1. An axon. Nerve fibers are identified by intrinsic characteristics (such as diameter, presence of myelin, or neurotransmitter type) or by extrinsic characteristics, such as whether they conduct sensory or motor information.

   SEE: axon.

2. An axon or a small group of axons bundled together by glia or Schwann cells.
3. In informal usage, a small peripheral nerve.

Nerve fiber, or simply fiber is more often used in anatomical descriptions, and fiber tract is more commonly used than axon tract. Axon is more commonly used in histological descriptions.

A-delta n.f.A lightly myelinated sensory fiber and nociceptor that relays sudden, sharp, fast pain.

adrenergic n.f.A nerve fiber that releases norepinephrine at its synapse when an impulse is transmitted. Most postganglionic fibers of the sympathetic division are adrenergic.

B n.f.A myelinated nerve fiber that transmits autonomic impulses.

C n.f.An unmyelinated nerve fiber that slowly relays sensations of pain from the periphery or viscera to the central nervous system.

cholinergic n.f.A nerve fiber that releases acetylcholine at its synapse when an impulse is transmitted. Sympathetic and parasympathetic preganglionic fibers, parasympathetic postganglionic fibers, and efferent somatic fibers ending in skeletal muscle are cholinergic.

collateral n.f.An axonal branch.

medullated n.f.Myelinated nerve fiber.

myelinated n.f.A nerve fiber with a myelin sheath. SYN: medullated nerve fiber.

nonmyelinated n.f.A nerve fiber without a myelin sheath.

postganglionic n.f.A nerve fiber of the autonomic nervous system that terminates in smooth or cardiac muscle or in a gland . Its cell body lies in an autonomic ganglion.

preganglionic n.f.A nerve fiber of the autonomic nervous system that terminates and synapses in one of the autonomic ganglia. Its cell body lies in the brain or spinal cord.