The Ranvier node as a chemo-electric pulsatory unit: a study of its structure-functions relations.

The Ranvier node of (Rana temporaria) frog nerve fibres is investigated by electron microscopy, Particular attention is given to the paranodal septate structures and to the extranodal junction of two Schwann cells. An interpretation of the functional meaning of these structures along with a quantitative analysis of the Schwann extranodal junction as regards the diffusion from/to the node is attempted. A 73 per cent reduction of the diffusion coefficient is obtained if the extranodal Schwann cell processes are considered impermeable to the diffusing vectors which indicates a protective role. Only 1 per cent reduction is obtained in the case of excitation-involved cations to which the Schwann cell membrane is considered to be permeable. This indicates the active role of the Schwann cell in the extranodal area ion diffusion, by minimizing the variations in ion concentration near the nodal membrane. Thus the nervous fibre-Schwann cell assembly may be regarded as a balanced pulsatory chemo-electric unit.