presynapseihin

The presynapse, or presynaptic terminal, is the part of a neuron that transfers information to a postsynaptic neuron at a chemical synapse. It is typically located at axon terminals or boutons and contains the machinery for neurotransmitter storage, release, and vesicle recycling. The presynaptic terminal houses synaptic vesicles filled with neurotransmitters, mitochondria, and a cytoskeletal network organized around an active zone where release occurs.

When an action potential arrives at the presynaptic terminal, it opens voltage-gated calcium channels and causes a rapid influx of calcium. The rise in intracellular calcium triggers the fusion of docked vesicles with the presynaptic membrane through a SNARE protein complex (including syntaxin-1, SNAP-25, and synaptobrevin/VAMP2), a process aided by calcium sensors such as synaptotagmin. The released neurotransmitter diffuses across the synaptic cleft and binds to receptors on the postsynaptic cell, conveying the signal.

Signal termination involves several mechanisms: reuptake of neurotransmitters into the presynaptic terminal via specific transporters (for example, SERT, NET, DAT), uptake into neighboring glial cells, enzymatic degradation in the synaptic cleft, or diffusion away from the synapse. The presynaptic membrane, after vesicle fusion, is recovered through endocytosis and vesicle refilling, completing the vesicle cycle.

Presynaptic activity is modulated by autoreceptors and various signaling pathways that adjust release probability. Short-term plasticity (facilitation or depression) and long-term changes in release contribute to learning and neural homeostasis. Dysfunction of presynaptic processes is implicated in multiple neurological and psychiatric conditions, making them a common target for pharmacological intervention.