The neuromuscular junction is a well-studied example of a chemical synapse. which of the following statements describes a critical event that occurs at the neuromuscular junction?
a) acetylcholine is released by axon terminals of the motor neuron.
b) when the action potential reaches the end of the axon terminal, voltage-gated sodium channels open and sodium ions diffuse into the terminal.
c) acetylcholine binds to its receptor in the junctional folds of the sarcolemma. its receptor is linked to a g protein.
d) acetylcholine is released and moves across the synaptic cleft bound to a transport protein.

A- Acetylcholine is released by axon terminals of the motor neuron.

Explanation:

The general mechanism of muscle contraction can be explained as follows:

a) The action potential which is generated, travels along the motor nerve to its ends on muscle fiber (or motor end plate) at the neuro-muscular junction.

b) The nerve endings then secrete small amounts of neurotransmitter called acetylcholine.

c) The actylecholine binds to the receptors on the membrane of muscle fiber and opens the voltage gated sodium channels.

d) Opening of these channels leads to flow of large quantities of sodium ions into the muscle fiber which ultimately results in initiation of action potential in muscle fiber.  

e) This action potential depolarizes the fiber including the T-tubules, triggering the release of calcium ions from sarcoplasmic reticulum.

f) The calcium ions thus released initiated the process of muscle contraction.

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