Synapse is the gap between the axon terminals of one neuron and the dendrite, or cell body, of another. It is at this cleft point that the electrochemical signals of one neuron, coming along its axon length, jumps the gap onto the other neuron's dendrite. The springboard for this signal jump is a chemical substance called neurotransmitter, which makes possible this synapse phenomenon. Synapse also exists between a neuron and a non-neuronal cell, such as those in muscles or glands.
The word synapse was first used at the end of the nineteenth century by the British neurophysiologist Charles Sherrington, who argued, on the basis of his own observations of reflex responses and the studies of the great Spanish anatomist, Ramón y Cajal, that a special form of transmission takes place at the contact between one nerve cell and the next.
Although most synapses connect axons to dendrites, there are also other types of connections, including axon-to-cell-body, axon-to-axon, and dendrite-to-dendrite. Synapses are generally too small to be recognizable using a light microscope. They can only be visualized clearly using an electron microscope.
Synapses send information-containing impulses directionally from a presynaptic neuron to a postsynaptic one. The presynaptic terminal, or synaptic button, is a specialized area within the axon of the presynaptic nerve cell which contains neurotransmitters enclosed in small membrane-bound spheres called synaptic vesicles. These synaptic vesicles are docked at the presynaptic plasma membrane at regions called active zones.
The synaptic process starts with a wave of electrochemical excitation called an action potential which travels along the membrane of the presynaptic cell, until it reaches the synaptic gap. The electrical depolarization of the membrane at the synapse causes channels to open that are permeable to calcium ions.
Calcium ions flow through the presynaptic membrane, rapidly increasing the calcium concentration in the interior. The high calcium concentration activates a set of calcium-sensitive proteins attached to vesicles that contain a neurotransmitter chemical. These proteins change shape, causing the membranes of some docked vesicles to fuse with the membrane of the presynaptic nerve cell, opening the vesicles and dumping their neurotransmitters into the synaptic cleft, the narrow space between the membranes of the pre- and post-synaptic cells.
The neurotransmitter diffuses within the cleft. Some of it escapes, but some of it binds to chemical receptor molecules located on the membrane of the postsynaptic neuron. The binding of neurotransmitter causes the receptor molecule to be activated in some way. Due to thermal shaking, neurotransmitter molecules eventually break loose from the receptors and drift away. The neurotransmitter is either reabsorbed by the presynaptic nerve cell, and then repackaged for future release, or else it is broken down metabolically.