Präsynapse

Präsynapse is the presynaptic component of a chemical synapse in neurons. It lies at the axon terminal opposite the postsynaptic membrane and contains synaptic vesicles, the active zone, and voltage-gated calcium channels. Its primary function is to release neurotransmitters into the synaptic cleft in response to action potentials, thereby influencing the postsynaptic cell.

Key structures include synaptic vesicles, which store neurotransmitters; the active zone, which organizes docking and fusion; and SNARE proteins (syntaxin-1, SNAP-25, and synaptobrevin/VAMP) that mediate vesicle fusion. Calcium enters through voltage-gated calcium channels (commonly P/Q- and N-type) at the active zone and triggers exocytosis of the vesicles.

Vesicle cycling involves docking, priming, fusion, and endocytosis. Vesicles are organized into pools, including the readily releasable pool, recycling pool, and reserve pool. After fusion, the membrane is retrieved by endocytosis, vesicles are refilled with neurotransmitter, and the cycle continues.

Release characteristics include fast, calcium-triggered transmitter release with both synchronous and, in some cases, asynchronous components. Release probability and timing are modulated by presynaptic receptors, kinases, and neuromodulators, contributing to short-term plasticity such as facilitation or depression.

Clinical and research relevance: presynaptic function is essential for learning and network modulation, and presynaptic dysfunction is linked to various neurological and psychiatric disorders. Toxins such as botulinum toxin disrupt presynaptic release by cleaving SNARE proteins, illustrating the crucial role of the Präsynapse in neurotransmission.

Compared with the postsynapse, which hosts neurotransmitter receptors, the Präsynapse is specialized for transmitter release and vesicle cycling.