Final answer:
Synaptic transmission at the NMJ involves the release of acetylcholine from the motor neuron, which binds to receptors on the muscle cell, leading to depolarization and muscle contraction.
Step-by-step explanation:
Synaptic transmission at the neuromuscular junction (NMJ) begins with a neuronal action potential traveling down a motor neuron axon to the axon terminals. When the presynaptic membrane of these terminals is depolarized, voltage-gated Ca²⁺ channels open, allowing Ca²⁺ to enter and causing synaptic vesicles to fuse with the membrane and release the neurotransmitter acetylcholine (ACh) into the synaptic cleft. The ACh then diffuses and binds to nicotinic acetylcholine receptors on the sarcolemma of the muscle cell, which are ligand-gated ion channels, causing either depolarization or hyperpolarization of the postsynaptic membrane. This depolarization, known as an end-plate potential, spreads and generates an action potential that initiates muscle contraction.