what is synaps

A synapse is a small gap at the end of a neuron that allows a signal to pass from one neuron to the next. It is the site of transmission of electric nerve impulses between two nerve cells (neurons) or between a neuron and a gland or muscle cell (effector) . Synapses most often form between axons and dendrites, which are the long, branching extensions of neurons that receive signals from other neurons. At a chemical synapse, each ending, or terminal, of a nerve fiber (presynaptic fiber) swells to form a knob-like structure that is separated from the fiber of an adjacent neuron, called a postsynaptic fiber, by a microscopic space called the synaptic cleft. The arrival of a nerve impulse at the presynaptic terminals causes the movement toward the presynaptic membrane of membrane-bound sacs, or synaptic vesicles, which fuse with the membrane and release a chemical substance called a neurotransmitter. This substance transmits the nerve impulse to the postsynaptic fiber by diffusing across the synaptic cleft and binding to receptor molecules on the postsynaptic membrane. The chemical binding action alters the shape of the receptors, initiating a series of reactions that open channel-shaped protein molecules. Electrically charged ions then flow through the channels into or out of the neuron. This sudden shift of electric charge across the postsynaptic membrane changes the electric polarization of the membrane, producing the postsynaptic potential, or PSP.