December (Final Notebook.2)

“reflex circuits.” Each neural circuit contains an afferent neuron that carries electrical activity from the periphery to a center of integration, either a ganglion or a specialized region of the brain. After afferent signals are processed, information flows back to the periphery via motoneurons to modify behavior. Thus afferent neurons were thought to be the sensors of the digestive state. Since the beginning of this century, it has been known that visceral nerves, including those supplying the gastrointestinal tract, contain afferent fibers that transmit information from the viscera to the central nervous system. Although early studies used sensation arising from the viscera as a means to study the afferent innervation, Ranson (11) noted that “the majority of the afferent impulses from the viscera never rise to the level of consciousness at all but expend themselves in the production of reflexes.” For example, placing acid in the intestine inhibits emptying of the stomach by activating a neural reflex without any conscious sensation. It is now well recognized that a number of mechanical and chemical stimuli applied to the lining or mucosa of the small intestine can exert feedback inhibition of both secretion and muscle contraction of the gastrointestinal tract. The importance of these regulatory mechanisms is to match the digestive and absorptive capacity of the intestine with the entry of nutrients from the stomach and to modify food intake. Despite nearly 50 years of neurophysiological recordings from afferent nerve fibers innervating the gastrointestinal tract, the transduction mechanisms in the nerve terminals responding to mechanical and chemical stimuli are largely unknown (7). These terminals are not readily identifiable, since they are unmyelinated, unencapsulated, and lacking in morphological specialization. Functional specificity therefore occurs in the absence of gross morphological differentiation. Gastrointestinal afferents are classified according to their presumed location in the gut wall. Thus mucosal afferents respond to mechanical and chemical stimuli applied to the mucosa or epithelial lining, and their activity is lost if the mucosa is perfused with local anesthetic or is stripped from the overlying muscle layers. Receptors with terminal fields in the smooth muscle generally respond to changes in tension, generated by either active contraction or passive distension of the muscle.