What a neurotransmitter actually does
Brain cells (neurons) don't quite touch — there is a tiny gap between them. To pass a signal across that gap, one neuron releases a small chemical called a neurotransmitter. That molecule drifts across, lands on a receptor on the next cell, and either excites it (makes it more likely to fire) or calms it (makes it less likely). Almost every CNS drug works by changing this one event: how much messenger is present, or how the receptor responds to it.
A drug that mimics the messenger and switches the receptor on is an agonist. A drug that blocks the receptor so the natural messenger can't act is an antagonist. Hold onto those two words — they explain more of this track than any other pair.
The big three: GABA, dopamine, serotonin
GABA is the brain's main calming signal. Wherever GABA acts, neurons quiet down. Sedatives, sleeping pills, and many antiepileptics work by amplifying GABA — that is why they make you drowsy and stop runaway electrical storms. Dopamine handles reward, movement, and motivation; too little in one pathway causes the stiffness of Parkinson's, while too much in another is linked to psychosis. Serotonin shapes mood, sleep, and appetite, and is the main target of most antidepressants.
There is a clever trick the body and many drugs use: after a messenger has done its job, a pump on the releasing neuron sucks it back in to be recycled. Block that pump and the messenger lingers longer in the gap, strengthening its signal. A drug that does this is a reuptake inhibitor — the whole logic of SSRIs in guide 3.
- A signal arrives and the neuron releases its messenger into the gap.
- The messenger binds a receptor on the next cell — exciting or calming it.
- The messenger is cleared: pumped back in (reuptake) or broken down by an enzyme.
- A CNS drug changes one of these three steps — release, binding, or clearance.