PERSPECTIVES Myelin—More than Insulation NEUROSCIENCE

Unusual features of myelin in the mammalian cerebral cortex permit more complex forms of network integration. M yelin is often compared to elec-trical insulation on nerve fi bers. However, nerve impulses are not transmitted through neuronal axons the way electrons are conducted through a copper wire, and the myelin sheath is far more than an insulator. Myelin fundamentally changes the way neural impulses (information) are generated and transmitted, and its damage causes dysfunction in many nervous system disorders including multiple sclerosis, cere-bral palsy, stroke, spinal cord injury, and cognitive impairments. A detailed under-standing of myelin structure is therefore imperative, but is lacking. On page 319 of this issue, Tomassy et al. ( 1) provide a high-resolution global view of myelin structure spanning the six layers of mammalian cere-bral cortex. The fi ndings are likely to spark new concepts about how information is transmitted and integrated in the brain. New techniques of automating col-lection of electron microscopic images taken in series through layers of tissue are becoming available to analyze neuron ultrastructure in large volumes ( 2). Using such methods, Tomassy et al. reveal myelin structure in the mouse cerebral cortex along individual nerve fi bers, providing a coherent picture. Myelin is a coating of compacted cell membrane that is wrapped around the axon by non-neuronal cells called oligodendro-cytes. These multipolar cells extend slender cellular processes to grip axons and spin up to dozens of layers of membrane around it like electrical tape. Many oligodendrocytes grasp a single axon to span its full length. The tiny space exposed between each grasp-ing “hand ” corresponds to a node of Ran-vier, where voltage-gated sodium channels are concentrated. When the electrical poten-tial across the axon membrane depolarizes by about 20 mV, these channels allow rapid influx of sodium ions that discharges the transmembrane potential, creating a volt-age transient of ~0.1 V—the action poten