Choice C is consistent because blocked receptors lower EPSP amplitude despite normal release. A distractor like choice B fails due to the misconception that postsynaptic blockers affect presynaptic propagation, which they do not. For antagonists, distinguish pre- vs. postsynaptic sites. Note that AMPA mediates fast excitation.

Question 19

1. Loss of myelin increases membrane capacitance and decreases membrane resistance in the affected region, increasing current leak and slowing depolarization of the next node. (correct answer)
2. Loss of myelin prevents neurotransmitter from binding postsynaptic receptors at the next synapse, delaying synaptic transmission rather than axonal conduction.
3. Loss of myelin increases intracellular Na+ at rest, strengthening the Na+ gradient and causing spontaneous action potentials that collide and cancel downstream signals.
4. Loss of myelin eliminates voltage-gated Na+ channels from dendrites, preventing graded potentials from reaching the soma and stopping action potentials from initiating.

Explanation: This question tests understanding of myelin's role in maintaining efficient action potential propagation. Myelin acts as an insulator that increases membrane resistance and decreases capacitance, allowing depolarizing current to spread efficiently to the next node of Ranvier. When myelin is lost, the exposed axonal membrane has higher capacitance and lower resistance, causing more current to leak out and charge to be stored in the membrane capacitor, slowing the depolarization of downstream nodes and potentially preventing threshold from being reached. The correct answer (A) accurately describes how demyelination increases capacitance and decreases resistance, leading to current loss and slowed conduction. Answer B incorrectly invokes synaptic transmission and neurotransmitter binding, which are not involved in axonal conduction between nodes of Ranvier. When analyzing conduction deficits, distinguish between axonal propagation (affected by myelin loss) and synaptic transmission (involving neurotransmitter release) - demyelinating diseases affect the former, not the latter.

Question 20

1. Blocking voltage-gated K+ channels slows repolarization, lengthening the action potential and potentially increasing the relative refractory period due to delayed return toward resting potential. (correct answer)
2. Blocking voltage-gated K+ channels prevents initial depolarization because K+ influx is the primary current that drives the upstroke of the action potential.
3. Blocking voltage-gated K+ channels prevents synaptic vesicle fusion because K+ entry into the presynaptic terminal is required to trigger neurotransmitter release.
4. Blocking voltage-gated K+ channels increases conduction velocity by increasing dendritic integration, allowing signals to propagate from axon terminals back to the soma.

Explanation: This question tests understanding of voltage-gated K+ channel function during action potential repolarization. Voltage-gated K+ channels open during the action potential to allow K+ efflux, which repolarizes the membrane back toward resting potential after Na+ channel-mediated depolarization. Blocking these K+ channels slows repolarization because the membrane must rely on passive K+ leak and Na+ channel inactivation alone, prolonging the action potential duration and potentially extending the relative refractory period. The correct answer (A) accurately describes how K+ channel blockade affects repolarization and refractory periods. Answer B incorrectly identifies K+ influx as driving depolarization, when actually Na+ influx drives the upstroke while K+ efflux drives repolarization. When analyzing action potential phases, remember that Na+ channels drive depolarization (upstroke) while K+ channels drive repolarization (downstroke) - blocking either channel type affects its respective phase.