A synapse is a contact point between two neurons, or between a neuron and a muscle cell, which serves the transmission of stimuli. The term synapse was coined by Sir Charles Sherrington (British physiologist; 1857-1952).
A synapse consists of 3 elements:
Synapses are classified according to neurotransmission and kinds of stimulus:
The electrical synapse or gap junction transmits the impulse through very close cell contact via ion channels directly from nerve cell to nerve cell. The synaptic cleft measures only about 3.5 nm. Electrical synapses work without delay. The neurotransmission is possible bidirectionally. They mainly occur where a rapid stimulus transmission is necessary (e.g. lid reflex).
In the chemical synapse, the presynapse releases neurotransmitters from vesicles to transmit the impulse. This process, which makes necessary a complex interaction of various proteins, is also called exocytosis. The neurotransmitters diffuse through the synaptic cleft and bind to suitable receptors of the postsynapse. These receptors are either ionotropic or metabotropic. The synaptic cleft is wider than in the gap junctions and measures approx. 20-40 nm. Chemical synapses act with a slight time delay (about 1 millisecond). The neurotransmission is possible only unidirectionally.
Three types of synapses can be distinguished according to their exact contact point:
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