The relations between two neurons are of the nature of contact rather than complete structural continuity. This junction of two neurons is called a synapse. The synapse may be defined as a point of contact of the cell membrane (plasma membrane) of an axon terminal with the cell membrane of another neuron. The synapse may be between the axon termination of one neuron and the cell body of another (axo-somatic synapse) or it may be between the axon terminals and the dendrites of another neuron (axo-dendritic synapse). In the CNS, interwoven telodendria of Axon and dentrite arborization forms a dense feltwork of fibres which termed neuropli.
- The axon terminals of a single axon may from synapses with both the body and dendrites of another neuron. Various methods of contact occur among which are;
- by neuropodia or terminal boutons,
- by gemmules,
- by parallel opposition.
- In the first method the axon terminals end in small bulb like expansions or neuropodium consists of neurofibrillae. Loops embedded in perifibrillar substance, some times there are simply small neurofibrillar rings. In the gemmule type of synapsis axon processes running at right angles to the dendrites of another neuron appear to come in contact with their gemmules.
- In the third type the axon terminal comes into length wise opposition with the dendrite cell body or in some cases, the amyelinated portion of the axon of the receiving neuron. It is obvious that one axon may carry impulses to a number of neurons and that each neuron may receive impulses from the axons of a number of other neurons.
