DAVID M. PHILLIPS / SCIENCE PHOTO LIBRARY DAVID M. PHILLIPS / SCIENCE PHOTO LIBRARY
Composite transmission electron micrographs (TEMs) showing unmyelinated nerve fibers (top) and myelinated ones (bottom). Peripheral nervous system nerves consist of axons enveloped in specialized metabolic and support cells (Schwann cells). Larger axons are wrapped in concentric layers of Schwann cell plasma membrane, the myelin sheath (in red). The rate of conduction of nerve impulses (action potential) is proportional to axon diameter. Myelination greatly increases axon conductivity velocity. Magnification = 25,000x at 5x7 inch print size.
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