The Node of Ranvier in the Central Nervous System

Abstract

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

Electron Microscopy of the Mouse Optic Nerve: A Quantitative Study of the Total Optic Nerve Fibers

“…However, they regarded these axons as myelinated axons which were cut at the node of Ranvier, because of the presence of electron dense granular material beneath the axolemma, which was thought to be a characteristic of an axon at a node of Ranvier by PETERS (1966). According to the present observation, the Table 1.…”

Morphogenesis of the Node of Ranvier: Co-Clusters of Ankyrin and Ankyrin-Binding Integral Proteins Define Early Developmental Intermediates

“…In addition, other ankyrin-binding proteins are also localized at the node, including isoforms of neurofascin and NrCAM (Davis et al, 1993(Davis et al, , 1996, ankyrin-binding cell adhesion molecules related to L1 (for review, see Hortsch, 1996). Ankyrin itself is localized at the 1-to 2-m-size nodal axonal plasma membrane (Kordeli et al, 1990) and is a component of the electron dense undercoating observed beneath the nodal membrane (Peters, 1966;Ichimura and Ellisman, 1991). Ankyrins are a family of spectrin-binding proteins that associate with diverse integral proteins, including ion channels, calcium-release channels, and cell adhesion molecules, and link these proteins to the spectrin-based membrane skeleton (for review, see Bennett and Gilligan, 1993;Lambert and Bennett, 1993).…”

Structural complexes in the squid giant axon membrane sensitive to ionic concentrations and cardiac glycosides

“…In this way, the structural complexes may be related to the so-called gap junctions (18) which represent the morphological correlates for the low resistance junctions that serve as sites of electrotonic coupling between cells (14). The existence, at the surfaces of the apposed membranes, of a lattice formed by globules or particles closely packed in a hexagonal array is, at present, the well-known ultrastructural feature of the gap junction (17,18). These junctions have also been pointed out in nerve tissue of vertebrates and invertebrates (15,23).…”

“…These postsynaptic densities have been related to the sites where permeability changes occur (4). The opaque axoplasmic material of the nodes of Ranvier has been thought to be related to specialized zones of membrane permeability (5), to areas of activity (17), or to diffusion barriers (I). A dense layer of fine granular material undercoating the plasma membrane of the axon initial segment, resembling that found at the node of Ranvier, has also been described in different neurons (13,22).…”

“…The axolemma thickenings have been compared to the postsynaptic density (7,12), as well as to the opaque axoplasmic material underlining the nodes of Ranvier of the peripheral and central nervous systems (1,5,17). These postsynaptic densities have been related to the sites where permeability changes occur (4).…”