Transmission electron micrograph (TEM) of a longitudinal section of an axon with many microtubules. Axons are easily recognized by their content of smooth endoplasmic reticulum and microtubules. Peripheral nervous system nerves consist of axons enveloped in specialized metabolic and support cells called 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: x100,000 when printed at 5x7".