Action Potentials and Synaptic Transmission (3A)
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MCAT Biological and Biochemical Foundations of Living Systems › Action Potentials and Synaptic Transmission (3A)
In a neuromuscular junction preparation, a toxin was applied that cleaves a SNARE protein required for synaptic vesicle fusion. End-plate potentials (EPPs) were recorded in the muscle fiber in response to motor neuron stimulation. The concept being tested is vesicle fusion as a prerequisite for neurotransmitter release. Which observation best explains the expected change in EPPs?
EPP amplitude decreases because fewer acetylcholine-containing vesicles fuse and release transmitter per stimulus
EPP amplitude increases because vesicles accumulate and release more acetylcholine when stimulated
EPP amplitude is unchanged because acetylcholine release occurs through membrane transporters, not vesicles
EPP amplitude decreases because postsynaptic nicotinic receptors become inactivated by the toxin
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.
In an experiment on myelinated peripheral axons, a compound was applied that selectively blocks voltage-gated Na+ channels at nodes of Ranvier but does not affect internodal membrane properties. The concept being tested is saltatory conduction and the requirement for nodal Na+ currents. Which outcome is most consistent with this manipulation?
Conduction velocity decreases but action potentials still propagate because myelin can generate action potentials in internodes
Action potentials become larger in amplitude because blocking nodal Na+ channels reduces Na+ channel inactivation
Propagation is unchanged because K+ channels, not Na+ channels, determine whether a spike travels down an axon
Propagation fails because action potentials require regenerative Na+ influx at successive nodes
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.
Investigators recorded from a postsynaptic neuron receiving inhibitory synaptic input. They applied a drug that increases postsynaptic Cl− conductance by increasing the open probability of ligand-gated Cl− channels, without changing presynaptic firing. The concept being tested is how changes in ion permeability influence postsynaptic potentials. Which change is most expected at the postsynaptic membrane near resting potential?
An increased action potential peak because Cl− channels increase Na+ driving force
A larger depolarizing EPSP because increased Cl− conductance raises input resistance
No change in excitability because Cl− conductance affects only neurotransmitter synthesis, not membrane voltage
A larger hyperpolarizing IPSP and reduced likelihood of reaching threshold due to increased Cl− influx (or shunting)
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.
A team studied synaptic integration by evoking two identical excitatory synaptic inputs onto the same dendritic branch. Condition 1: the two presynaptic spikes were separated by 2 ms. Condition 2: the spikes were separated by 50 ms. The concept being tested is temporal summation of postsynaptic potentials. Which outcome is most consistent with physiological principles?
Condition 1 produces a smaller peak depolarization because closely spaced EPSPs cancel due to the absolute refractory period
Both conditions produce identical peak depolarization because EPSP amplitude depends only on presynaptic spike amplitude
Condition 1 produces a larger peak depolarization because EPSPs overlap in time and summate
Condition 2 produces a larger peak depolarization because EPSPs require time to recruit voltage-gated Na+ channels
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.
In cultured neurons, extracellular K+ was increased from 4 mM to 10 mM while all other ions were held constant. Investigators monitored spontaneous firing rate. The concept being tested is how changes in extracellular ion concentrations alter resting membrane potential and excitability. Which effect is most expected?
Depolarization of the resting membrane potential, but with reduced excitability because threshold becomes more positive
Depolarization of the resting membrane potential, increasing excitability and potentially increasing firing rate
Hyperpolarization of the resting membrane potential, decreasing firing rate
No change in resting membrane potential because K+ is impermeant at rest
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.
A research group perfused hippocampal slices with artificial cerebrospinal fluid in which extracellular Na+ was reduced by 40% (osmolarity maintained by replacing with an impermeant cation). They then stimulated an axon bundle and recorded somatic action potentials in downstream neurons. The concept being tested is how extracellular ion gradients influence action potential initiation and propagation. Which change is most expected in the recorded action potentials?
Shortened absolute refractory period because fewer Na+ channels open per spike
Increased conduction velocity because lower extracellular Na+ reduces membrane capacitance
Decreased action potential upstroke and peak amplitude due to reduced Na+ electrochemical driving force
Increased action potential peak amplitude due to a larger Na+ driving force at threshold
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.
A study examined synaptic transmission under a drug that inhibits acetylcholinesterase at cholinergic synapses, without directly affecting presynaptic release probability. Postsynaptic responses were recorded following single presynaptic action potentials. The concept being tested is neurotransmitter clearance and its effect on postsynaptic potentials. Which change is most consistent with this manipulation?
Shorter postsynaptic response because acetylcholine is removed more rapidly from the cleft
Longer-lasting postsynaptic depolarization because acetylcholine persists in the synaptic cleft
No change in postsynaptic response because acetylcholine breakdown occurs only inside the presynaptic terminal
Reduced presynaptic action potential amplitude because acetylcholinesterase controls Na+ channel opening
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.
A neuron expresses a toxin-insensitive Na$^+$ channel variant only in the soma, while the axon expresses normal toxin-sensitive Na$^+$ channels. After adding TTX to the bath, the concept tested is compartment-specific excitability and action potential initiation. Which result is most consistent?
Current injection is delivered at the soma to attempt to evoke spikes.
Normal action potentials propagate down the axon because somatic Na$^+$ channels are sufficient for axonal regeneration
Resting membrane potential becomes 0 mV because blocking axonal Na$^+$ channels eliminates ionic gradients
Synaptic transmission increases because TTX enhances Ca$^{2+}$ influx at presynaptic terminals
Somatic depolarization may occur, but action potentials fail to propagate along the axon because axonal Na$^+$ channels required for regeneration are blocked
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.
A lab engineered neurons expressing a mutant voltage-gated Na$^+$ channel that inactivates more slowly, without changing activation threshold. The concept tested is Na$^+$ channel inactivation and refractory period. During a sustained depolarizing current injection, which firing pattern is most expected compared with wild-type?
Resting potential and extracellular ion concentrations are unchanged.
Lower maximum firing frequency because prolonged inactivation delays recovery of Na$^+$ channels between spikes
No change in firing frequency because only K$^+$ channels set the refractory period
Higher maximum firing frequency because slower inactivation shortens the absolute refractory period
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.
A presynaptic terminal was exposed to a toxin that cleaves SNARE proteins required for vesicle docking and fusion, without affecting presynaptic action potential waveform. The concept tested is vesicular neurotransmitter release. Which observation is most consistent with this manipulation?
Postsynaptic receptors and membrane potential are otherwise normal.
Evoked postsynaptic potentials increase due to accumulation of neurotransmitter in the synaptic cleft
Postsynaptic potentials persist because neurotransmitter diffuses out of the presynaptic cytosol without vesicles
Evoked postsynaptic potentials are reduced, while action potentials in the presynaptic axon still occur
Presynaptic action potentials fail because SNARE proteins are required for Na$^+$ channel opening
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.