Final answer:
In a normal fish, when nerve cells release acetylcholine, it causes depolarization and triggers an action potential in muscles, leading to muscle contraction.
Step-by-step explanation:
When nerve cells in a normal fish release acetylcholine (ACh), several events occur leading to muscle movement. Nerve impulses cause the release of ACh into the synapse between the neuron and muscle fiber. ACh diffuses across the synaptic cleft and binds with receptors on the motor end plate of the muscle fiber. This binding opens ion channels, allowing Na+ ions to enter the muscle cell, causing depolarization. This electrical change is known as an end-plate potential, which triggers an action potential along the muscle fiber, leading to muscle contraction.
All ganglionic neurons of the autonomic system, in both sympathetic and parasympathetic ganglia, are also activated by ACh released from preganglionic fibers. These receptors are of the nicotinic type, which are ligand-gated ion channels that when activated by ACh, cause depolarization of the neuron.