Glossary · Sinh học
Neurotransmitter
A molecule a neuron releases into the synaptic cleft to pass a signal to the next cell. It diffuses across the gap and binds to receptors on the far membrane, making the receiving cell either more or less likely to fire.
Neurotransmitters sit ready inside small membrane-bound sacs called synaptic vesicles. When an electrical impulse reaches the end of a neuron, the vesicles release them into the synaptic cleft — the space between the presynaptic and postsynaptic membranes.
The molecules diffuse across the cleft and bind to receptor proteins on the postsynaptic membrane, like a key into a lock. The result depends on the transmitter and the receptor: at the synapse between a nerve and a muscle, acetylcholine opens Na⁺ channels and depolarises the postsynaptic membrane, pushing the receiving cell toward firing; GABA opens Cl⁻ channels and hyperpolarises it, pulling the other way.
The signal does not last indefinitely. The transmitter leaves the cleft by three routes: it diffuses away, it is degraded by enzymes in the cleft, or it is recycled by the sending neuron — transporter proteins carry it back, thereby limiting or shutting off the signal.