How does a chemical synapse pass a signal between cells?

A signal reaching the end of one neuron can affect another across a tiny gap by releasing chemical messengers. At a chemical synapse, an electrical event in the presynaptic terminal triggers a sequence that changes the postsynaptic membrane's electrical behavior. The synaptic cleft separates the two cells, making release and reception essential steps.

An arriving action potential changes the terminal’s membrane voltage and opens voltage-gated calcium channels. Calcium is more concentrated outside the terminal, and its electrochemical gradient favors entry under these conditions. Opening the channels therefore produces a local rise in calcium inside. The voltage controls a gate; the existing gradient drives ion movement.

Calcium triggers molecular machinery that helps transmitter-filled vesicles fuse with the sending membrane. Vesicles are small membrane-bound packages. Fusion releases neurotransmitter molecules, which diffuse across the cleft. Some bind matching receptors on the receiving cell. This recognition makes the response depend on the transmitter and receptor, not just on proximity.

In the pictured pathway, the receptor directly controls an ion channel. Binding opens a route through which selected ions can move. Their direction depends on ion concentrations and membrane voltage. The resulting current can move the receiving cell closer to the threshold for firing, or oppose firing; channel opening does not universally mean excitation.

A receiving neuron combines many inputs, so one transmitter release need not produce another action potential. Uptake, breakdown or diffusion later removes or inactivates transmitter, letting the signal change again. The illustration ends at receptor-channel activation; its separated membranes make the chemical relay visible without representing every synapse or the full recycling machinery.