How would application of a Ca2+ channel blocker affect the neuromuscular junction synapse after stimulation?

Question

BlackTom AI · Accepted Answer

Voltage-gated Ca2+ entry into the presynaptic terminal triggers neurotransmitter release, so blocking those channels prevents evoked synaptic transmission at the neuromuscular junction.

1) Relevant knowledge points and concepts
- An action potential arriving at the motor nerve terminal depolarizes the membrane and opens voltage-gated Ca2+ channels. 
- Ca2+ influx is the essential trigger for synaptic vesicle fusion (via Ca2+-sensing proteins like synaptotagmin and the SNARE machinery), releasing acetylcholine (ACh) into the synaptic cleft.
- ACh binds postsynaptic nicotinic receptors on the muscle end plate, producing an end-plate potential (EPP) by opening ligand-gated Na+/K+ channels. Evoked EPPs therefore require presynaptic Ca2+ influx.
- (Note: spontaneous miniature end-plate potentials can occur at low frequency without action potentials, but evoked responses require Ca2+ entry.)

2) Step-by-step reasoning
Step 1: Apply a Ca2+ channel blocker to the motor neuron terminal. 
Step 2: When the nerve is stimulated, depolarization occurs but voltage-gated Ca2+ channels are blocked → presynaptic Ca2+ current is effectively eliminated. 
Step 3: Without Ca2+ entry, synaptic vesicles do not undergo the Ca2+-dependent fusion process → negligible or no ACh is released in response to stimulation. 
Step 4: With no evoked ACh in the cleft, nicotinic receptors on the muscle are not activated by that stimulation → no evoked postsynaptic end-plate potential is produced.

3) Intermediate logical steps (explicit)
- Stimulus → depolarization (intact)  
- Depolarization + Ca2+ channels blocked → Ca2+ influx ≈ 0  
- Ca2+ ≈ 0 → evoked vesicle fusion ≈ 0 → evoked ACh release ≈ 0  
- Evoked ACh ≈ 0 → evoked postsynaptic potential ≈ 0

4) Why the correct answer is correct
Because evoked neurotransmitter release at the neuromuscular junction is strictly Ca2+-dependent, eliminating the presynaptic Ca2+ current abolishes the chain of events that produce the postsynaptic potential. Thus both the presynaptic Ca2+ current and the evoked postsynaptic potential are eliminated.

5) Why the other options are incorrect
- b) Incorrect: the postsynaptic EPP magnitude depends on presynaptic Ca2+; blocking Ca2+ reduces/abolishes it. 
- c) Incorrect: blocking Ca2+ cannot increase transmitter release or the postsynaptic potential. 
- d) Incorrect: although presynaptic Ca2+ current would be eliminated, that would necessarily reduce evoked ACh release and thus abolish the evoked postsynaptic potential. 
- e) Incorrect: eliminating the postsynaptic potential without affecting presynaptic Ca2+ is inconsistent with the causal sequence—evoked postsynaptic potentials depend on presynaptic Ca2+-triggered release.

Conclusion: The correct answer is (a) — the Ca2+ channel blocker would eliminate the presynaptic Ca2+ current in the motor neuron and the evoked postsynaptic potential in the muscle fiber, which matches the provided answer.

How would application of a Ca2+ channel blocker affect the neuromuscular junction synapse after stimulation?单项选择题

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