unmyelinierten

Unmyelinierten, or unmyelinated, describes nerve fibers that lack a myelin sheath. Myelin is a lipid-rich insulating layer produced by oligodendrocytes in the central nervous system (CNS) and by Schwann cells in the peripheral nervous system (PNS). It accelerates electrical signaling by enabling saltatory conduction, in which impulses jump between gaps called nodes of Ranvier. By contrast, unmyelinated fibers conduct impulses via continuous conduction, resulting in slower transmission.

Anatomically, many small-diameter axons in the PNS are unmyelinated. These axons are often associated with non-myelinating Schwann cells that envelop multiple fibers in a single Schwann cell membrane but do not form the layered myelin sheath. In the CNS, some axons remain unmyelinated or only sparsely myelinated, particularly within gray matter or certain tracts, while most long-range signaling pathways are myelinated.

Functionally, conduction velocity in unmyelinated fibers is typically lower than in myelinated fibers of comparable diameter. Small-diameter unmyelinated fibers, such as C fibers, mediate slower sensations like dull, diffuse pain, as well as some autonomic signals. Larger-diameter unmyelinated fibers, when present, still conduct more slowly than their myelinated counterparts.

Clinical relevance includes demyelinating diseases, in which loss of myelin slows or blocks conduction along formerly insulated fibers, leading to neurologic deficits. Unmyelinated fibers, though inherently slower, remain essential for a range of sensory and autonomic functions.