AFasern

AFasern, or A-fibers, are a subset of nerve fibers in the peripheral nervous system characterized by large diameter and heavy myelination. Their myelin and size enable fast conduction of action potentials, allowing rapid transmission of both sensory and motor information. AFasern are subdivided by diameter and conduction velocity into Aα, Aβ and Aδ fibers.

Aα fibers are the largest and fastest, typically conducting at about 80–120 m/s. They include alpha motor neurons that innervate skeletal muscle fibers and Ia proprioceptive afferents from muscle spindles, playing a key role in proprioception and motor control. Aβ fibers are slightly smaller and conduct at roughly 35–75 m/s. They carry discriminative touch and proprioceptive information from mechanoreceptors such as Merkel discs, Meissner corpuscles, Pacinian corpuscles, and Ruffini endings. Aδ fibers are smaller still and conduct at about 5–30 m/s, transporting fast pain and temperature signals, as well as some light touch.

Anatomically, AFasern are myelinated by Schwann cells, and their impulses travel via saltatory conduction between nodes of Ranvier. Functionally, they contribute to rapid reflexes, precise touch perception, and proprioceptive feedback that guides movement. Clinically, damage or demyelination of A-fibers can impair touch, position sense, and fast pain perception, and nerve conduction studies often help assess their integrity. AFasern contrast with smaller, unmyelinated C-fibers and lightly myelinated B-fibers, which conduct more slowly and subserve other sensory and autonomic functions.