A class of nerve fibers as defined by their structure, specifically the nerve sheath arrangement. The AXONS of the myelinated nerve fibers are completely encased in a MYELIN SHEATH. They are fibers of relatively large and varied diameters. Their NEURAL CONDUCTION rates are faster than those of the unmyelinated nerve fibers (NERVE FIBERS, UNMYELINATED). Myelinated nerve fibers are present in somatic and autonomic nerves.

Type B fibers are the small myelinated fibers with a diameter up to 3 um and NEURAL CONDUCTION rates of 3-15 m/sec. They are mainly associated with the visceral autonomic nerves.

Type A fibers are the myelinated fibers ranging from 1-22 um in diameter with NEURAL CONDUCTION rates of 5-120 m/sec. They are the typical myelinated fibers of spinal nerves. Type A fibers are further subdivided into alpha, beta, gamma, and delta fibers.

Synonyms: Nerve Fiber, Myelinated, Fibers, Myelinated Nerve, Fibers, B, Myelinated Nerve Fibers, B Fiber, Fibers, A, Myelinated Nerve Fiber, A Fiber, B Fibers, Fiber, Myelinated Nerve, A Fibers, Fiber, B, Fiber, A, Nerve Fibers, Myelinated