Synapsenspalt

Synapsenspalt, or synaptic cleft, is the narrow extracellular space between the presynaptic axon terminal of one neuron and the postsynaptic membrane of another cell, typically a neuron or muscle cell, at chemical synapses. It is about 20-40 nanometers wide.

The cleft lies within the active zone of the presynaptic terminal; it contains extracellular matrix molecules and can be filled with fluid that facilitates diffusion of neurotransmitters. The postsynaptic side has a density of receptors and scaffold proteins that organize signaling complexes.

Transmission across the synapsenspalt is chemical. An action potential triggers voltage-gated calcium channels to open; Ca2+ influx leads to fusion of synaptic vesicles with the presynaptic membrane via SNARE proteins, releasing neurotransmitters into the cleft. The neurotransmitters diffuse across the cleft and bind to receptors on the postsynaptic membrane, producing excitatory or inhibitory postsynaptic potentials. Termination occurs by neurotransmitter reuptake, enzymatic degradation, or diffusion away from the cleft.

Variations include different transmitter systems (acetylcholine at the neuromuscular junction; glutamate, GABA, glycine in the CNS). The cleft is essential for synaptic specificity and plasticity; its dysfunction is linked to neurological diseases and developmental disorders.