Chemical synapse

A synapse is a microscopic physical junction either between two neurons or betwen a neuron and another "electrically excitable" cell (see action potential). By way of the synapse, which is an asymmetric and directional connection, an action potential or impulse in the "pre-synaptic" cell may trigger an impulse or lesser electrical response in the "post-synaptic" cell. The responsiveness of a post-synaptic cell to impulses in the pre-synaptic neuron is determined largely by the "strength" of the synapse. Neuroscientists describe this synaptic strength as "plastic," meaning that it is subject to change or regulation. One mechanism involves the simple association or coincidence of action potentials in the two cells. Consequently, researchers hypothesize that changes in synaptic strength underlie associative learning and memory (see Hebbian theory).

Generally, if a synapse is strong, an action potential in the pre-synaptic neuron will make the post-synaptic cell likewise "fire;" whereas a neuron coupled through a weak synapse will fail to elicit an action potential post-synaptically. Excitatory synapses between two neurons connect the tip or "terminus" of the pre-synaptic axon with a dendrite or with the cell body of the post-synaptic cell. In the brain, typically a single neuron connects or "synapses" to many post-synaptic cells and receives "inputs" via synapse from many other neurons. The coincident firing of many neurons that synapse on the same post-synaptic cell may elicit an action potential in that cell even if the individual synapses are weak.

While nerve impulses propagate electrically within neurons, across the synaptic gap these signals are conveyed by neurotransmitter molecules. An action potential arriving at the axon terminus of the pre-synaptic cell causes vesicles containing neurotransmitter to fuse with the cell membrane at the synapse, a process which releases their contents into the gap. Diffusing to the opposite side the gap, the molecules bind receptors that stimulate the flow of electric current into or out of the post-synaptic cell by opening ion channels. If enough current flows to bring the transmembrane potential in the post-synaptic cell to a certain threshold, an action potential is initiated. At a weak synapse, a rapid succession or "tetanus" of action potentials in the pre-synaptic neuron may be necessary to bring the post-synaptic cell to threshold.

Typically 90% of transmitter molecules are recovered rapidly by specialized membrane proteins. Called re-uptake, this process is the target of SSRIs.

The interaction area between an antigen presenting cell and lymphocyte by analogy is sometimes called an immunological synapse. This kind of synapse is formed between cells of the immune system to make specific antibodies directed against a certain antigen.