Excitatory and Inhibitory Neurotransmitters

A brain that could only say go would be a disaster — every neuron would fire nonstop until the whole system turned to noise. Inhibitory signals are the brakes that make the gas pedal useful. Excitatory inputs make a neuron more likely to fire by nudging its internal voltage toward threshold; inhibitory inputs make it less likely to fire by pushing voltage away from threshold. A neuron does not just listen to one input — it listens to thousands at once, arriving at the dendrites and cell body simultaneously. It adds up all the excitatory nudges and subtracts all the inhibitory ones. If that net sum crosses the threshold, the neuron fires an action potential. If not, it stays silent. This is summation, and it is simple algebra, not chemistry — counts and sizes of inputs matter more than which specific one arrived.

Different neurotransmitters specialize in different jobs even though they all work through this same add-and-subtract logic. Dopamine is tied to reward, motivation, and movement — too little is linked to Parkinson's disease, too much to some symptoms of schizophrenia. Serotonin regulates mood, hunger, and sleep; many antidepressants work by keeping more of it available at the synapse. GABA is the brain's main inhibitory neurotransmitter, calming neural activity — too little is linked to anxiety and seizures. Acetylcholine enables muscle action at the junction between nerve and muscle, and also supports learning and memory in the brain.

This topic sets up everything after it: once you know neurons fire or don't fire based on these chemical pushes and pulls, drugs, disorders, and brain damage all become stories about tipping that balance.