Neuromuscular junctions are synapses between neurons and skeletal muscle cells. They are excellent models for how chemical synaptic transmission works. Neurons that control muscles are called motor neurons. Like other neurons, a motor neuron has only one axon, but that axon can branch into numerous axon terminals that form many synapses with muscle cells. At each axon terminal an enlarged knob or buttonlike structure contains membrane-bound vesicles filled with neurotransmitter molecules. The neurotransmitter used by all vertebrate neuromuscular synapses is acetylcholine (ACh). ACh is released by exocytosis when the membrane of a vesicle containing this neurotransmitter fuses with the presynaptic membrane of the axon terminal.

The postsynaptic membrane of the neuromuscular junction is a modified part of the muscle cell membrane called a motor end plate. It appears as a depression in the muscle cell membrane, and the terminals of the motor neuron sit in the depression. The space between the presynaptic membrane and the postsynaptic membrane is the synaptic cleft, which in chemical synapses is about 20–40 nanometers wide. ACh released into the cleft by the presynaptic cell diffuses across to the postsynaptic membrane (Focus: Key Figure 44.11).