The synapse is the functional connection between two neurons. The synapse is the place where the axonal termination of one nerve fiber form a close contact with the dendrite of the next nerve cell. In general pre-synaptic fiber divides up into numerous find branches which end in greatly expanded terminations called pre-synaptic knob. It makes contact with the post synaptic cell. Electrons microscopy has clearly shown that nerve cells at the synapse are separated by a synaptic cleft of several 100A degree wide. The transmission of impulse through synapse is called synaptic transmission.
When a nerve cell forms a synapse with a muscle cell then it is called neuro-muscular junction. The cells which send the impulse are called pre-synaptic cells while the cell which receives impulse is called post-synaptic cell. There are two theories regarding synaptic transmission.
Theory of Electrical Transmission: According to this theory though the neurons are separated but an effective local circuit connection is found which allows enough current to pass from one nerve to another nerve through sysnaptic cleft. This type of transmission is found in cary fish giant motor synapse.
Theory of Chemical Transmission: According to this theory as soon as the nerve impulse reaches at synapse it activates to secrete a transmitter substance from the tip of axon. This transmitter substance diffuses across the synaptic cleft and attaches to the special chemo-receptor on the surface of the dendride of the adjacent nerve cell. The combination of this chemical with chemo-receptor leads to a change in the permeability of membrane which depolarizes and sets up a new action potential. This action potential or nerve impulse passes along the length of the neuron to the next synapse where it stimulates to release again the transmitter substance. Chemical Transmission of the synapse involves two processes. They are as follows.
Neurosecretion: When a nerve impulse reaches the axonal termination substance is attached to the special molecular site in the dendrite and produces a change in the permeability of the cell membrane so that a new nerve impulse is set up.
Chemo-reception: The transmitter substance is attached to the special molecular site in the dendrite and produces a change in the permeability of the cell membrane so that a new impulse is set up.
The passage of chemical transmitter from axon to dendrite takes place by diffusion. It has been observed that acetyl choline is released by motor nerve. Calcium ions are essential for the release of acetyl choline while Mg++ inhibits it. Electron micro-graph of the tip of the neuron at the synapse showed the presence of tiny synaptic vesicles which are regarded to be the site of storage of transmitter substance.
Normally the transmission of the nerve impulse across the synapse in both sympathetic and para-sympathetic system takes place by release of transmitter substance. In para-sympathetic system acetyl choline is released and such fiber are called cholinergic fiber. When a nerve impulse arrives at the motor end plate, the acetyl choline is released and transmits impulse to the muscle fiber which starts contraction; after a brief contraction the nerve releases another chemical substance called acetyl choline esteraze which breaks acetyl choline and causes the muscle fiber to relax.
In the sympathetic system, sympathetin releases at the nerve ending and such fibers are known as adrenergic fiber because sympathin is similar to adrenalin hormone secreted by the medulla of adrenal gland.