Functional Classification of Afferent Subtypes

Much of the terminology used in the classification of visceral afferents has been translated from that used in the study of cutaneous sensation. These physiological classifications are based on afferent conduction velocities, which in turn, relate to axon diameter and the degree of myeli-nation and their responsiveness to mechanical and thermal stimuli (61). Cutaneous afferents can be subdivided into three classes based on conduction velocity alone: Ap fibers, AS fibers, and C-fibers. Each of these classes has subclasses of afferents based on mechanosensory responses. Large diameter myelinated Ap fibers can be subclassified into rapidly adapting mechanoreceptors, which respond exclusively to movement of the skin but not to static indentation and slowly, adapting mechanoreceptors, which respond to both (61). AS fibers have thin axons and a thin myelination and can be subclassified into either be low-threshold down hair (D-hair) mechanoreceptors, which have relatively large receptive fields, nociceptive neurons high-threshold (AM) mechanoreceptors (61,62). Small-diameter unmyelinated C-fibers can be subclassified into one of two classes. C-mechanonociceptors, have high mechanical thresholds and respond to mechanical but not thermal stimuli. Polymodal C-fibers that respond to mechanical and thermal stimuli are termed C-mechanoheat receptors. These C-fibers are designed to transmit exclusively noxious information in response to noxious stimuli (61,62). Thus cutaneous afferents have highly specific functions and as such different classes of sensory neurons carry information for distinct sensory modalities. By contrast, studies of visceral afferents throughout the gastrointestinal tract have demonstrated that conduction velocities are limited to either small diameter unmyelinated C-fibers or thinly myelinated AS fibers (35,36,63-68). However, visceral afferent fibers differ considerably in their basic physiological properties as they can signal normal functional events in addition to signaling pain in noxious environments (36). Moreover, studies of colonic afferents have shown little correlation between conduction velocities and functional properties either in the lumbar or in the sacral colonic afferents (67,69). In contrast to cutaneous afferents, visceral afferents lack a standardized nomenclature of afferent subclasses. As such, visceral afferents have been classified based on the layer of gut containing their receptive field, on the type of mechanical stimuli that they are responsive to, or their general response properties. However, the location of the receptive fields of the endings of their receptive field is crucial in determining their mechanical sensitivity and responsiveness to varying mechanical stimuli.

Combinations of in vivo and in vitro electrophysiological techniques have led to the identification and classification of three distinct patterns of afferent endings distributed within the wall of the gastrointestinal tract. Recent in vitro preparations have allowed manipulation of isolated afferent receptive fields resulting in a more controlled application of mechanical and chemical stimuli. Vagal and spinal afferents can be loosely divided into four classes: distension/tension sensitive, mucosal, serosal/mesenteric, and silent nociceptors.

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