Myelin and Action Potential Propagation

Transcription

1 Myelin and Action Potential Propagation Matthew N Rasband, State University of New York, Stony Brook, New York, USA The high action potential conduction velocities achieved in some vertebrate axons are a consequence of myelin, an insulating sheath made by glial cells, and clustered sodium ion (Na 1 ) channels found at regularly spaced gaps in the myelin sheath. Introduction Vertebrates have solved the problem of how to successfully and rapidly transmit electrical signals over the immense distances between the neuronal cell body and the axon terminal, while minimizing space requirements, by the development of both myelin and heterogeneous ion channel localization. Myelin is a bimolecular leaflet of glyco- and phospholipids, sandwiched between two protein layers, that wraps an axon with numerous concentric lamellae. These layers of membrane are extensions of satellite cells called glia. Myelin acts like an insulating sheath, preventing the loss of current as the action potential propagates along the axon. Sodium ion (Na 1 ) channels, on the other hand, are found clustered at regularly spaced gaps in the myelin sheath, called nodes of Ranvier. These channels are responsible for the inward currents that allow for the generation and propagation of the action potential. Together, myelin and nodal Na 1 channels allow for conduction of electrical signals as fast as 120 m s 2 1, and discharge frequencies of hundreds of impulses per second. Wrapping of Glial Cells Around Axons Only certain kinds of glial cells are responsible for making myelin. Other, nonmyelinating glia, such as astrocytes and microglia, perform neuroimmune, nutritive, ionic buffering, developmental and structural roles. In the peripheral nervous system (PNS), myelin is made by Schwann cells. In contrast, oligodendrocytes elaborate myelin in the central nervous system (CNS). Myelination in the peripheral nervous system During early development of the PNS, Schwann cells proliferate and associate both with those axons that are destined to be myelinated and those that remain unmyelinated. Initially, all axons are enveloped by Schwann cells until completely surrounded. The glia continue to divide, and if an axon is to be myelinated, it is transferred to daughter Schwann cells until only one glial cell wraps a. Introduction Secondary article Article Contents. Wrapping of Glial Cells Around Axons. Glial Cell Membranes as Electrical Insulators Around Axons. Interruption of the Glial Cell Wrapping at Nodes of Ranvier. Clustering of Sodium Channels at the Nodes of Ranvier. Saltatory Conduction of Action Potentials from One Node to Another. Speed of Action Potential Propagation in Myelinated and Nonmyelinated Axons. Summary single axon or fibre. In contrast, if the axon is to remain unmyelinated, the Schwann cell still surrounds the axon, but a single Schwann cell may enclose many fibres (Figure 1a). Myelination commences after a one-to-one association has been achieved between the myelinating Schwann cells and axons, and as the latter reach a diameter of about 1 2 mm. At this point, Schwann cells begin to flatten, extend longitudinally along the length of the axon, and wrap the fibre with concentric, spiralling layers of cytoplasm (Figure 1a); the inner tongue of the Schwann cell process is responsible for the wrapping. Compact myelin is formed as the cytoplasm of each layer is gradually extruded, until adjacent cytoplasmic faces of the membrane are directly apposed one to another. This process continues until many layers of compact myelin are formed; large fibres may have as many as concentric wraps of myelin, although small fibres have as few as five. The longitudinal extension of Schwann cells also continues during myelin formation and compaction until adjacent myelinating Schwann cells are contacted. The narrow gap between the cells defines a new node of Ranvier. Each Schwann cell delineates a single internode, the segment between nodes of Ranvier, and the length of internodes may increase, both by the growth of the animal and by competitive elimination of some Schwann cells, such that the myelin sheath is continually modified during early development. In adult animals, the internodal length varies greatly, anywhere from 200 to 2000 mm, and is roughly 100 times the diameter of the fibre (Hildebrand and Johansson, 1991). Myelination in the central nervous system In contrast to Schwann cells, single oligodendrocytes in the CNS extend numerous thin cytoplasmic processes, each of which may contact a different axon, elongate, and then wrap and ensheathe their associated axons (Figure 1a). The most pronounced and obvious difference between central ENCYCLOPEDIA OF LIFE SCIENCES /&2001 Nature Publishing Group / 1

2 PNS CNS Unmyelinated Myelinated (a) PNS Juxtaparanode Kv1.1 Kv1.2 Kvβ2 Paranode Caspr Node NaCh Ankyrin-G NrCAM Neurofascin CNS (b) Figure 1 Myelination (a) and morphology of the node of Ranvier (b) in the peripheral nervous system (PNS) and central nervous system (CNS). (a) Cartoon of myelination shows that Schwann cells ensheathe unmyelinated axons in the PNS, but form myelin only after a one-to-one association has occurred. Oligodendrocytes myelinate numerous axons in the CNS, and form shorter internodal lengths than in the PNS. The box at the bottom of the axon contains a node of Ranvier and is shown at higher magnification in (b). (b) The myelin sheath is interrupted at regularly spaced nodes of Ranvier, where Na 1 channels are clustered in high density, and other molecules are discretely localized in the paranodal and juxtaparanodal subcellular zones. The top half of the figure shows a node of Ranvier in the PNS; the lower shows a node in the CNS. 2 ENCYCLOPEDIA OF LIFE SCIENCES /&2001 Nature Publishing Group /

3 and peripheral myelination is that a single oligodendrocyte myelinates as many as axons (Peters and Vaughn, 1970). Thus, myelinated segments from an oligodendrocyte can form distinct internodes that may be on the same or different axons. One pathophysiological consequence of this difference is that the loss or death of a single Schwann cell only affects a single axon, whereas the death of an oligodendrocyte is detrimental to conduction in numerous axons. In contrast to the PNS, myelin compaction occurs almost immediately after ensheathement. In general, however, the processes of myelination in both the CNS and PNS are very similar. This might also be expected, as the role of myelin, both from functional and structural perspectives, is identical. Glial Cell Membranes as Electrical Insulators Around Axons Electrophysiologically, the myelin sheath serves a dual role in support of action potential conduction and can be thought of as a string of parallel resistances and capacitances in series. First, the numerous lamellae of protein and lipid create a high resistance barrier to the flow of transverse current through internodal regions, and electrically isolate the internodal axonal membrane. Secondly, myelin decreases the internodal membrane capacitance, as the latter is a function of the distance between the two charge carriers (intracellular and extracellular spaces), thereby reducing the internodal capacitative current lost to membrane charging during action potential propagation. Measurement of the internodal resistance and capacitance of frog myelinated nerve fibres has shown that they are 8000 and times that for nodal resistance and capacitance, respectively (Aidley, 1991). As a result, the vast majority of the membrane current (both ionic and capacitative) in myelinated axons is seen only at nodes. Interruption of the Glial Cell Wrapping at Nodes of Ranvier The myelin sheath is interrupted at regularly spaced, highly specialized sites, called nodes of Ranvier, where a diverse population of ion channels and other proteins are clustered and localized to support rapid action potential conduction (Figure 1b). The structure of the node of Ranvier differs only slightly between CNS and PNS tissues. In particular, the Schwann cell has several outermost layers of myelin which do not terminate on the axonal membrane itself, but instead form finger-like projections that extend across the nodal gap and interdigitate with similar projections from the opposite Schwann cell. In the CNS, all myelin lamellae terminate in axoglial junctions adjacent to the node of Ranvier, leaving the node itself exposed to the extracellular space. The similarities and differences in nodal architecture are shown in Figure 1b, where the top half of the axon is shown myelinated by a Schwann cell, and the bottom half by an oligodendrocyte. On average, the length of the nodal gap in both CNS and PNS tissues is about 1 mm. Immediately adjacent to the node itself is a region called the paranode, where sequential layers of myelin terminate in specialized, closely apposed axoglial junctions. These junctions form a high resistance barrier to the flow of longitudinal currents in the narrow ( 15-nm) internodal periaxonal space between the axon and myelin, and may also serve as a structural barrier to the lateral diffusion of nodal and internodal molecules. Just beyond the innermost axoglial junction is another specialized region called the juxtaparanode, where, in large fibres, the axonal diameter increases and flutes out (Figure 1b). This region extends for approximately 5 20 mm into the internode, depending on the diameter of the fibre. The majority of the axonal membrane, beginning at the edge of the juxtaparanode, is found beneath myelin, and is called the internode. Molecular composition of the node of Ranvier The node of Ranvier represents one of the most highly specialized sites in the nervous system with respect to both its molecular and structural organization. Ion channels and adhesion molecules are organized in very discrete locations that correspond precisely with the previously described nodal, paranodal and juxtaparanodal structures. In particular, Na 1 channels, ankyrin-g (ankyrin-3; the cytoskeletal-binding protein thought to anchor Na 1 channels), and the ankyrin-g-binding cell adhesion molecules neurofascin and NrCAM are all found localized to the node (Davis et al., 1996). The cell adhesion molecule Caspr (paranodin) has been found in, and restricted to, the paranode and is an important component of paranodal axoglial junctions, although the proteins with which Caspr associate remain to be elucidated (Einheber et al., 1997). In mammals, the juxtaparanode is the exclusive domain of voltage-gated potassium ion (K 1 ) channels. Specifically, the Shaker-like Kv1.1 and Kv1.2 subunits, and the cytoplasmic Kvb2 subunit, form heteromultimeric K 1 channels at these sites (Rhodes et al., 1997). Despite structural and cellular differences in myelination between the CNS and PNS, the molecules found in nodal zones are conserved in these two neuronal tissues. Figures 2a and 2b show nodes of Ranvier from the PNS and CNS, respectively. Na 1 channels can be seen focally clustered between flanking zones of paranodal Caspr immunoreactivity. This entire nodal and paranodal apparatus is further bounded by juxtaparanodal K 1 channels. A pronounced feature that is readily apparent is the precise localization of each of these molecules to specific ultrastructural domains. ENCYCLOPEDIA OF LIFE SCIENCES /&2001 Nature Publishing Group / 3

4 the mechanisms whereby these high-density aggregates arise. Most experiments have shown that nodes of Ranvier have Na 1 channels in densities of several thousand channels per square micrometre. As a result, the nodal membrane is highly excitable and, upon depolarization, is able to conduct a substantial amount of current. How are these ion channels clustered in such highdensity aggregates and at regularly spaced sites, in some cases more than a metre from the neuronal cell body? As described above, during early development axons are initially devoid of any overlying myelin but, importantly, these same axons are able to support action potential propagation at very slow velocities because they have a uniformly distributed population of Na 1 channels (Waxman et al., 1989). Studies of myelination and ion channel clustering have shown that these two processes are intimately related. In particular, as myelination begins, and as Schwann cells or oligodendrocytes ensheathe axons and extend processes longitudinally along the length of axons, Na 1 channel clusters form at the very edges of the elongating cell. These channel aggregates migrate as the glial cells continue to myelinate and grow. Finally, as adjacent glial cells or processes come into close proximity, the Na 1 channel clusters fuse, and form a new node of Ranvier (Figure 2c). When myelination is pharmacologically, genetically or surgically disrupted, clusters do not form. Instead, broad, diffuse Na 1 channel distributions result. Thus, the formation of high-density aggregates at nodes of Ranvier is dependent on myelinating glial cells (Vabnick et al., 1996; Rasband et al., 1999). One other significant consequence of this clustering process is apparent in Figure 2c: Na 1 channels are markedly absent from internodal regions. Consequently, the internodal membrane lacks sufficient Na 1 channels to support action potential conduction alone, and if myelin is experimentally removed, conduction fails. The specific neuronal and glial molecular mechanisms and events that occur to cause clustering of node-specific molecules, and that target these essential components of conduction to the axon and the node of Ranvier, have not yet been determined and are an active area of investigation. Figure 2 The discrete molecular organization of ion channels in the rat peripheral and central nervous systems. (a, b) Nodes of Ranvier in the peripheral and central nervous systems, respectively, labelled for Na 1 channels (green), Caspr (red) and Kv1.2 K 1 channel a subunits (blue). (c) Four myelinated axons from the peripheral nervous system, visualized using Hoffman optics and immunofluorescence, two of which have Na 1 channels clustered in the nodal gap (green). Bars, 10 mm. Clustering of Sodium Channels at the Nodes of Ranvier Nodal Na 1 channels have been the subject of many studies to determine the density of channels, their function, and Saltatory Conduction of Action Potentials from One Node to Another How do clustered, nodal Na 1 channels and myelin work together to facilitate action potential conduction? As described above, several factors inhibit the flow of transmembrane currents in internodal regions. First, the myelin sheath forms a high-resistance, low-capacitance, barrier. Second, regions covered by myelin have a very low density of Na 1 channels. And third, the paranodal axoglial junctions and the small periaxonal space make continuous conduction through the internode difficult 4 ENCYCLOPEDIA OF LIFE SCIENCES /&2001 Nature Publishing Group /

5 because of the very high external resistance through the periaxonal space. Consequently, axoplasmic currents are dissipated minimally in internodal regions. When a depolarizing action potential is sufficient to excite a node of Ranvier, the currents generated propagate down the low resistance axoplasm until the next node is reached, depolarizing it so that it too becomes excited, allowing for inward Na + currents that restore the amplitude of the action potential and cause currents to continue to the next node. This process continues as node after node is depolarized and the action potential propagates down the length of the axon (Figure 3). Since all transmembrane currents occur at nodes of Ranvier, the site of active excitation appears to jump or skip from node to node. This type of action potential propagation is called saltatory conduction, from the latin word saltare, to jump. Figure 3 shows that since there is very little internodal membrane capacitance, the charge is found primarily at nodes of Ranvier, and as positive charge travels along the axon, nodes are activated one after the other, allowing for the action potential to propagate, with the currents themselves travelling through the axoplasm and the extracellular space. Further, were the transmembrane currents to be recorded at both nodal and internodal locations, currents would be detected only at nodes, as shown at the bottom of Figure 3. Speed of Action Potential Propagation in Myelinated and Nonmyelinated Axons Experiments to measure the conduction velocities in myelinated nerves have shown a wide range of values. For example, in the mammalian PNS, myelinated sensory fibres may vary in diameter from 1 to 20 mm, and correspondingly have conduction velocities from 4.5 to 120 m s 2 1. Similarly, the action potential velocity in small (1-mm) CNS myelinated axons are as high as 12 m s 2 1 (Rasband et al., 1999). In contrast, the very small unmyelinated axons in the PNS that conduct action potentials in response to nociceptive stimuli (pain), also known as C fibres, have conduction velocities that range from only 0.5 to 3.0 m s 2 1 (Paintal, 1978). Since signal propagation depends on the ability of the action potential to depolarize consecutive nodes, the conduction velocity must be dependent on the distance between nodes. Further, since the internodal length is directly proportional to the diameter of the axon, the conduction velocity is also proportional to the diameter. In contrast, both empirical measurement and mathematical models of conduction in unmyelinated axons have shown that the speed of action potential propagation is proportional to the square root of the axon diameter. Although myelin s most noticeable contribution is an increase in the conduction velocity, there are several Direction of propagation Figure 3 Saltatory conduction in myelinated axons. Transmembrane currents appear to jump from node to node, as each depolarizing action potential causes the next node to be excited (top). (1,2 ) show the relative charges at each point along the axon during action potential propagation. Further, were transmembrane currents to be recorded at each of the sites indicated by the green arrows, they would be detected only at nodes of Ranvier (bottom). ENCYCLOPEDIA OF LIFE SCIENCES /&2001 Nature Publishing Group / 5

6 additional important benefits of saltatory conduction. First, since little charge is lost through internodal transmembrane or capacitative currents, and the only site of current loss is the node itself, the metabolic energy required to recover excitability is reduced dramatically. Second, the conduction velocity in myelinated axons is roughly 10 times that of unmyelinated axons of equivalent size. Therefore, since the conduction velocity in unmyelinated axons is proportional to the square root of the axonal diameter, the number of myelinated fibres that can be used in the same volume of space required by an unmyelinated axon that conducts with an equivalent velocity increases by 100 times. As a result, myelination increases the speed of action potential propagation, and decreases the space required to achieve the increase in conduction velocity. Finally, myelinated nerve fibres are able to conduct trains of action potentials at higher frequencies than unmyelinated fibres, as the frequency depends largely on the conduction velocity. Summary It is difficult to overstate the impact that myelination has on action potential conduction and propagation. Myelin functions not only to decrease the internodal transmembrane currents, but also to direct the incredibly precise and regulated molecular organization of the membrane, such that Na 1 channels and other important molecules are clustered at, or near, regularly spaced nodes of Ranvier. Consequently, the formation of myelin by glial cells and the establishment of heterogeneous ion channel distributions allows efficient, faithful and rapid conduction velocities in the vertebrate nervous system. However, since myelin is so important, demyelinating diseases, like multiple sclerosis and Guillain Barré syndrome, or injuries are accompanied by failure of action potential conduction. Efforts to find therapies and drugs to either repair demyelinated axons, or restore conduction through modulation of ion channels, have met with some limited success and may promise a day when clinical intervention can treat these debilitating diseases. References Aidley DJ (1991) The Physiology of Excitable Cells, 3rd edn. Cambridge: Cambridge University Press. Davis JQ, Lambert S and Bennett V (1996) Molecular composition of the node of Ranvier: identification of ankyrin-binding cell adhesion molecules neurofascin (mucin/third FNIII domain) and NrCAM at nodal axon segments. Journal of Cell Biology 135: Einheber S, Zanazzi G, Ching W et al. (1997) The axonal membrane protein Caspr, a homologue of neurexin IV, is a component of the septate-like paranodal junctions that assemble during myelination. Journal of Cell Biology 139: Hildebrand C and Johansson CS (1991) Nodal spacing in the developing, young adult and ageing rat inferior alveolar nerve. Brain Research. Developmental Brain Research 64: Paintal AS (1978) Conduction properties of normal peripheral mammalian axons. In: Waxman SG (ed.) Physiology andpathobiology of Axons, pp New York: Raven Press. Peters A and Vaughn JE (1970) Morphology and development of the myelin sheath. In: Davison AN and Peters A (eds) Myelination,pp3 79. Springfield, IL: Thomas. Rasband MN, Peles E, Trimmer JS et al. (1999) Dependence of nodal sodium channel clustering on paranodal axoglial contact in the developing CNS. Journal of Neuroscience 19: Rhodes KJ, Strassle BW, Monaghan MM et al. (1997) Association and colocalization of the Kvbeta1 and Kvbeta2 beta-subunits with Kv1 alpha-subunits in mammalian brain K channel complexes. Journal of Neuroscience 17: VabnickI, Novakovic SD, Levinson SR, Schachner M and Shrager P (1996) The clustering of axonal sodium channels during development of the peripheral nervous system. Journal of Neuroscience 16: Waxman SG, BlackJA, Kocsis JD and Ritchie JM (1989) Low density of sodium channels supports action potential conduction in axons of neonatal rat optic nerve. Proceedings of the National Academy of Sciences of the USA 86: Further Reading Morell P (ed.) (1984) Myelin, 2nd edn. New York: Plenum Press. Peters A, Palay SL and Webster HD (1976) The Fine Structure of the Nervous System: The Neurons andsupporting Cells. Philadelphia: WB Saunders. VabnickI and Shrager P (1998) Ion channel redistribution and function during development of the myelinated axon. Journal of Neurobiology 37: Waxman SG and Ritchie JM (1993) Molecular dissection of the myelinated axon. Annals of Neurology 33: Zagoren JC and Fedoroff S (eds) (1984) The Node of Ranvier. Orlando: Academic Press. 6 ENCYCLOPEDIA OF LIFE SCIENCES /&2001 Nature Publishing Group /