Transmission electron micrograph (TEM) of myelinated nerve cell and Schwann cell showing outer (blue) and inner (light blue) mesaxon in cat. 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). The rate of conduction of nerve impulses (action potential) is proportional to axon diameter. Myelination greatly increases axon conductivity velocity. Magnification: 25,000x when printed at 5x7".