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Mittal RK. Motor Function of the Pharynx, Esophagus, and its Sphincters. San Rafael (CA): Morgan & Claypool Life Sciences; 2011.

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Motor Function of the Pharynx, Esophagus, and its Sphincters.

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Central Program Generator and Brain Stem

A swallow can be induced by mechanical stimulation of the pharynx or electrical stimulation of the superior laryngeal nerve (SLN) in decerebrate animals [1,2]. Events elicited by such stimulation show sequential contractions in the pharynx and esophagus along with relaxation of sphincters, similar to what one observes with a spontaneous swallow. Above implies that a programmed set of impulses that coordinate swallow-related events must emanate from the brain stem, often referred to as the central program generator (CPG) [3] or swallow program generator (SPG) [4]. It is not to say that the supramedullary sites cannot or do not influence the CPG [5,6]. Detailed discussion of the CPG can be found in several reviews [3,7]. Briefly, the critical elements of CPG are nucleus tractus solitarius (NTS), adjacent reticular formation, nucleus ambiguus and dorsomotor nucleus of vagus (Figure 1). All of these nuclei are not very well demarcated and have been studied in detail in mice, rat, cat, and sheep. The neurons of these nuclei, in addition to swallow function are also involved in respiration, cardiovascular reflexes, and possibly other functions. There is topographical representation of sensory input into the NTS such that the dense labeling of pharyngeal and laryngeal fibers is found in the intermediate and interstitial subnuclei and esophageal afferent labeling is observed primarily in the central subnucleus of NTS [8]. Local medullary and ascending supramedullary projections connect with the NTS. Local medullary connections either directly or through the reticular formation project to the premotor nucleus of vagus. In addition, NTS receives descending projections from the supramedullary and cortical centers. NTS is not a simple relay station of the vagus and sympathetic afferents; sequential activity in the NTS neurons can be recorded following stimulation of the SLN. Micro-injection of excitatory amino acid (EAA) agonist into the NTS induces peristaltic motor events in the pharynx and esophagus and these can be blocked by antagonist of EAA. N-methyl-D-aspirate (NMDA) receptor agonist and antagonist induce rhythmic “swallow like” contractions in the pharynx and esophagus. Micro-injection of GABA-A agonist and antagonist inhibits and facilitates motor events associated with swallowing and peristalsis, respectively, suggesting that the NTS must also exert tonic inhibitory control over premotor neurons of the DMV [9,10]. It is likely that the swallow reflex is a polysynaptic reflex at the level of brain stem and several neurotransmitters appear to be involved at these synaptic sites; i.e., acetylcholine (through nicotinic and muscrinic [11] receptors), epinephrine (alpha receptors), monoamines, serotonin, vasopressin, oxytocin, somatostatin, thyrotropine, nitric oxide [12,13], and possibly others [3].

FIGURE 1. Central pattern generator (CPG).

FIGURE 1

Central pattern generator (CPG). CPG includes two main groups of neurons located with the medulla oblongata: a dorsal group (DSG) located within the nucleus tractus solitarii (NTS) and the adjacent reticular formation and a ventral group (VSG) located (more...)

Copyright © 2011 by Morgan & Claypool Life Sciences.
Bookshelf ID: NBK54278

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