Axonterminale

An axon terminal, also called a synaptic terminal or bouton, is the distal end of an axon where a neuron forms a chemical or electrical synapse with another neuron, a muscle cell, or a glandular cell. It contains synaptic vesicles filled with neurotransmitters, mitochondria, and cytoskeletal elements. The presynaptic membrane at these terminals features a specialized region called the active zone, where vesicle docking and fusion occur.

The terminal's primary function is to convert an electrical signal into a chemical signal. When an action potential arrives, voltage-gated calcium channels open and Ca2+ influx increases intracellular calcium. Calcium triggers the fusion of synaptic vesicles with the presynaptic membrane via SNARE proteins and calcium sensors such as synaptotagmin, releasing neurotransmitters into the synaptic cleft. These chemicals then bind to receptors on the postsynaptic membrane, generating excitatory or inhibitory postsynaptic potentials that influence the activity of the postsynaptic cell. Neurotransmitter action is terminated by reuptake, enzymatic degradation, or diffusion away from the synapse.

Axon terminals exhibit diversity in transmitter types. They can be excitatory (commonly glutamatergic), inhibitory (such as GABAergic or glycinergic), or modulatory (cholinergic, among others). They also contain distinct vesicle pools: readily releasable vesicles that respond immediately to stimulation and reserve pools that replenish the pool during sustained activity. In addition to small, clear vesicles, terminals can contain dense-core vesicles carrying neuropeptides.

Overall, axon terminals are essential for synaptic transmission, plasticity, and the integration of neural networks.