NEURAL CONTROL OF SWALLOWING

Abstract

ABSTRACT BACKGROUND: Swallowing is a motor process with several discordances and a very difficult neurophysiological study. Maybe that is the reason for the scarcity of papers about it. OBJECTIVE: It is to describe the chewing neural control and oral bolus qualification. A review the cranial nerves involved with swallowing and their relationship with the brainstem, cerebellum, base nuclei and cortex was made. METHODS: From the reviewed literature including personal researches and new observations, a consistent and necessary revision of concepts was made, not rarely conflicting. RESULTS AND CONCLUSION: Five different possibilities of the swallowing oral phase are described: nutritional voluntary, primary cortical, semiautomatic, subsequent gulps, and spontaneous. In relation to the neural control of the swallowing pharyngeal phase, the stimulus that triggers the pharyngeal phase is not the pharyngeal contact produced by the bolus passage, but the pharyngeal pressure distension, with or without contents. In nutritional swallowing, food and pressure are transferred, but in the primary cortical oral phase, only pressure is transferred, and the pharyngeal response is similar. The pharyngeal phase incorporates, as its functional part, the oral phase dynamics already in course. The pharyngeal phase starts by action of the pharyngeal plexus, composed of the glossopharyngeal (IX), vagus (X) and accessory (XI) nerves, with involvement of the trigeminal (V), facial (VII), glossopharyngeal (IX) and the hypoglossal (XII) nerves. The cervical plexus (C1, C2) and the hypoglossal nerve on each side form the ansa cervicalis, from where a pathway of cervical origin goes to the geniohyoid muscle, which acts in the elevation of the hyoid-laryngeal complex. We also appraise the neural control of the swallowing esophageal phase. Besides other hypotheses, we consider that it is possible that the longitudinal and circular muscular layers of the esophagus display, respectively, long-pitch and short-pitch spiral fibers. This morphology, associated with the concept of energy preservation, allows us to admit that the contraction of the longitudinal layer, by having a long-pitch spiral arrangement, would be able to widen the esophagus, diminishing the resistance to the flow, probably also by opening of the gastroesophageal transition. In this way, the circular layer, with its short-pitch spiral fibers, would propel the food downwards by sequential contraction.