MCAT Biological and Biochemical Foundations of Living Systems Quiz: 3a Action Potentials Synaptic Transmission

What this quiz covers

This quiz focuses on 3a Action Potentials Synaptic Transmission, 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. EPP amplitude decreases because postsynaptic nicotinic receptors become inactivated by the toxin
2. EPP amplitude increases because vesicles accumulate and release more acetylcholine when stimulated
3. EPP amplitude is unchanged because acetylcholine release occurs through membrane transporters, not vesicles
4. EPP amplitude decreases because fewer acetylcholine-containing vesicles fuse and release transmitter per stimulus (correct answer)

Explanation: This question assesses understanding of action potentials and synaptic transmission, focusing on SNARE-mediated vesicle fusion in neurotransmitter release. Action potentials trigger Ca2+ influx that drives SNARE-dependent fusion of synaptic vesicles with the presynaptic membrane, releasing neurotransmitter into the synaptic cleft. In this scenario, cleaving SNARE proteins prevents vesicle fusion, reducing the number of vesicles that can release acetylcholine per stimulus. Choice D is correct because it aligns with the principle that disrupting vesicle fusion machinery reduces quantal release and EPP amplitude. Choice C fails as it incorrectly suggests neurotransmitter release occurs through transporters rather than vesicular exocytosis. To avoid similar mistakes, always remember that fast synaptic transmission requires SNARE-mediated vesicle fusion for neurotransmitter release.

Question 2

1. Conduction velocity decreases but action potentials still propagate because myelin can generate action potentials in internodes
2. Propagation fails because action potentials require regenerative Na+ influx at successive nodes (correct answer)
3. Propagation is unchanged because K+ channels, not Na+ channels, determine whether a spike travels down an axon
4. Action potentials become larger in amplitude because blocking nodal Na+ channels reduces Na+ channel inactivation

Explanation: This question assesses understanding of action potentials and synaptic transmission, focusing on saltatory conduction in myelinated axons. Action potentials in myelinated axons jump between nodes of Ranvier where voltage-gated Na+ channels are concentrated, with passive current spread through myelinated internodes. In this scenario, blocking nodal Na+ channels prevents regeneration of action potentials at nodes, causing propagation failure despite intact myelin. Choice B is correct because it aligns with the principle that saltatory conduction requires active regeneration at each node through Na+ channel activation. Choice A fails as it incorrectly suggests myelin can generate action potentials. To avoid similar mistakes, always remember that nodes of Ranvier are essential sites for action potential regeneration in myelinated axons.

Question 3

1. A larger depolarizing EPSP because increased Cl− conductance raises input resistance
2. A larger hyperpolarizing IPSP and reduced likelihood of reaching threshold due to increased Cl− influx (or shunting) (correct answer)
3. An increased action potential peak because Cl− channels increase Na+ driving force
4. No change in excitability because Cl− conductance affects only neurotransmitter synthesis, not membrane voltage

Explanation: This question assesses understanding of action potentials and synaptic transmission, focusing on inhibitory synaptic mechanisms. Action potentials at inhibitory synapses release neurotransmitters that open Cl- channels, causing Cl- influx (if ECl is negative to resting potential) or shunting inhibition through decreased input resistance. In this scenario, increasing Cl- conductance enhances inhibitory effects by allowing more Cl- influx and/or reducing membrane resistance. Choice B is correct because it aligns with the principle that increased Cl- conductance produces larger IPSPs and reduces excitability through hyperpolarization or shunting. Choice A fails as it incorrectly suggests Cl- conductance increases input resistance. To avoid similar mistakes, always consider that opening inhibitory channels decreases membrane resistance and opposes depolarization.

Question 4

1. Condition 1 produces a larger peak depolarization because EPSPs overlap in time and summate (correct answer)
2. Condition 2 produces a larger peak depolarization because EPSPs require time to recruit voltage-gated Na+ channels
3. Both conditions produce identical peak depolarization because EPSP amplitude depends only on presynaptic spike amplitude
4. Condition 1 produces a smaller peak depolarization because closely spaced EPSPs cancel due to the absolute refractory period

Explanation: This question assesses understanding of action potentials and synaptic transmission, focusing on temporal summation of excitatory postsynaptic potentials. Action potentials trigger EPSPs that decay over tens of milliseconds, allowing closely-timed inputs to summate before the first EPSP decays. In this scenario, 2 ms separation allows the second EPSP to add to the first before significant decay, while 50 ms separation means the first EPSP has largely decayed. Choice A is correct because it aligns with the principle that temporal summation occurs when EPSPs overlap in time. Choice D fails as it incorrectly applies the concept of refractory periods to postsynaptic potentials rather than action potentials. To avoid similar mistakes, always distinguish between summation of graded potentials (EPSPs) and refractory periods of action potentials.

Question 5

1. Hyperpolarization of the resting membrane potential, decreasing firing rate
2. Depolarization of the resting membrane potential, increasing excitability and potentially increasing firing rate (correct answer)
3. No change in resting membrane potential because K+ is impermeant at rest
4. Depolarization of the resting membrane potential, but with reduced excitability because threshold becomes more positive

Explanation: This question assesses understanding of action potentials and synaptic transmission, focusing on how extracellular K+ affects membrane potential and excitability. Action potentials depend on the resting membrane potential, which is largely determined by K+ equilibrium potential (EK = -RT/F × ln[K+]out/[K+]in). In this scenario, increasing extracellular K+ from 4 to 10 mM makes EK less negative, depolarizing the resting potential closer to threshold. Choice B is correct because it aligns with the principle that elevated extracellular K+ depolarizes neurons, increasing excitability and firing rate. Choice C fails as it incorrectly suggests K+ is impermeant at rest when K+ conductance dominates resting potential. To avoid similar mistakes, always apply the Nernst equation to predict how ion concentration changes affect membrane potential.

Question 6

1. Increased action potential peak amplitude due to a larger Na+ driving force at threshold
2. Decreased action potential upstroke and peak amplitude due to reduced Na+ electrochemical driving force (correct answer)
3. Shortened absolute refractory period because fewer Na+ channels open per spike
4. Increased conduction velocity because lower extracellular Na+ reduces membrane capacitance

Explanation: This question assesses understanding of action potentials and synaptic transmission, focusing on how extracellular Na+ concentration affects action potential characteristics. Action potentials involve Na+ influx driven by both concentration gradient and electrical gradient (electrochemical driving force). In this scenario, reducing extracellular Na+ by 40% decreases the Na+ concentration gradient, reducing the driving force for Na+ entry at any given membrane potential. Choice B is correct because it aligns with the principle that reduced Na+ driving force leads to slower depolarization rate and lower peak amplitude. Choice A fails as it incorrectly suggests increased amplitude when the driving force is actually reduced. To avoid similar mistakes, always calculate how changes in ion concentrations affect electrochemical gradients and resulting current magnitudes.

Question 7

1. Shorter postsynaptic response because acetylcholine is removed more rapidly from the cleft
2. Longer-lasting postsynaptic depolarization because acetylcholine persists in the synaptic cleft (correct answer)
3. Reduced presynaptic action potential amplitude because acetylcholinesterase controls Na+ channel opening
4. No change in postsynaptic response because acetylcholine breakdown occurs only inside the presynaptic terminal

Explanation: This question assesses understanding of action potentials and synaptic transmission, focusing on neurotransmitter clearance mechanisms. Action potentials release acetylcholine into the synaptic cleft, where acetylcholinesterase rapidly breaks it down, terminating the postsynaptic response. In this scenario, inhibiting acetylcholinesterase prevents acetylcholine breakdown, allowing it to persist and continue activating postsynaptic receptors. Choice B is correct because it aligns with the principle that neurotransmitter persistence in the cleft prolongs receptor activation and postsynaptic depolarization. Choice D fails as it incorrectly places acetylcholine breakdown inside the presynaptic terminal rather than in the synaptic cleft. To avoid similar mistakes, always remember that synaptic enzymes like acetylcholinesterase function in the extracellular space to terminate signaling.

Question 8

1. Normal action potentials propagate down the axon because somatic Na$^+$ channels are sufficient for axonal regeneration
2. Somatic depolarization may occur, but action potentials fail to propagate along the axon because axonal Na$^+$ channels required for regeneration are blocked (correct answer)
3. Synaptic transmission increases because TTX enhances Ca$^{2+}$ influx at presynaptic terminals
4. Resting membrane potential becomes 0 mV because blocking axonal Na$^+$ channels eliminates ionic gradients

Explanation: This question assesses understanding of action potentials and synaptic transmission, focusing on compartment-specific excitability and action potential initiation. Action potentials involve the rapid influx of sodium ions through voltage-gated channels to depolarize the membrane, with initiation typically occurring at the axon hillock and propagation requiring regenerative opening of axonal sodium channels. In this scenario, TTX blocks toxin-sensitive sodium channels in the axon but spares the toxin-insensitive variants in the soma, allowing somatic depolarization from current injection while preventing axonal propagation. Choice B is correct because it aligns with the principle that blocked axonal sodium channels halt action potential regeneration despite somatic excitability. Choice A fails as it misapplies the principle by assuming somatic channels suffice for axonal propagation, ignoring the need for local axonal channel function. To avoid similar mistakes, always consider the distinct roles of somatic versus axonal ion channels in spike initiation and conduction. Additionally, verify how pharmacological agents selectively affect neuronal compartments based on channel expression.

Question 9

1. Higher maximum firing frequency because slower inactivation shortens the absolute refractory period
2. Lower maximum firing frequency because prolonged inactivation delays recovery of Na$^+$ channels between spikes (correct answer)
3. No change in firing frequency because only K$^+$ channels set the refractory period
4. Higher firing frequency because slower inactivation increases K$^+$ efflux during the upstroke

Explanation: This question assesses understanding of action potentials and synaptic transmission, focusing on Na+ channel inactivation and refractory period. Action potentials involve Na+ channel activation followed by inactivation, which sets the refractory period limiting firing frequency. In this scenario, slower Na+ inactivation prolongs the inactivated state, extending the time before channels recover. Choice B is correct because it aligns with the principle that delayed recovery reduces maximum firing frequency during sustained depolarization. Choice A fails as it misapplies inactivation kinetics by suggesting a shortened refractory period, when slower inactivation actually lengthens it. To avoid similar mistakes, always consider how channel state transitions influence the timing between consecutive action potentials.

Question 10

1. Evoked postsynaptic potentials are reduced, while action potentials in the presynaptic axon still occur (correct answer)
2. Evoked postsynaptic potentials increase due to accumulation of neurotransmitter in the synaptic cleft
3. Presynaptic action potentials fail because SNARE proteins are required for Na$^+$ channel opening
4. Postsynaptic potentials persist because neurotransmitter diffuses out of the presynaptic cytosol without vesicles

Explanation: This question assesses understanding of action potentials and synaptic transmission, focusing on vesicular neurotransmitter release. Action potentials involve Ca2+-triggered vesicle fusion mediated by SNARE proteins for exocytosis. In this scenario, cleaving SNARE proteins prevents vesicle docking, blocking evoked release despite intact presynaptic action potentials. Choice A is correct because it aligns with the principle that SNARE disruption reduces postsynaptic responses without affecting axonal propagation. Choice D fails as it misapplies release mechanisms by suggesting diffusion without vesicles, which is not typical for quantal transmission. To avoid similar mistakes, always consider the molecular machinery required for regulated vesicular release at synapses.

Question 11

1. Reduced presynaptic action potential amplitude because AMPA receptors are voltage-gated Na$^+$ channels
2. Increased EPSP amplitude because blocking AMPA receptors removes Mg$^{2+}$ block from NMDA receptors
3. No change in EPSP amplitude because competitive antagonists only affect receptor synthesis, not receptor function
4. Reduced EPSP amplitude at a given glutamate release because fewer receptors are activated (correct answer)

Explanation: This question assesses understanding of action potentials and synaptic transmission, focusing on postsynaptic receptor activation and EPSP generation. Action potentials involve ligand binding to receptors, opening channels for ion flow. In this scenario, a competitive antagonist blocks AMPA receptors, reducing available binding sites. Choice D is correct because it aligns with the principle that fewer activated receptors decrease EPSP amplitude. Choice B fails as it misapplies receptor interactions by confusing AMPA with NMDA properties. To avoid similar mistakes, always consider the specific pharmacology and downstream effects of receptor subtypes in synaptic signaling.

Question 12

1. Increased mEPP frequency because Ca$^{2+}$ channel blockade increases spontaneous vesicle release
2. Increased evoked end-plate potential amplitude due to prolonged action potentials in the presynaptic terminal
3. No change in evoked end-plate potential because Ca$^{2+}$ is required only for postsynaptic receptor opening
4. Decreased evoked end-plate potential amplitude due to reduced vesicle fusion probability (correct answer)

Explanation: This question assesses understanding of action potentials and synaptic transmission, focusing on neurotransmitter release at chemical synapses. Action potentials involve presynaptic depolarization opening Ca2+ channels, triggering vesicle fusion and neurotransmitter release. In this scenario, blocking presynaptic voltage-gated Ca2+ channels reduces Ca2+ influx, lowering release probability at the neuromuscular junction. Choice D is correct because it aligns with the principle that decreased vesicle fusion reduces evoked end-plate potential amplitude. Choice B fails as it misapplies channel roles by suggesting prolonged action potentials increase release, ignoring the direct dependence on Ca2+ entry. To avoid similar mistakes, always consider the critical role of specific ion channels in linking presynaptic depolarization to synaptic vesicle exocytosis.

Question 13

1. Membrane hyperpolarizes, making action potential initiation less likely
2. Membrane depolarizes, bringing the neuron closer to threshold and potentially increasing firing probability (correct answer)
3. No change in resting potential because only Na$^+$ sets the resting membrane potential
4. Membrane depolarizes and action potential amplitude increases because the K$^+$ equilibrium potential becomes more negative

Explanation: This question assesses understanding of action potentials and synaptic transmission, focusing on how extracellular K+ affects resting membrane potential and excitability. Action potentials involve initiation from resting potential, governed by ion permeabilities. In this scenario, increasing extracellular K+ shifts E_K less negative, depolarizing the resting potential. Choice B is correct because it aligns with the principle that depolarization brings the membrane closer to threshold, enhancing excitability. Choice A fails as it misapplies the Goldman equation by predicting hyperpolarization, ignoring the shift in E_K. To avoid similar mistakes, always consider the impact of extracellular ion changes on equilibrium potentials and resting voltage.

Question 14

1. Decreased membrane time constant (e.g., increased leak conductance), reducing EPSP overlap
2. Increased membrane time constant, allowing EPSPs to decay more slowly and sum (correct answer)
3. Increased absolute refractory period, increasing the chance of reaching threshold
4. Reduced synaptic cleft width, converting the synapse into an electrical synapse

Explanation: This question assesses understanding of action potentials and synaptic transmission, focusing on synaptic integration and membrane time constant. Action potentials involve summation of EPSPs, influenced by passive membrane properties. In this scenario, temporal summation at high frequencies depends on EPSP decay rate. Choice B is correct because it aligns with the principle that a longer time constant allows EPSPs to overlap and reach threshold. Choice A fails as it misapplies time constant effects by suggesting reduced summation, when shorter tau decreases overlap. To avoid similar mistakes, always consider how membrane time constants affect the temporal integration of synaptic inputs.

Question 15

1. Longer synaptic delay because diffusion slows at higher temperature
2. Shorter synaptic delay because vesicle fusion and channel kinetics generally speed up (correct answer)
3. No change in synaptic delay because chemical synapses have fixed delays independent of kinetics
4. Shorter synaptic delay because action potentials propagate faster only in the postsynaptic dendrite

Explanation: This question assesses understanding of action potentials and synaptic transmission, focusing on rate-limiting steps in chemical synaptic transmission. Action potentials involve delays from vesicle fusion, diffusion, and receptor activation in chemical synapses. In this scenario, higher temperature accelerates kinetic processes like fusion and channel opening. Choice B is correct because it aligns with the principle that temperature speeds up enzymatic and diffusion rates, reducing delay. Choice A fails as it misapplies temperature effects by suggesting slower diffusion, contrary to thermodynamic principles. To avoid similar mistakes, always consider how temperature influences the kinetics of biophysical processes in synaptic transmission.

Question 16

1. Synapse X most likely uses ligand-gated ion channels activated by neurotransmitter diffusion across a cleft
2. Synapse X most likely consists of gap junctions permitting direct ionic current flow between cells (correct answer)
3. Synapse X most likely requires presynaptic Ca$^{2+}$ influx to trigger vesicle fusion
4. Synapse X most likely exhibits quantal release events that sum to produce a postsynaptic potential

Explanation: This question assesses understanding of action potentials and synaptic transmission, focusing on differences between electrical and chemical synapses. Action potentials involve signal transmission that can occur via direct ionic flow or neurotransmitter-mediated mechanisms. In this scenario, Synapse X's near-zero delay and bidirectional signaling distinguish it from Synapse Y's delayed, unidirectional nature. Choice B is correct because it aligns with the principle that gap junctions enable fast, electrical coupling between cells. Choice A fails as it misapplies chemical synapse features like ligand-gated channels to the properties observed in Synapse X. To avoid similar mistakes, always consider the temporal and directional characteristics that differentiate electrical from chemical synaptic transmission.

Question 17

1. Larger somatic EPSP because increased leak conductance increases the length constant
2. Smaller somatic EPSP because increased leak conductance shunts current and reduces voltage spread (correct answer)
3. No change at the soma because EPSPs propagate without decrement like action potentials
4. Larger somatic EPSP because dendritic leak conductance increases neurotransmitter release

Explanation: This question assesses understanding of action potentials and synaptic transmission, focusing on passive spread of postsynaptic potentials. Action potentials involve electrotonic propagation of subthreshold signals along dendrites. In this scenario, increased dendritic leak conductance enhances current shunting, reducing voltage decrement to the soma. Choice B is correct because it aligns with the principle that higher leak diminishes signal amplitude at distant sites. Choice A fails as it misapplies length constant effects, as increased leak actually decreases it. To avoid similar mistakes, always consider how conductance changes influence the space constant and signal attenuation.

Question 18

1. Progressive reduction in action potential amplitude and eventual conduction failure as more channels enter and remain in the blocked/inactivated state (correct answer)
2. Progressive increase in action potential amplitude because blocked channels increase Na$^+$ driving force
3. No frequency dependence because Na$^+$ channels do not inactivate during physiological firing
4. Immediate complete block of the first action potential because the drug binds only the resting state

Explanation: This question assesses understanding of action potentials and synaptic transmission, focusing on state-dependent channel block and firing. Action potentials involve Na+ channel cycling through states, susceptible to use-dependent drugs. In this scenario, high-frequency stimulation accumulates inactivated channels, enhancing block. Choice A is correct because it aligns with the principle that progressive block reduces amplitude and leads to failure. Choice B fails as it misapplies block effects by suggesting increased amplitude, ignoring reduced availability. To avoid similar mistakes, always consider how firing frequency influences state-dependent drug interactions with channels.

Question 19

1. Increased conduction velocity because current can spread more freely across the membrane
2. Decreased conduction velocity because increased membrane capacitance and leak reduce effective saltatory conduction (correct answer)
3. No change in conduction velocity because myelin affects only synaptic transmission, not axonal propagation
4. Increased action potential amplitude because Na$^+$ channels are concentrated in internodes after demyelination

Explanation: This question assesses understanding of action potentials and synaptic transmission, focusing on myelin's effect on signal propagation. Action potentials involve efficient conduction in myelinated axons due to reduced capacitance and leak. In this scenario, demyelination increases exposed membrane, elevating capacitance and current leak. Choice B is correct because it aligns with the principle that these changes impair saltatory conduction, slowing velocity. Choice A fails as it misapplies myelin's role by suggesting faster conduction, ignoring increased passive losses. To avoid similar mistakes, always consider how insulation affects electrotonic spread and regenerative efficiency in axons.

Question 20

1. The drug decreases vesicle fusion probability, reducing event frequency while leaving quantal size (postsynaptic response per vesicle) unchanged (correct answer)
2. The drug decreases postsynaptic ligand-gated channel conductance, reducing event frequency without changing event amplitude
3. The drug increases neurotransmitter reuptake, which reduces mEPSC amplitude but not event frequency
4. The drug hyperpolarizes the postsynaptic membrane by increasing Na$^+$/K$^+$-ATPase activity, eliminating quantal release events

Explanation: This question assesses understanding of action potentials and synaptic transmission, focusing on neurotransmitter release probability vs postsynaptic receptor responsiveness. Synaptic transmission involves Ca2+-dependent vesicle fusion for quantal release, with mEPSCs reflecting spontaneous single-vesicle events. In this scenario, blocking presynaptic Ca2+ channels reduces fusion probability, decreasing mEPSC frequency without altering postsynaptic response per vesicle. Choice A is correct because it aligns with physiological principles by explaining reduced event frequency with unchanged quantal size. Choice B fails as it misapplies the principle by attributing reduced frequency to postsynaptic changes, which would affect amplitude instead. To avoid similar mistakes, always consider whether presynaptic or postsynaptic mechanisms differentially impact event rate versus amplitude in miniature currents.