Transmission

Transmission is the process by which information travels from one neuron to another — the fundamental unit of communication in the brain and nervous system. Each neuron is separated from its neighbour by a tiny gap called a synapse, and bridging that gap is what transmission accomplishes.

Most transmission is chemical: the sending neuron releases signalling molecules called neurotransmitters, which drift across the synapse and bind to receptor proteins on the receiving cell. That binding is what carries the message forward. A small subset of synapses use electrical transmission instead, passing current directly through gap-junction channels — but chemical synapses dominate the brain and are the primary target of psychoactive substances.

How it works · its role

The sequence unfolds in distinct steps. An electrical impulse travels down the axon of the sending neuron and triggers small membrane sacs — vesicles — to fuse with the cell wall and release their neurotransmitter payload into the synapse. Those molecules diffuse across and bind to receptors on the receiving cell, shifting its electrical state in a way that either promotes or suppresses its own firing.

Once the signal is delivered, the synapse clears itself. Transporter proteins in the sending cell's membrane pump most neurotransmitters back in — a process called reuptake — where they are repackaged for future release. Others are broken down on the spot by enzymes; monoamine oxidase (MAO), for instance, degrades dopamine, norepinephrine, and serotonin in the cleft.

The net result at any synapse — whether a signal passes, falters, or amplifies — depends on the balance among release, receptor binding, reuptake, and enzymatic breakdown. Psychoactive substances alter that balance.

Relevance to substances & effects

Virtually every psychoactive substance works by disrupting one or more of these steps. The diversity of drug effects maps closely onto the diversity of intervention points.

Reuptake inhibitors — including SSRIs, SNRIs, and cocaine — block transporter proteins, causing neurotransmitters to linger in the synapse and prolong the signal. Releasers such as MDMA and amphetamines flood the synapse by pushing neurotransmitters out of the presynaptic cell in greater quantities than normal. Receptor agonists — including opioids, benzodiazepines, and classic psychedelics such as LSD and psilocin — bind directly to receptors and activate them; antagonists occupy the same sites without activating them, muting the signal instead.

MAO inhibitors remove enzymatic breakdown from the equation, amplifying whatever neurotransmitter activity is already present. This is part of why MAOIs interact dangerously with so many other substances — one of the main brakes on transmission is simply gone.

Knowing which step a substance targets, and in which brain circuits, is what explains why a stimulant, a psychedelic, and an antidepressant can converge on overlapping pathways yet produce profoundly different experiences.