Neuroregulation of Deglutition
The main center for control of motility in the esophageal body and lower esophageal sphincter is the myenteric plexus. This plexus is located between the layers of the outer longitudinal and inner circular muscle of the muscularis propria. As the esophagus changes proximally from striated to smooth muscle distally, the myenteric plexus exhibits greater control over motor function. The plexuses connect to fibers of the vagus nerve which provide modulatory function. In the proximal esophagus, central control through these vagal fibers originates in the nucleus ambiguus whereas vagal fibers for the smooth muscle esophagus originate from the dorsal motor nucleus, both in the brainstem. The vagus nerves also carry sensory nerves originating from the muscle and the mucosa. In the cervical esophagus, efferent fibers of the vagus course through the recurrent laryngeal nerve. The thoracic esophagus receives fibers directly off the vagus. Afferent fibers course through the superior and recurrent laryngeal nerve and the vagus.
Peristalsis with aborad movement of the bolus from the proximal esophageal segment into the stomach is the main function of the esophagus. Primary peristalsis is initiated with pharyngeal contraction, whereas secondary peristalsis occurs in response to esophageal distention and originates from the point of stimulation.
This latter type of peristalsis is particularly important in clearing a reflux of gastric content. Peristalsis is coordinated by the neural network to achieve a pattern of descending proximal segment contraction to propel the bolus and distal segment relaxation to receive the bolus. This reflex is achieved through coordinated release of excitatory neuropeptides proximally and inhibitory neuropeptides distally. Excitatory peptides in the esophagus include acetylcholine contained by neurons more proximally and nonadrenergic noncholinergic neurons distally. Nitric oxide and vasoactive intestinal peptide are the major inhibitory neuropeptides contained in neurons. Descending coordination is achieved through a combination of local, myenteric, and central vagal control.
Concordant with peristalsis, coordinated opening of the gastroesophageal highpressure zone must occur to complete bolus propulsion into the stomach. This involves the neural coordination of the two structures that form the lower esophageal sphincter (LES), the intrinsic LES and the crural diaphragm. The intrinsic LES maintains a baseline tone likely through both cholinergic and adrenergic activity. Similar to the esophageal body, the bulk of control is local through the myenteric plexus but can be modulated by vagal reflexes, particularly in reaction to modifying events such as increased abdominal strain. Crural diaphragm contraction is mediated through the phrenic nerve.