Question at position 36 Which of the following best describes the situation at the neuromuscular junction?Glutamate is being released from the muscle fiber.Acetylcholine is being released from the presynaptic terminal.Each axon is innervating only one muscle fiber.Each muscle fiber is being innervated by multiple axons.Clear my selection

Question

BlackTom AI · Accepted Answer

At the neuromuscular junction (NMJ) the presynaptic motor neuron releases the neurotransmitter acetylcholine (ACh) into the synaptic cleft; ACh binds to nicotinic receptors on the motor end plate of the muscle fiber, producing an end-plate potential that triggers muscle contraction.

1. Relevant knowledge points and concepts
- The NMJ is a chemical synapse between a motor neuron's axon terminal and a skeletal muscle fiber.
- The primary neurotransmitter at the vertebrate NMJ is acetylcholine, not glutamate.
- Nicotinic ACh receptors on the motor end plate are ligand-gated ion channels that depolarize the muscle membrane.
- Motor unit organization: a single motor neuron axon branches to innervate many muscle fibers (one axon → many fibers), and in adult skeletal muscle each muscle fiber is typically innervated by a single motor neuron (mononeuronal innervation).

2. Step-by-step reasoning
- Identify the presynaptic element: motor neuron axon terminal. Determine its transmitter: ACh is packaged in synaptic vesicles and released by Ca2+-dependent exocytosis.
- Identify the postsynaptic element: muscle fiber motor end plate with nicotinic receptors responding to ACh to produce end-plate potentials and action potentials in the muscle.
- Compare alternatives regarding neurotransmitter identity and innervation patterns.

3. Intermediate steps/details
- Action potential arrives at axon terminal → voltage-gated Ca2+ channels open → ACh vesicles released → ACh diffuses across cleft → ACh binds nicotinic receptors → Na+ influx → end-plate potential → muscle AP → contraction.

4. Why the correct answer is correct
- “Acetylcholine is being released from the presynaptic terminal” matches the established physiology: ACh is the neurotransmitter released by motor neuron terminals at the NMJ in vertebrates and causes muscle excitation.

5. Why other options are incorrect
- “Glutamate is being released from the muscle fiber.” Glutamate is a CNS excitatory transmitter, not released from muscle fibers at the NMJ.
- “Each axon is innervating only one muscle fiber.” False: motor axons branch and innervate many muscle fibers (a motor unit).
- “Each muscle fiber is being innervated by multiple axons.” False for adult skeletal muscle; each fiber is typically innervated by a single motor neuron (polyneuronal innervation occurs transiently during development but is later pruned).

6. Conclusion
The correct choice is “Acetylcholine is being released from the presynaptic terminal,” which matches the provided answer.

类似问题

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Question at position 35 The site where a motor neuron's terminal meets the muscle fiber is called the ___________.Motor unitSynaptic cleftAxon terminalNeuromuscular junction (NMJ)

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