Evolution of the mechanisms of connection between neurons: electrical, mixed, and chemical synapses.

Investigation of the mechanisms of transmission of stimuli in synapses of isolated perfused spinal cord of cyclostomes, amphibia, reptiles, and mammals demonstrated that the ratio between electrical and chemical synapses decreased progressively in favor of the latter in the transition from primitive toward more highly developed forms. Electrical transmission was not detected in synapses of spinal cords of reptiles and mammals. On the basis of the data, the result of analysis of elementary synaptic processes in synapses uniting electrical and chemical mechanisms of transmission and dendrodendrite electronic interdependences of the motor neurons, a hypothesis is formulated that the number of electrical connections characterizes the union of similar neurons, while in a sequential union of nerve cells of various functions and origins, there are mixed or chemical synapses. The possible cellular mechanisms which are the basis of this phenomenon are discussed.