The Node of Ranvier in the Central Nervous System

The axolemma of myelinated nerve fibres of the central nervous system has been examined both at nodes and in the paranodal region. In the paranodal region, the outside of the axolemma is separated from the outer surface of the plasma membrane bounding the inner aspect of the paranodal cytoplasm by a distance of only 25–30 A, so that the two membranes form a complex. In longitudinal sections, a periodicity may be observed within this complex. It has been shown that this is mainly due to the form of the outer dense layer of the axolemma, which is modified into a series of bands or rings that circumscribe the axon. Each band is 120–150 A wide and the centre to centre spacing is 250–300 A. At the node, the axolemma does not show a periodicity and is not covered by a myelin sheath, but in the axoplasm beneath it there is a layer of osmiophilic granular material, about 250 A thick.