MCAT Biological and Biochemical Foundations of Living Systems Quiz: 3a Nervous System Organization Function
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3a Nervous System Organization FunctionQuestion 1 of 20

A neuron receives two simultaneous synaptic inputs: an excitatory synapse on a distal dendrite and an inhibitory synapse located on the soma near the axon hillock. Both synapses are activated at the same time with equal transmitter release probability. The inhibitory receptor is a ligand-gated Cl−^- channel whose reversal potential is near the resting membrane potential. Based on the scenario, which function is most consistent with the somatic inhibitory synapse?

Assume: action potentials initiate at the axon hillock when local depolarization reaches threshold.

It increases the likelihood of firing by summating with the distal excitatory input to depolarize the hillock
It reduces the likelihood of firing by shunting depolarizing current near the hillock, limiting the excitatory input's effect on threshold
It triggers firing by opening Cl−^- channels that depolarize the membrane above threshold at the soma
It reduces firing only by decreasing acetylcholine secretion into the bloodstream, lowering global neuronal excitability
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MCAT Biological and Biochemical Foundations of Living Systems Quiz

MCAT Biological and Biochemical Foundations of Living Systems Quiz: 3a Nervous System Organization Function

Practice 3a Nervous System Organization Function in MCAT Biological and Biochemical Foundations of Living Systems with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

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This quiz focuses on 3a Nervous System Organization Function, 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.

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Question 1

A neuron receives two simultaneous synaptic inputs: an excitatory synapse on a distal dendrite and an inhibitory synapse located on the soma near the axon hillock. Both synapses are activated at the same time with equal transmitter release probability. The inhibitory receptor is a ligand-gated Cl−^- channel whose reversal potential is near the resting membrane potential. Based on the scenario, which function is most consistent with the somatic inhibitory synapse?

Assume: action potentials initiate at the axon hillock when local depolarization reaches threshold.

  1. It increases the likelihood of firing by summating with the distal excitatory input to depolarize the hillock
  2. It reduces the likelihood of firing by shunting depolarizing current near the hillock, limiting the excitatory input's effect on threshold (correct answer)
  3. It triggers firing by opening Cl−^- channels that depolarize the membrane above threshold at the soma
  4. It reduces firing only by decreasing acetylcholine secretion into the bloodstream, lowering global neuronal excitability

Explanation: This question tests understanding of shunting inhibition and synaptic integration at the axon hillock. When an inhibitory synapse opens Cl- channels near the axon hillock, it creates a low-resistance pathway (shunt) that diverts depolarizing current from reaching threshold at the spike initiation zone. Even though the Cl- reversal potential is near rest (not hyperpolarizing), the increased conductance reduces the effectiveness of excitatory inputs by providing an alternate current path. The correct answer (B) recognizes this shunting mechanism where somatic inhibition strategically positioned near the hillock can veto distal excitation. Answer A incorrectly suggests inhibition would summate with excitation to promote firing, missing the shunting effect. When analyzing synaptic integration, consider both the location of inputs and the conductance changes, not just the reversal potentials.

Question 2

A cultured excitatory synapse was voltage-clamped at the postsynaptic neuron. The presynaptic terminal was stimulated with a single action potential. Under control conditions, a fast postsynaptic current (PSC) is recorded. When extracellular Ca2+^{2+} is reduced from 2 mM to 0.2 mM (Mg2+^{2+} unchanged), the PSC amplitude decreases markedly, while presynaptic action potential shape is unchanged.

Which mechanism best explains the reduced PSC amplitude?

(Assume the same number of postsynaptic receptors are present and the neurotransmitter is cleared normally.)

  1. Reduced Ca2+^{2+} converts neurotransmitter release into endocrine signaling, delaying PSCs beyond the recording window
  2. Reduced Ca2+^{2+} increases the driving force for Na+^+ influx through postsynaptic receptors, decreasing PSC amplitude
  3. Reduced Ca2+^{2+} directly blocks postsynaptic ligand-gated channels, preventing neurotransmitter binding
  4. Reduced Ca2+^{2+} entry decreases synaptic vesicle fusion probability, lowering neurotransmitter release per presynaptic spike (correct answer)

Explanation: This question tests understanding of synaptic transmission mechanisms in the nervous system. Neurotransmitter release at chemical synapses is triggered by calcium influx through voltage-gated channels, promoting vesicle fusion. Reducing extracellular calcium limits this influx, decreasing the probability of vesicle release without altering presynaptic action potentials. The correct answer aligns because lower calcium entry reduces fusion events, leading to smaller postsynaptic currents from less neurotransmitter. A common distractor like option B fails by misunderstanding that reduced calcium affects presynaptic release, not postsynaptic driving forces for sodium. For similar questions, confirm if the manipulation targets presynaptic calcium-dependent processes. Remember that postsynaptic currents depend on quantal release modulated by calcium levels.

Question 3

In a sensory pathway, a peripheral mechanoreceptor afferent synapses onto a second-order neuron in the spinal cord. The second-order neuron then projects to the thalamus. In a lesion experiment, the synapse between the afferent and the second-order neuron is pharmacologically silenced, but the second-order neuron and its axon remain intact and excitable. Mechanical stimulation of the skin still generates action potentials in the afferent.

What outcome would be expected in the thalamic neuron that normally receives input from the second-order neuron during skin stimulation?

  1. Unchanged stimulus-evoked activity because action potentials in the afferent propagate directly to the thalamus
  2. Increased stimulus-evoked activity because silencing the synapse removes refractory periods in the afferent
  3. Reduced or absent stimulus-evoked activity because the second-order neuron is no longer driven by afferent synaptic input (correct answer)
  4. Delayed stimulus-evoked activity because the signal is rerouted through endocrine circulation to reach the thalamus

Explanation: This question tests understanding of neural pathway organization in the nervous system. Sensory pathways rely on synaptic transmission to relay signals through second-order neurons to higher centers. Silencing the synapse isolates the second-order neuron, preventing afferent-driven activity. The correct answer aligns because without synaptic input, the thalamic response to stimulation is lost. A common distractor like option B fails by misunderstanding that blocking removes excitation, not refractory constraints. For similar questions, trace signal flow through synapses. Confirm that pathway disruptions abolish downstream activity if synaptic drive is essential.

Question 4

At a glutamatergic synapse, an investigator measures miniature excitatory postsynaptic currents (mEPSCs) in the postsynaptic neuron in the presence of tetrodotoxin to block presynaptic action potentials. A drug is applied that decreases the number of functional postsynaptic receptors without affecting presynaptic vesicle fusion.

Which change is most consistent with the drug's effect on the mEPSCs?

  1. Decreased mEPSC amplitude with unchanged mEPSC frequency (correct answer)
  2. Decreased mEPSC frequency with unchanged mEPSC amplitude
  3. Increased mEPSC frequency because fewer receptors increase presynaptic release probability
  4. No change in mEPSCs because receptor changes affect only endocrine signaling, not synaptic currents

Explanation: This question tests understanding of quantal synaptic transmission in the nervous system. Miniature EPSCs reflect single vesicle releases, with amplitude depending on postsynaptic receptor density. Reducing receptors decreases current per vesicle without affecting spontaneous release frequency. The correct answer aligns because amplitude drops while frequency remains unchanged. A common distractor like option B fails by misunderstanding that postsynaptic changes do not alter presynaptic fusion rates. For similar questions, differentiate quantal size from release probability. Confirm that mEPSC amplitude probes postsynaptic sensitivity.

Question 5

A postsynaptic neuron is voltage-clamped at −70-70 mV. Activation of a ligand-gated cation channel produces an inward current. When the membrane is clamped at 0 mV, activation of the same channel produces ~0 net current. The channel is permeable to Na+^+ and K+^+.

Which mechanism best explains the near-zero current at 0 mV?

  1. At 0 mV the membrane potential is near the channel's reversal potential, so Na+^+ influx and K+^+ efflux balance to yield minimal net current (correct answer)
  2. At 0 mV the channel becomes permanently inactivated, preventing ion flow regardless of ligand binding
  3. At 0 mV Na+^+ and K+^+ concentrations become equal across the membrane, eliminating driving force
  4. At 0 mV the neurotransmitter is converted into a hormone that cannot open ion channels

Explanation: This question tests understanding of ion channel reversal potentials in synaptic signaling. Non-selective cation channels have reversal potentials near 0 mV, balancing sodium influx and potassium efflux. At 0 mV, driving forces equalize, yielding zero net current. The correct answer aligns because clamping at reversal eliminates current flow. A common distractor like option B fails by misunderstanding that ligand-gated channels do not inactivate like voltage-gated ones. For similar questions, calculate reversal using permeabilities. Confirm that current direction reverses around Erev in voltage-clamp.

Question 6

A neuron is stimulated repeatedly with identical depolarizing current pulses. In one condition, extracellular K+^+ is increased modestly (with extracellular Na+^+ unchanged). The resting membrane potential becomes less negative, and the neuron initially fires more easily, but during sustained stimulation it begins to fail to generate action potentials.

Which mechanism best explains the failure to fire during sustained stimulation in elevated extracellular K+^+?

  1. Elevated extracellular K+^+ increases the Na+^+ equilibrium potential, eliminating the action potential upstroke
  2. Elevated extracellular K+^+ hyperpolarizes the membrane, preventing Na+^+ channel activation
  3. Chronic depolarization promotes voltage-gated Na+^+ channel inactivation, reducing available channels for action potential initiation (correct answer)
  4. Elevated extracellular K+^+ triggers endocrine release of K+^+ from glia, which blocks action potentials

Explanation: This question tests understanding of ion effects on neuronal excitability in the nervous system. Elevated extracellular potassium depolarizes rest potential, initially easing firing but promoting sodium inactivation during sustained activity. Chronic depolarization inactivates channels, causing failures. The correct answer aligns because fewer available channels impair AP generation. A common distractor like option B fails by misunderstanding that high potassium depolarizes, not hyperpolarizes. For similar questions, consider Nernst potential shifts. Confirm that accommodation arises from inactivation in depolarized states.

Question 7

In a simple neural pathway, two excitatory presynaptic neurons (P1 and P2) converge onto a postsynaptic neuron (N). P1 and P2 each produce a subthreshold EPSP in N when stimulated alone. When P1 and P2 are stimulated simultaneously, N reaches threshold and fires an action potential.

Which mechanism best explains N firing only during simultaneous stimulation?

  1. Simultaneous stimulation releases neurotransmitter into blood, allowing endocrine summation to trigger firing
  2. Simultaneous stimulation causes Na+^+ channel inactivation to reverse, lowering depolarization and triggering firing
  3. Simultaneous stimulation depletes neurotransmitter, increasing postsynaptic depolarization
  4. Spatial summation of EPSPs increases net depolarization at the axon initial segment to reach threshold (correct answer)

Explanation: This question tests understanding of synaptic summation in neuronal integration. Spatial summation combines subthreshold EPSPs from multiple inputs to reach firing threshold. Simultaneous activation sums depolarizations axially. The correct answer aligns because net depolarization triggers the AP. A common distractor like option B fails by misunderstanding that summation adds, not reverses, inactivation. For similar questions, distinguish spatial from temporal summation. Confirm that convergent inputs enable threshold crossing via addition.

Question 8

A neuron is exposed to a toxin that selectively blocks voltage-gated Ca2+^{2+} channels in presynaptic terminals but does not affect voltage-gated Na+^+ channels along the axon. Presynaptic action potentials still invade the terminal normally.

Based on the scenario, which function is most consistent with the toxin's effect on synaptic transmission?

  1. Faster neurotransmitter diffusion because Ca2+^{2+} channels regulate synaptic cleft width like a hormone
  2. Increased neurotransmitter release because Ca2+^{2+} influx normally inhibits vesicle fusion
  3. Unchanged neurotransmitter release because Na+^+ entry at the terminal is sufficient to fuse vesicles
  4. Reduced neurotransmitter release because Ca2+^{2+} influx is required to trigger synaptic vesicle fusion (correct answer)

Explanation: This question tests understanding of calcium's role in synaptic release in the nervous system. Presynaptic calcium influx is essential for vesicle fusion and neurotransmitter exocytosis. Blocking calcium channels impairs release despite normal action potential invasion. The correct answer aligns because absent calcium entry halts transmission. A common distractor like option B fails by misunderstanding that calcium triggers, not inhibits, fusion. For similar questions, identify calcium-dependent steps in neurotransmission. Verify that axonal conduction persists but terminal release fails without calcium.

Question 9

In a neuron, voltage-gated Na+^+ channels open rapidly when the membrane is depolarized to threshold, producing the rising phase of the action potential. An experimental mutation slows Na+^+ channel inactivation but does not alter activation threshold. When a brief depolarizing current is injected, the neuron produces an action potential with a prolonged depolarized phase.

Which mechanism best explains the prolonged depolarization?

  1. Delayed Na+^+ channel inactivation triggers neurotransmitter release into blood, prolonging depolarization via hormones
  2. Delayed Na+^+ channel inactivation reduces Na+^+ influx, accelerating repolarization
  3. Delayed Na+^+ channel inactivation increases Cl−^- influx, prolonging depolarization
  4. Delayed Na+^+ channel inactivation sustains inward Na+^+ current, opposing repolarization (correct answer)

Explanation: This question tests understanding of action potential duration control in the nervous system. Sodium channel inactivation terminates inward current, allowing repolarization via potassium efflux. Slowing inactivation sustains sodium influx, prolonging the depolarized phase. The correct answer aligns because persistent current opposes repolarization, extending the plateau. A common distractor like option B fails by misunderstanding that delayed inactivation prolongs, not reduces, depolarization. For similar questions, examine channel kinetics in AP phases. Verify that inactivation defects lead to extended APs in channelopathies.

Question 10

In a patch-clamp experiment on a neuron, an investigator injects a constant depolarizing current step. Under control conditions, the neuron fires an action potential. After applying a drug that selectively prolongs the open time of voltage-gated K+^+ channels during the falling phase of the action potential, the neuron still fires but the action potential is narrower and the after-hyperpolarization is larger.

Which mechanism best explains these changes in action potential shape?

  1. Prolonged K+^+ channel opening raises intracellular K+^+ enough to reverse the K+^+ gradient, decreasing after-hyperpolarization
  2. Enhanced K+^+ efflux slows repolarization by preventing Na+^+ channel inactivation, widening the spike
  3. Prolonged K+^+ channel opening increases neurotransmitter release from the soma, narrowing the spike via hormonal feedback
  4. Enhanced K+^+ efflux accelerates repolarization and deepens after-hyperpolarization by driving VmV_m toward EKE_K (correct answer)

Explanation: This question tests understanding of action potential repolarization in the nervous system. Voltage-gated potassium channels facilitate repolarization by allowing potassium efflux, restoring membrane potential to rest. Prolonging their open time enhances efflux, speeding repolarization and deepening after-hyperpolarization. The correct answer aligns because increased potassium conductance narrows the spike and amplifies hyperpolarization toward EK. A common distractor like option B fails by misunderstanding that enhanced potassium efflux accelerates, not slows, repolarization. For similar questions, analyze ion currents during AP phases. Verify that potassium modulation affects falling phase duration and refractory properties.

Question 11

A neuron is stimulated repeatedly while intracellular recordings are made. After a train of action potentials, the afterhyperpolarization phase becomes larger and lasts longer, yet the action potential upstroke speed is unchanged. A selective blocker of Ca2+^{2+}-activated K+^+ channels is then applied, which reduces the prolonged afterhyperpolarization without changing the resting membrane potential. Which mechanism best explains the activity-dependent increase in afterhyperpolarization before the blocker is applied?

  1. Accumulated intracellular Ca2+^{2+} during repetitive firing activates Ca2+^{2+}-dependent K+^+ conductance, increasing K+^+ efflux and hyperpolarization (correct answer)
  2. Repeated firing inactivates voltage-gated K+^+ channels, reducing K+^+ efflux and thereby deepening the afterhyperpolarization
  3. Repeated firing increases extracellular K+^+, which increases the K+^+ driving force and accelerates Na+^+ influx during repolarization
  4. Repeated firing increases secretion of K+^+ as a circulating hormone, which binds receptors to hyperpolarize the neuron over minutes

Explanation: This question tests understanding of calcium-activated potassium channels and activity-dependent afterhyperpolarization. During repetitive firing, calcium enters through voltage-gated calcium channels and accumulates intracellularly, activating calcium-dependent potassium channels that produce an enhanced afterhyperpolarization following the spike train. The correct answer (A) identifies this mechanism, which is confirmed by the selective blocker reducing the prolonged afterhyperpolarization. Option B incorrectly suggests K+ channel inactivation would deepen afterhyperpolarization, when inactivation would actually reduce K+ efflux and decrease afterhyperpolarization. For questions about activity-dependent changes in neuronal excitability, consider whether calcium accumulation could activate calcium-dependent conductances that modulate firing patterns.

Question 12

At a synapse, neurotransmitter is normally cleared from the cleft by reuptake transporters on the presynaptic terminal. A reuptake inhibitor is applied that slows neurotransmitter clearance but does not alter vesicle release.

What outcome would be expected in the postsynaptic response to a single presynaptic action potential?

  1. A delayed postsynaptic response because neurotransmitter must enter the bloodstream before binding receptors
  2. Smaller postsynaptic current because neurotransmitter remains in the cleft and cannot bind receptors
  3. No postsynaptic response because reuptake is required for vesicle fusion during the same action potential
  4. Prolonged postsynaptic current due to extended receptor activation from slower neurotransmitter removal (correct answer)

Explanation: This question tests understanding of neurotransmitter clearance in synaptic function. Reuptake removes transmitter from the cleft, terminating receptor activation. Inhibiting reuptake prolongs exposure, extending postsynaptic currents. The correct answer aligns because slower clearance sustains signaling. A common distractor like option B fails by misunderstanding that accumulated transmitter enhances, not reduces, binding. For similar questions, evaluate clearance kinetics. Confirm that inhibitors prolong responses in reuptake-dependent synapses.

Question 13

A myelinated axon is exposed to a focal toxin that selectively blocks voltage-gated Na+^+ channels at a single node of Ranvier, without affecting internodal membrane properties. Stimulation is applied upstream of the blocked node. The axon proximal to the node generates a normal action potential. What outcome would be expected at recording sites distal to the blocked node?

  1. Action potential amplitude distal to the blocked node increases due to reduced Na+^+ influx at the node, which decreases refractoriness
  2. No action potential is recorded distal to the blocked node because saltatory conduction requires regenerative depolarization at each node (correct answer)
  3. A delayed but full-amplitude action potential is recorded distal to the blocked node because current spreads passively and regenerates at the next node without Na+^+ channels
  4. A sustained depolarization is recorded distal to the blocked node because Na+^+ channels remain open longer when blocked, prolonging inward current

Explanation: This question tests understanding of saltatory conduction in myelinated axons. In myelinated fibers, action potentials regenerate only at nodes of Ranvier where voltage-gated sodium channels are concentrated, with passive current spread through the myelinated internodes. When sodium channels at a single node are blocked, the action potential cannot regenerate at that location, and the passive current decaying through the internode is insufficient to depolarize the next node to threshold. The correct answer (B) recognizes that saltatory conduction fails when regeneration is prevented at any node. Option C incorrectly suggests passive spread alone can trigger regeneration at distant nodes, ignoring the cable properties that cause signal decay. For myelinated axon questions, remember that each node must actively regenerate the action potential - the system cannot skip a non-functional node.

Question 14

A presynaptic terminal is stimulated with a train of action potentials at 50 Hz. Initially, postsynaptic responses increase in amplitude, but after several seconds they decrease despite continued presynaptic firing. Presynaptic Ca2+^{2+} entry per spike is unchanged, but the readily releasable pool of vesicles becomes depleted.

Which mechanism best explains the late decrease in postsynaptic response amplitude during sustained stimulation?

  1. Vesicle depletion reduces neurotransmitter release per spike, decreasing postsynaptic responses despite ongoing Ca2+^{2+} entry (correct answer)
  2. Vesicle depletion increases neurotransmitter release per spike because fewer vesicles compete for fusion sites
  3. Vesicle depletion increases postsynaptic receptor number within seconds, decreasing response amplitude
  4. Vesicle depletion causes neurotransmitter to be secreted as a hormone, increasing synaptic delay and reducing amplitude

Explanation: This question tests understanding of synaptic depression in the nervous system. High-frequency stimulation depletes releasable vesicles, reducing release despite sustained calcium entry. Late amplitude decline reflects fewer vesicles available for fusion. The correct answer aligns because depletion limits neurotransmitter output per spike. A common distractor like option B fails by misunderstanding that depletion decreases, not increases, release. For similar questions, assess stimulation frequency and vesicle pool dynamics. Confirm that depression follows initial facilitation in sustained trains.

Question 15

A postsynaptic neuron expresses a ligand-gated cation channel that is permeable to both Na+^+ and K+^+. At rest, the membrane potential is −70-70 mV. The channel's reversal potential is measured as approximately 0 mV. During synaptic transmission, neurotransmitter binding opens these channels briefly. Which outcome would be expected if the postsynaptic membrane potential is experimentally clamped at +10+10 mV during neurotransmitter application?

  1. A net inward current flows because Na+^+ influx dominates at positive potentials when the channel opens
  2. A net outward current flows because the membrane potential is above the channel reversal potential, driving positive charge out (correct answer)
  3. No current flows because ligand-gated channels conduct only at negative membrane potentials
  4. A net inward current flows because neurotransmitter acts as a hormone that activates Na+^+-K+^+ ATPase to import Na+^+

Explanation: This question tests understanding of reversal potentials and current flow through ligand-gated channels. The reversal potential (0 mV) is the membrane potential at which no net current flows through the channel because inward and outward driving forces balance. When the membrane is clamped above the reversal potential (+10 mV), the electrochemical gradient drives net positive charge outward through the open channel. The correct answer (B) correctly identifies this outward current flow based on the driving force (Vm - Erev = +10 - 0 = +10 mV). Option A incorrectly suggests inward current at positive potentials, ignoring that current direction depends on the difference between membrane potential and reversal potential. For synaptic current questions, always compare the membrane potential to the channel's reversal potential to determine current direction.

Question 16

A neuron is tested for excitability while extracellular Na+^+ is reduced, with osmolarity maintained by replacing Na+^+ with an impermeant cation. The threshold for firing increases, and action potentials that do occur have reduced peak amplitude.

Which mechanism best explains these observations?

  1. Lower extracellular Na+^+ reduces the Na+^+ electrochemical driving force, decreasing inward current during the upstroke and reducing spike amplitude (correct answer)
  2. Lower extracellular Na+^+ increases Na+^+ influx by increasing the concentration gradient, raising spike amplitude
  3. Lower extracellular Na+^+ increases K+^+ equilibrium potential, making repolarization impossible and reducing peak amplitude
  4. Lower extracellular Na+^+ decreases hormone secretion from neurons, which directly lowers action potential peaks

Explanation: This question tests understanding of ion gradients in action potential generation. Sodium driving force determines inward current magnitude during the upstroke. Reducing extracellular sodium weakens this force, lowering peak amplitude. The correct answer aligns because diminished gradient reduces current and spike height. A common distractor like option B fails by misunderstanding that lower external sodium decreases, not increases, influx. For similar questions, apply Goldman equation principles. Verify that gradient manipulations affect amplitude proportionally.

Question 17

In a cortical circuit, an excitatory neuron synapses onto a target neuron. A drug is applied that selectively blocks GABAA_A receptors but does not affect glutamate receptors, voltage-gated channels, or neurotransmitter release probability. In response to the same excitatory presynaptic firing pattern, the target neuron shows a higher probability of firing action potentials. Which mechanism best explains this change in target neuron output?

  1. Blocking GABAA_A receptors reduces inhibitory Cl−^- conductance, increasing membrane resistance and allowing excitatory inputs to produce larger depolarizations (correct answer)
  2. Blocking GABAA_A receptors increases presynaptic glutamate synthesis, increasing quantal size at excitatory synapses
  3. Blocking GABAA_A receptors increases Cl−^- influx, hyperpolarizing the neuron and thereby increasing firing probability
  4. Blocking GABAA_A receptors prevents release of GABA hormone into the bloodstream, which normally suppresses cortical neuron firing

Explanation: This question tests understanding of synaptic integration and the role of GABAergic inhibition. GABA_A receptors are ligand-gated chloride channels that typically produce inhibitory postsynaptic potentials by increasing membrane conductance and shunting excitatory currents. When GABA_A receptors are blocked, the inhibitory conductance is removed, increasing the neuron's input resistance and allowing excitatory postsynaptic potentials to produce larger depolarizations that more readily reach action potential threshold. The correct answer (A) accurately describes this disinhibition mechanism. Option C incorrectly states that blocking receptors increases Cl- influx, when blocking actually prevents chloride conductance. Understanding synaptic integration requires recognizing that inhibition works both by hyperpolarization and by conductance changes that shunt excitatory currents.

Question 18

At a glutamatergic synapse, miniature EPSCs (mEPSCs) are recorded in the postsynaptic neuron in the presence of tetrodotoxin (TTX) to block action potentials. A presynaptic-targeted drug is then applied that decreases the probability of vesicle fusion but does not change vesicle neurotransmitter content. Compared with baseline, mEPSC frequency decreases while mEPSC amplitude is unchanged. Which mechanism best explains this pattern?

  1. Reduced presynaptic release probability decreases circulating glutamate hormone concentration, reducing both frequency and amplitude of mEPSCs
  2. Reduced presynaptic release probability decreases postsynaptic receptor conductance, lowering mEPSC amplitude but not frequency
  3. Reduced presynaptic release probability increases vesicle neurotransmitter loading, increasing mEPSC amplitude while decreasing frequency
  4. Reduced presynaptic release probability lowers event frequency without altering quantal size, leaving individual mEPSC amplitudes unchanged (correct answer)

Explanation: This question tests understanding of quantal analysis and the distinction between presynaptic and postsynaptic mechanisms. Miniature EPSCs represent the postsynaptic response to single vesicle release events, with frequency reflecting release probability and amplitude reflecting quantal size (vesicle content and postsynaptic receptor response). A drug that decreases vesicle fusion probability reduces how often vesicles spontaneously release, decreasing mEPSC frequency without affecting the neurotransmitter content per vesicle or postsynaptic response, leaving amplitude unchanged. The correct answer (D) accurately describes this presynaptic mechanism. Option B incorrectly suggests presynaptic changes affect postsynaptic receptor conductance, conflating pre- and postsynaptic mechanisms. For quantal analysis questions, remember that frequency changes typically indicate presynaptic effects while amplitude changes can be either pre- or postsynaptic.

Question 19

In a simplified spinal reflex circuit, a stretch receptor afferent makes an excitatory synapse onto a motor neuron and also excites an inhibitory interneuron that synapses onto a second motor neuron controlling an antagonist muscle. During tendon tap, recordings show that the agonist motor neuron depolarizes while the antagonist motor neuron hyperpolarizes. Based on the scenario, which function is most consistent with the inhibitory interneuron in this circuit?

  1. It produces reciprocal inhibition to reduce antagonist muscle activation, improving the efficiency of the reflex movement (correct answer)
  2. It amplifies sensory receptor firing by releasing neurotransmitter into the bloodstream to recruit additional afferents
  3. It generates the action potential in the afferent by pumping Na+^+ into the axon, initiating depolarization at the receptor ending
  4. It causes antagonist motor neuron depolarization by opening ligand-gated Na+^+ channels, increasing firing of the antagonist muscle

Explanation: This question tests understanding of spinal reflex circuits and reciprocal inhibition. In the stretch reflex, the inhibitory interneuron receives excitation from the stretch receptor afferent and inhibits the motor neuron controlling the antagonist muscle, preventing it from opposing the reflex contraction of the agonist muscle. The correct answer (A) identifies this reciprocal inhibition function, which improves movement efficiency by relaxing opposing muscles during reflex activation. Option D incorrectly suggests the inhibitory interneuron would depolarize its target, when inhibitory synapses typically hyperpolarize postsynaptic cells through chloride or potassium conductances. When analyzing reflex circuits, consider how coordinated activation of agonists and inhibition of antagonists produces efficient movement.

Question 20

In an isolated frog sciatic nerve preparation, a single suprathreshold stimulus is delivered at one end of a myelinated axon. The extracellular solution initially contains (in mM): Na+^+ 145, K+^+ 4, Cl−^- 110, Ca2+^{2+} 2. The axon is then perfused with a solution in which extracellular Na+^+ is reduced to 20 mM and replaced with an impermeant cation to maintain osmolarity; all other ions are unchanged. Resting membrane potential remains near −70-70 mV, but the recorded action potential peak amplitude decreases substantially and conduction fails in some trials. Which mechanism best explains the reduced action potential amplitude and intermittent propagation failure under low extracellular Na+^+ conditions?

  1. Decreased Na+^+ electrochemical driving force reduces inward Na+^+ current through voltage-gated Na+^+ channels, limiting depolarization and impairing regenerative propagation (correct answer)
  2. Increased Na+^+-K+^+ ATPase activity hyperpolarizes the membrane and prevents voltage-gated Na+^+ channel opening during the stimulus
  3. Reduced extracellular Na+^+ increases K+^+ efflux through voltage-gated K+^+ channels, producing a larger overshoot during the action potential
  4. Low extracellular Na+^+ decreases acetylcholine release from presynaptic terminals, preventing axonal depolarization by blocking endocrine signaling

Explanation: This question tests understanding of action potential generation and the role of sodium ions in neural conduction. Action potentials depend on the electrochemical gradient for sodium ions, which drives Na+ influx through voltage-gated sodium channels during depolarization. When extracellular Na+ is reduced from 145 mM to 20 mM, the driving force for sodium (Vm - ENa) decreases substantially because ENa becomes less positive. The correct answer (A) explains that this reduced driving force limits the inward Na+ current during depolarization, resulting in smaller action potentials that may fail to reach threshold at subsequent nodes. Option C incorrectly suggests increased K+ efflux would cause a larger overshoot, when actually the overshoot depends on Na+ influx. A key principle for similar questions is that action potential amplitude and propagation depend critically on the sodium electrochemical gradient, not just the presence of functional channels.