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Synapses are specialized junctions through which cells of the nervous system signal to one another and to non-neuronal cells such as muscles or glands.
Synapses form the circuits in which the neurons of the central nervous system interconnect. They are thus crucial to the biological computations that underlie perception and thought. They also provide the means through which the nervous system connects to and controls the other systems of the body.
The word "synapse" comes from "synaptein" which Sir Charles Scott Sherrington and his colleagues coined from the Greek "syn-" meaning "together" and "haptein" meaning "to clasp".
At a prototypical synapse, such as a dendritic spine, a mushroom-shaped bud projects from each of two cells and the caps of these buds press flat against one another. At this interface, the membranes of the two cells flank each other across a slender gap, the narrowness of which enables signaling molecules known as neurotransmitters to pass rapidly from one cell to the other by diffusion. This gap is sometimes called the synaptic cleft.
Such synapses are asymmetric both in structure and in how they operate. Only the so-called pre-synaptic neuron secretes the neurotransmitter, which binds to receptors facing into the synapse from the post-synaptic cell. The pre-synaptic nerve terminal generally buds from the tip of an axon, while the post-synaptic target surface typically appears on a dendrite, a cell body, or another part of a cell. The parts of synapses where neurotransmitter is released are called the active zones. At active zones the membranes of the two adjacent cells are held in close contact by cell adhesion proteins.
Note: There also exists a less elaborate form of junction called an electrical synapseAn electical synapse is a mechanical and electrically conductive link between two abutting neurons that is formed by proteins known as gap junctions. Each junction consists of proteins called connexins. Six connexins combine to form one channel called a c.
The release of neurotransmitter is triggered by the arrival of a nerve impulse (or action potential) and occurs through an unusually rapid process of cellular secretion: Within the pre-synaptic nerve terminal, vesicleIn cell biology, a vesicle is a relatively small and enclosed compartment, separated from the cytosol by at least one lipid bilayer. Vesicles store, transport, or digest cellular products and wastes. This biomembrane enclosing the vesicle is the same as ts containing neurotransmitter sit "docked" and ready at the synaptic membrane. The arriving action potential produces an influx of calcium ionsIn biology, second messengers are low-weight diffusible molecules that are used in signal transduction to relay a signal within a cell. They are synthesized or released by specific enzymatic reactions, usually as a result of an external signal that was re through voltage-dependent, calcium-selective ion channels, at which point the vesicles fuse with the membrane and release their contents to the outside. Receptors on the opposite side of the synaptic gap bind neurotransmitter molecules and respond by opening nearby ion channels in the post-synaptic cell membrane, causing ions to rush in or out and changing the local transmembrane potential of the cell. The result is excitatory, in the case of depolarizing currents, or inhibitory in the case of hyperpolarizing currents. Whether a synapse is excitatory or inhibitory depends on what type(s) of ion channel conduct the post-synaptic current, which in turn is a function of the type of receptors and neurotransmitter employed at the synapse.
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