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.NODES of Ranvier occur at intervals along the myelinated nerve fibres of both the peripheral and central nervous systems. They occur at the junctions between adjacent lengths of myelin, so that when sections of central nervous tissue are stained specifically to show the myelin and then examined with the light microscope [Hess and Young, 1952;Allison and Feindel, 1949], the nodes of Ranvier appear as short segments from which the myelin is absent.Any doubts that previously existed about the presence of nodes of Ranvier in the central nervous system were finally dispelled by electron microscope pictures of these structures [Metuzals, 1960; b; Uzman and Villegas, 1960]. These electron microscope studies clearly showed that as it approaches a node, the myelin sheath becomes thinner, the spiralled lamellae of the sheath [Maturana, 1960; a] terminating in such a way that each succeeding turn of the spiral overlaps, and projects beyond, the lamella lying inside it. In effect then, the lamellae terminate in a helical fashion. As they do so, the lamellae open up along the major dense line to enclose cytoplasm, the paranodal cytoplasm which, because of the mode of termination of the lamellao, has itself the form of a helix [Peters, 1960 b]. Consequently, in longitudinal sections through the paranodal region the appearance is given of a series of pockets of cytoplasm, lying on each side of the axon and separating it from the gradually thinning sheath ( fig. 1).Recently, it has become apparent that the unmyelinated portion of the axon at a node is surrounded by an extracellular space that in sections is somewhat larger than that occuring elsewhere within the central nervous system [Metuzals, 1965;Robertson, Bodenheimer and Stage, 1963], but little attention has been paid to the characteristics of the plasma membrane of the axon, the axolemma, at the node. This will be described in the present 229