Final answer:
Nerve impulses cause the release of acetylcholine from vesicles in the axon terminus, which binds to receptors on the muscle fiber's membrane, allowing for muscle contraction. Calcium ions play a crucial role in this process. Acetylcholinesterase removes excess acetylcholine from the synaptic cleft.
Step-by-step explanation:
Nerve impulses traveling down motor neurons cause the release of the acetylcholine from vesicles in the axon terminus into the synapse between the neuron and the muscle fiber. Acetylcholine binds to receptors on the plasma membrane (sarcolemma) of the muscle fiber, allowing an action potential to move to the endoplasmic reticulum where calcium ions are stored.
When Ca²+ is released in response to the change in voltage, it binds to actin, causing actin filaments to shift position and revealing myosin binding sites for the cross-bridges. The muscle contracts until the nerve impulse stops and Ca²+ returns to its storage sites. The enzyme acetylcholinesterase removes lingering acetylcholine from the synaptic cleft.