Synapse is the site of nerve impulse transmission from one neuron to another, usually from the axon terminal endings to the dendrite of another nerve cell. The site is not a mere physical contact point but a functional contact between excitable nerve cells. This site or junction is a microscopic space, which is called the synaptic cleft between the teledendrons of a neuron axon and the dendritic branches of another nerve cell as the electrochemical nerve impulse, or jump across the gap, is always from axon to dendrite (or nerve cell body in some cases).
There are two types of synapses: excitatory or inhibitory, depending on whether they activate or suppress the activity of a given cell. In either case, transmission through a synapse may be effected by means of a chemical or electrical mechanism. There are also mixed synapses, which combine chemical and electrical transmission mechanisms. Most commonly found are synapses with chemical mechanisms, in which signals are transmitted from pre-synaptic to postsynaptic membranes by means of mediators — chemical compounds whose molecules are capable of reacting with the specific receptors of a postsynaptic membrane. Mediators change the permeability of a postsynaptic membrane to ions, generating a local, nonregenerative potential. In an electrical synapse the current from an activated presynaptic membrane acts directly on a postsynaptic membrane.
Synapses with chemical and electrical transmission mechanisms are characterized by specific structural features. In synapses with the chemical transmission mechanisms, the presynaptic ending includes synaptic vesicles, which contain high concentrations of a mediator called neurotransmitter. Presynaptic and postsynaptic membranes are separated by a synaptic cleft, which is usually 150–200 angstroms (Å) in width. Synaptic vesicles have a tendency to become concentrated at the internal surface of a pre-synaptic membrane, opposite a synaptic cleft. They may emerge from a synapse’s presynaptic ending at breaks in the membrane, penetrate the synaptic cleft, and make contact with the postsynaptic membrane. The arrangement and number of synaptic vesicles vary as a result of nervous activity.
Excitatory or inhibitory effects in a synapse with a chemical reaction are produced by a series of processes. The nerve impulse, upon arriving at a presynaptic ending, depolarizes the presynaptic membrane, whose permeability to calcium ions is increased. The entry of calcium ions into a synapse’s presynaptic ending releases the neurotransmitter, which diffuses through the synaptic cleft and reacts with the receptors of the postsynaptic membrane. This reaction usually results in an increase in the permeability of the postsynaptic membrane to one or more ions and to the generation of a postsynaptic potential. In excitatory synapses sodium conductivity increases, sometimes simultaneously with potassium conductivity, which leads to the depolarization and excitation of a postsynaptic cell. In inhibitory synapses the permeability of the postsynaptic membrane to chloride ions increases, sometimes simultaneously with potassium-ion permeability; this effect is usually accompanied by hyper-polarization.
In electrotonic synapses channels permit the molecules of low-molecular-weight compounds to pass from the cytoplasm of one cell to the cytoplasm of another. The channels do not communicate with the extracellular space and are absent in other areas of the membrane. Most neural activity can be effected by means of chemical and electrotonic synapses. Electrotonic synapses ensure the rapidity and stability of transmission and are less sensitive to fluctuations in temperature. A chemical mechanism more reliably ensures unidirectional conduction and makes it possible to change the effectiveness of a synapse as a result of preceding activity.
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| Above, a schematic picture of a synapse between axon teledendron (terminal branch) and dendrite. |
Synapse animation (video)