Nerve terminal

A nerve terminal (also called a presynaptic terminal or synaptic bouton) is the bulb-shaped tip at the end of an axon — the long projection that carries electrical signals away from a neuron's cell body. It is where chemical communication takes over from electrical: the terminal houses clusters of membrane-bound pouches called vesicles, each packed with neurotransmitter molecules ready to be emptied into the narrow gap (the synapse) separating two neurons.

A single neuron may branch into thousands of terminals, each forming its own synaptic contact. Though each terminal is only a few micrometres across, the density of terminals in a brain region is a reliable index of how heavily that region depends on a given transmitter system.

How it works · its role

When an electrical impulse (action potential) reaches the terminal, it triggers a rapid influx of calcium ions through voltage-gated channels in the membrane. Calcium is the releasing switch: it causes vesicles to fuse with the terminal's outer membrane and empty their contents into the synaptic cleft — a process called exocytosis.

The released neurotransmitter diffuses across the cleft and binds to receptors on the neighbouring neuron. The signal is then curtailed: transporter proteins embedded in the terminal membrane pump most of the transmitter back inside, a recycling process called reuptake. Recovered molecules are either repackaged into new vesicles or broken down by enzymes such as monoamine oxidase.

Relevance to substances & effects

The nerve terminal is a primary site of action for many psychoactive substances because storage, release, and reuptake all converge there.

Releasing agents — MDMA, amphetamines, and related stimulants — enter the terminal by hijacking the same reuptake transporters (SERT for serotonin, DAT for dopamine, NET for noradrenaline). Once inside, they reverse the transporter's normal direction, flooding the synapse with transmitter. The resulting surge produces the stimulant, empathogenic, or euphoric effects characteristic of these compounds.

Reuptake inhibitors — SSRIs, cocaine, many tricyclic antidepressants — block the transporters from outside the terminal without entering it, leaving neurotransmitter in the cleft longer. The distinction matters: a releasing agent exhausts vesicular stores, which partly explains the comedown after MDMA; a reuptake inhibitor leaves stores largely intact.

Some drugs act inside the terminal at the vesicular monoamine transporter (VMAT2), which is responsible for refilling vesicles. Reserpine and tetrabenazine block VMAT2, preventing storage and progressively depleting available transmitter — a mechanism that can precipitate low mood or depressive episodes with sustained use.

Classic psychedelics such as LSD and psilocin exert most of their effects postsynaptically, at 5-HT₂A receptors on the receiving neuron. Even so, presynaptic serotonin autoreceptors on the terminal sense the transmitter level and can throttle further release — one reason tolerance to psychedelics develops quickly and cross-tolerance between them is well established.