A synapse is a specialized junction through which one cell influences another. In a chemical synapse, the presynaptic cell releases a signaling molecule called a neurotransmitter, which crosses a narrow extracellular gap and binds receptors on the postsynaptic cell.

The main structural regions are:

• the presynaptic terminal, containing neurotransmitter-filled synaptic vesicles and release machinery;
• the synaptic cleft, the narrow extracellular space between the cells;
• the postsynaptic membrane, enriched in receptors and associated signaling proteins.

The basic direction of information flow is

presynaptic action potential
        ↓
neurotransmitter release
        ↓ diffusion across cleft
postsynaptic receptor activation
        ↓
postsynaptic response

Chemical transmission converts signal type twice

An action potential arriving at the terminal is an electrical signal. Vesicle release converts it into a chemical signal carried by neurotransmitter. Postsynaptic receptors then convert neurotransmitter binding into another cellular response, often an electrical change or a slower signaling cascade.

Thus a chemical synapse is an electrical → chemical → cellular-response converter.

Synaptic directionality comes from molecular asymmetry

The two sides of a chemical synapse are not interchangeable. Vesicle-release machinery is concentrated presynaptically, while appropriate neurotransmitter receptors are concentrated postsynaptically.

This structural polarity usually makes transmission strongly directional from the presynaptic cell to the postsynaptic cell.

A synapse is not simply extracellular diffusion

Neurotransmitter crosses only a very short distance, but reliable signaling also requires controlled release, receptor specificity and rapid termination. The cleft alone does not determine the signal.

Chemical synapses therefore combine spatial organization, regulated secretion and receptor-mediated detection to transfer information between cells.