Axon terminal

Functionally, the axon terminal converts an electrical signal into a chemical signal. When an action potential arrives at an axon terminal (A), the neurotransmitter is released and diffuses across the synaptic cleft. If the postsynaptic cell (B) is also a neuron, neurotransmitter receptors generate a small electrical current that changes the postsynaptic potential. If the postsynaptic cell (B) is a muscle cell (neuromuscular junction), it contracts.

Neurotransmitter release

Axon terminals are specialized to release neurotransmitters very rapidly by exocytosis. Neurotransmitter molecules are packaged into synaptic vesicles called quanta that cluster beneath the axon terminal membrane on the presynaptic side (A) of a synapse. Some of these vesicles are docked, i.e., connected to the membrane by several specialized proteins, such as the SNARE complex. The incoming action potential activates voltage-gated calcium channels, leading to an influx of calcium ions into the axon terminal. The SNARE complex reacts to these calcium ions. It forces the vesicle's membrane to fuse with the presynaptic membrane, releasing their content into the synaptic cleft within 180 μs of calcium entry. When receptors in the postsynaptic membrane bind this neurotransmitter and open ion channels, information is transmitted between neurons (A) and neurons (B). To generate an action potential in the postsynaptic neuron, many excitatory synapses must be active at the same time.