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.
