Question at position 4 Neurotransmitters are released upon stimulation from a nerve impulse by the: axon terminals of the motor neuronmotor unitmyofibrilssarcolemma of the muscle cell

Question

Question at position 4 Neurotransmitters are released upon stimulation from a nerve impulse by the: axon terminals of the motor neuronmotor unitmyofibrilssarcolemma of the muscle cell

BlackTom AI · Accepted Answer

Neurotransmitters are released from the axon terminals of the motor neuron.

1) Relevant knowledge points and concepts
- Motor neuron anatomy: the axon terminates in axon terminals (synaptic boutons) at the neuromuscular junction.
- Synaptic vesicles: axon terminals contain vesicles filled with the neurotransmitter acetylcholine (ACh).
- Excitation–secretion coupling: an action potential arriving at the axon terminal opens voltage-gated Ca2+ channels; Ca2+ influx triggers vesicle fusion and exocytosis of ACh into the synaptic cleft.
- Postsynaptic response: ACh binds nicotinic receptors on the sarcolemma (motor end plate), producing a muscle action potential and subsequent contraction.

2) Step-by-step reasoning process
1. A nerve impulse (action potential) travels down the motor neuron's axon to its axon terminals.
2. Depolarization opens voltage-gated calcium channels in the terminal membrane.
3. Extracellular Ca2+ enters the terminal, causing synaptic vesicles to fuse with the presynaptic membrane.
4. Acetylcholine is released into the synaptic cleft by exocytosis.
5. ACh diffuses and binds to receptors on the sarcolemma (motor end plate), initiating muscle fiber depolarization and contraction cascade.

3) Intermediate steps / mechanistic detail
- Arrival of action potential → opening of Ca2+ channels → Ca2+ binds to synaptotagmin → SNARE-mediated vesicle fusion → ACh release → receptor activation → muscle action potential → Ca2+ release from sarcoplasmic reticulum → actin–myosin cross-bridge cycling.

4) Why the correct answer is correct
The axon terminal is the presynaptic specialization specifically adapted for neurotransmitter storage and regulated release. All molecular steps of stimulus-induced neurotransmitter release occur in the axon terminal, so it is the precise location where release happens.

5) Why other options are incorrect
- Motor unit: this is the functional group (one motor neuron plus all muscle fibers it innervates), not the microscopic site of release.
- Myofibrils: contractile filaments inside muscle fibers (actin/myosin); they do not store or release neurotransmitters.
- Sarcolemma of the muscle cell: the sarcolemma is the postsynaptic membrane that receives neurotransmitter and generates a muscle action potential, but it does not release neurotransmitter.

6) Conclusion confirming the answer
Therefore, the correct choice is "axon terminals of the motor neuron," which matches the provided answer.

Question at position 4 Neurotransmitters are released upon stimulation from a nerve impulse by the: axon terminals of the motor neuronmotor unitmyofibrilssarcolemma of the muscle cell单项选择题

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