B-Afferents: A fundamental division of the nervous system mediating homeostasis?

Abstract

The peripheral nervous system (PNS) has classically been separated into a somatic division composed of both afferent and efferent pathways and an autonomic division containing only efferents. J. N. Langley, who codified this asymmetrical plan at the beginning of the twentieth century, considered different afferents, including visceral ones, as candidates for inclusion in his concept of the “autonomic nervous system” (ANS), but he finally excluded all candidates for lack of any distinguishing histological markers. Langley's classification has been enormously influential in shaping modern ideas about both the structure and the function of the PNS. We survey recent information about the PNS and argue that many of the sensory neurons designated as “visceral” and “somatic” are in fact part of a histologically distinct group of afferents concerned primarily autonomic function. These afferents have traditionally been known as “small dark” neurons or B-neurons. In this target article we outline an association between autonomic and B-neurons based on ontogeny, cell phenotype, and functional relations, grouping them together as part of a common reflex system involved in homeostasis. This more parsimonious classification of the PNS, made possible by the identification of a group of afferents associated primarily with the ANS, avoids a number of confusions produced by the classical orientation. It may also have practical implications for an understanding of nociception, homeostatic reflexes, and the evolution of the nervous system.

Nociception and autonomic nervous system

“…This course of development supports the view that the smalldiameter afferents and lamina I constitute a cohesive homeostatic afferent system [6,8,9]. The ascending projections of lamina I project strongly to the sympathetic cell columns of the thoracolumbar spinal cord, thus forming a spino-spinal loop for somato-autonomic and visceraautonomic reflexes [10,11].…”

Pathobiological reactions of C‐fibre primary sensory neurones to peripheral nerve injury

“…Subsequent studies confirmed and extended these observations and provided evidence that the release of neuropeptides from stimulated endings of capsaicin-sensitive primary afferent neurones is involved in the mediation of antidromic vasodilatation and neurogenic inflammation (Lembeck & Holzer, 1979;Chahl, 1988 (Jancso, 1990 Jancso & Lynn, 1987;Holzer, 1988;Maggi & Meli, 1988;Prechtl & Powley, 1990). Several experimental protocols have been evolved on the basis of the selective neuroexcitatory and neurotoxic effects of capsaicin.…”

Evidence that antidromically stimulated vagal afferents activate inhibitory neurones innervating guinea‐pig trachealis.

“…Evidence for tachykinergic synaptic neurotransmission Tachykinins (substance P and neurokinin A in particular) are frequently associated with the capsaicin-sensitive afferents innervating the viscera (see Prechtl & Prowley, 1990 Petitet et al 1993;Ellis et al 1993), virtually abolish capsaicin-sensitive nerve-mediated relaxations of the trachealis. In any event, it seems unlikely that NK2 receptors are involved.…”