C h a p t e r 2 8
Structure and Function of the Gastrointestinal System
681
pelvic nerves. Preganglionic parasympathetic fibers can
synapse with intramural plexus neurons, or they can act
directly on intestinal smooth muscle. In addition, these
same nerve bundles provide many afferent nerves whose
receptors lie within the various tissues of the gut. Their
nerves project to the spinal cord and brain to provide
sensory input for integration. Most parasympathetic
innervation is excitatory. Numerous vagovagal reflexes
influence motility and secretions of the digestive tract.
Sympathetic innervation occurs through the thoracic
chain of sympathetic ganglia and the celiac, superior mes-
enteric, and inferior mesenteric ganglia. The sympathetic
nervous system exerts several effects on GI function. It
controls the extent of mucus secretion by the mucosal
glands, reduces motility by inhibiting the activity of intra-
mural plexus neurons, enhances sphincter function, and
increases the vascular smooth muscle tone of the blood
vessels that supply the GI tract. Sympathetic stimulation
suppresses the release of the excitatory neuromediators
in the intramural plexuses, inhibiting GI motility.
Swallowing and Esophageal Motility
Chewing begins the digestive process—it breaks the food
into particles of a size that can be swallowed, and lubri-
cates it by mixing it with saliva. Although chewing usually is
considered a voluntary act, it can be carried out involuntarily
by a person who has lost the function of the cerebral cortex.
The swallowing reflex is a rigidly ordered sequence
of events that results in the propulsion of food from the
mouth to the stomach through the esophagus. Although
swallowing is initiated as a voluntary activity, it becomes
involuntary as food or fluid reaches the pharynx. Sensory
impulses for the reflex begin at tactile receptors in the
pharynx and esophagus and are integrated with the motor
components of the response in an area of the reticular
formation of the medulla and lower pons called the
swal-
lowing center.
The motor impulses for the oral and pha-
ryngeal phases of swallowing are carried in the trigeminal
(V), glossopharyngeal (IX), vagus (X), and hypoglossal
(XII) cranial nerves, and impulses for the esophageal
phase are carried by the vagus nerve. Diseases that dam-
age these brain centers or their cranial nerves disrupt the
coordination of swallowing and predispose an individual
to food and fluid lodging in the trachea and bronchi, lead-
ing to the risk of asphyxiation or aspiration pneumonia.
Swallowing consists of three phases: an oral, or vol-
untary, phase; a pharyngeal phase; and an esophageal
phase. During the
oral phase
, the bolus of food is col-
lected at the back of the mouth so the tongue can lift
the food upward until it touches the posterior wall of
the pharynx (Fig. 28-5A). At this point, the
pharyngeal
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)
Esophagus
Bolus
Relaxed membrane
Circular muscles
contract
Longitudinal muscles
contract
Relaxed muscularis
Stomach
Lower esophageal
sphincter
Nasopharynx
Laryngopharynx
Oropharynx
Soft palate
Hard palate
Uvula
Epiglottis
Larynx
Esophagus
Tongue
Bolus
Bolus
A
B
C
FIGURE 28-5.
Steps in the swallowing reflex:
(A)
The oral or voluntary phase during which the
bolus is collected at the back of the mouth so the tongue can lift the food upward and into the
pharynx and the
(B)
pharyngeal phase during which food movement into the respiratory passages
is prevented as the tongue is elevated and pressed against the soft palate closing the epiglottis, the
upper esophageal sphincter relaxes, and the superior constrictor muscle contracts, forcing food
into the esophagus; and
(C)
the esophageal phase during which peristalsis moves food through the
esophagus and into the stomach.
