1. An axon. Nerve fibers are identified by intrinsic characteristics (e.g., diameter, presence of myelin, or neurotransmitter type) or by extrinsic characteristics, e.g., whether they conduct sensory or motor information (e.g., a myelinated, large-diameter, cholinergic motor fiber).
“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.

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

A delta nerve fiber

A myelinated sensory fiber that rapidly relays painful sensations to the central nervous system.

adrenergic nerve fiber

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

association nerve fiber

A nerve fiber that connects one region of the cerebral cortex with another region in the same hemisphere.

B nerve fiber

A myelinated nerve fiber that transmits autonomic impulses.

C nerve fiber

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

cholinergic nerve fiber

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.

climbing nerve fibers of the cerebellum

1. Afferent nerve fibers entering the cortex and synapsing with dendrites of Purkinje cells.
SYN: fiber, mossy
2. Collateral branches of Purkinje cell axons that return to the molecular layer terminating about Purkinje or basket cell dendrites.

collateral nerve fiber

A small branch extending at a right angle from an axon.

commissural nerve fiber

A nerve fiber that passes from one cerebral hemisphere to the other.

decussating nerve fiber

An axon that crosses from one side of the central nervous system to the other.

medullated nerve fiber

SEE: Myelinated nerve fiber.

myelinated nerve fiber

A nerve fiber with a myelin sheath.
SYN: medullated nerve fiber

nonmyelinated nerve fiber

A nerve fiber without a myelin sheath.

postganglionic nerve fiber

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 nerve fiber

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.

projection nerve fiber

1. A nerve fiber arising in the diencephalon and passing to the cerebral cortex.
2. A nerve fiber arising in the cerebral cortex and terminating in lower portions of the brain or in the spinal cord.