synapse

A synapse is a junction where signaling occurs between neurons or between a neuron and another cell type, such as a muscle or gland. In the nervous system, most signaling happens at chemical synapses, though electrical synapses formed by gap junctions also exist. A chemical synapse consists of a presynaptic terminal, the synaptic cleft, and a postsynaptic membrane bearing receptors.

Transmission begins when an action potential reaches the presynaptic terminal, triggering voltage-gated calcium channels to open and Ca2+ to enter. This influx prompts synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitters into the synaptic cleft. The neurotransmitters diffuse across the gap and bind to receptors on the postsynaptic membrane, causing ion channels to open or close and generating a postsynaptic potential. The response can be depolarizing (excitatory) or hyperpolarizing (inhibitory). Glutamate is a principal excitatory transmitter in the central nervous system; GABA and glycine are major inhibitory transmitters; acetylcholine is prominent at neuromuscular junctions and in certain brain pathways.

Electrical synapses use gap junctions that allow ions to pass directly between cells, enabling faster, bidirectional transmission and greater synchronization, though they are less adaptable than chemical synapses.

Synapses are dynamic structures. Short- and long-term changes in synaptic strength, such as long-term potentiation and long-term depression, underlie learning and memory. During development, synaptogenesis forms networks that are later refined through pruning. Glial cells and extracellular signals influence synapse formation and elimination.

Dysfunction at synapses is linked to numerous conditions, including Alzheimer's disease, autism spectrum disorders, schizophrenia, and neuromuscular junction disorders such as myasthenia gravis.