presynaptic

Presynaptic is an adjective used in neuroscience to describe the region of a chemical synapse that sends signals. In most neurons, the presynaptic element is the axon terminal (also called the presynaptic bouton or varicosity) that releases neurotransmitter onto the postsynaptic cell.

The presynaptic terminal contains synaptic vesicles filled with neurotransmitter, a specialized active zone, and voltage-gated calcium channels. Arrival of an action potential depolarizes the membrane, opening Ca2+ channels. The resulting calcium influx triggers vesicle docking and fusion with the plasma membrane through a SNARE complex (including syntaxin, SNAP-25, and synaptobrevin/VAMP) aided by synaptotagmin, leading to exocytotic release into the synaptic cleft.

Release is regulated by presynaptic receptors and intracellular signaling. Autoreceptors on the presynaptic membrane can inhibit or alter release in response to the transmitter itself. Short-term plasticity, such as facilitation or depression, reflects changes in calcium dynamics and vesicle availability in the readily releasable pool.

Neurotransmitter choice by the presynaptic neuron influences postsynaptic response, while neuromodulators can alter release probability. Common transmitters include acetylcholine, glutamate, GABA, dopamine, norepinephrine, and serotonin; peptides and gases can also act presynaptically.

Presynaptic function is a target for toxins and disease. Botulinum toxin and tetanus toxin disrupt SNARE-mediated release. Autoimmune Lambert-Eaton myasthenic syndrome impairs presynaptic calcium channel function, reducing transmitter release. Studying presynaptic mechanisms is central to understanding synaptic transmission and neurological disorders.