Bfibres

Bfibres, or B fibers, are a class of peripheral nerve fibers that are small in diameter and myelinated. They are among the thinly myelinated fibers that carry autonomic nervous system signals.

Anatomy and conduction: B-fibres are among the smallest myelinated fibers, typically with diameters around 1–3 micrometers. Their light myelination results in relatively slow conduction, approximately 3–15 meters per second, compared with larger myelinated fibers. In the common A–V fiber classification, B-fibres are identified as preganglionic autonomic fibers.

Function and distribution: B-fibres primarily convey preganglionic autonomic signals within both the sympathetic and parasympathetic divisions of the autonomic nervous system. They originate in the spinal cord or brainstem and project to autonomic ganglia, where they synapse with postganglionic neurons. Through these pathways, B-fibres influence functions of visceral organs, glands, and vascular tone, contributing to involuntary processes such as heart rate, digestion, and airway regulation.

Clinical relevance: In clinical neurophysiology, B-fibres are relevant for assessments of autonomic function and in the context of nerve blocks or injuries. Because they are small and myelinated, they can be selectively affected in certain neuropathies and during pharmacological anesthesia, and their function can be indicative of autonomic integrity.

Terminology: The designation B-fibre comes from historical classifications of nerve fiber types. In modern literature, they are described as small, myelinated preganglionic autonomic fibers involved in autonomic regulation.