Molecular dissection of the myelinated axon

The membrane of the myelinated axon expresses a rich repertoire of physiologically active molecules: (1) Voltagesensitive NA⁺ channels are clustered at high density (～1,000/μm²) in the nodal axon membrane and are present at lower density(<25/μm²) in the internodal axon membrane under the myelin. Na⁺ channels are also present within Schwann cell processes (in peripheral nerve) and perinodal astrocyte processes (in the central nervous system) which contact the Na⁺ channel–rich axon membrane at the node. In some demyelinated fibers, the bared (formerly internodal) axon membrane reorganizes and expresses a higher-than-normal Na⁺ channel density, providing a basis for restoration of conduction. The presence of glial cell processes, adjacent to foci of Na⁺ channels in immature and demyelinated axons, suggests that glial cells participate in the clustering of Na⁺ channels in the axon membrane. (2) “Fast” K⁺ channels, sensitive to 4-aminopyridine, are present in the paranodal of internodal axon membrane under the myelin; these channels may function to prevent reexcitation following action potentials, or participate in the generation of an internodal resting potential. (3) “Slow” K⁺ channels, sensitive to tetraethylammonium, are present in the nodal axon membrane and, in lower densities, in the internodal axon membrane; their activation produces a hyperpolarizing afterpotential which modulates repetitive firing. (4) The “inward rectifier” is activated by hyperpolarization. This channel is permeable to both Na⁺ and K⁺ ions and may modulate axonal excitability or participate in ionic reuptake following activity. (5) Na⁺/K⁺-ATPase and (6) Ca²⁺-ATPase are also present in the axon membrane and function to maintain transmembrane gradients of Na⁺, K⁺, and Ca²⁺. (7) A specialized antiporter molecule, the Na⁺/Ca²⁺ exchanger, is present in myelinated axons within central nervous system white matter. Following anoxia, the Na⁺/Ca²⁺ exchanger mediates an influx of Ca²⁺ which damages the axon. The molecular organization of the myelinated axon has important pathophysiological implications. Blockade of fast K⁺ channels and Na⁺/K⁺-ATPase improves action potential conduction in some demyelinated axons, and block of the Na⁺/Ca²⁺ exchanger protects white matter axons from anoxic injury. Modification of ion channels, pumps, and exchangers in myelinated fibers may thus provide an important therapeutic approach for a number of neurological disorders.