The relationship between the mode of operation and the dimensions of the junctional regions at synapses and motor end-organs

Abstract

An attempt has been made to correlate the dimensions of synapses as revealed by electron-microscopy with their functional operation. In particular the problems under consideration have concerned the synaptic cleft, which is the narrow space, 200 to 500 Å across, that separates the pre-synaptic from the sub-synaptic membrane. The synaptic transmitter substance is liberated from the pre-synaptic membrane into this space. By making simplifying assumptions it has been possible to derive equations relating to the elimination of transmitter substance from this space by diffusion into the surrounding medium. These equations apply to two types of synaptic contact, the long strip-like and the circular knob-like. It is shown that with some synapses the time course of this elimination is satisfactorily described by these equations for both geometrical types of synapses. For both geometrical types of synapses equations have also been derived that enable their efficiencies to be assessed in respect of the flow of electric current through the synaptic cleft and so to the whole membrane of the post-synaptic structure. Essentially synapses are devices for causing the flow of these currents, and they appear to be very efficiently designed for this purpose. Despite their narrowness the synaptic clefts probably offer a negligible resistance to the flow of these post-synaptic currents. There is a general discussion on the efficiency of design of synaptic structures of diverse types.