Porth's Essentials of Pathophysiology, 4e

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Structure and Function of the Gastrointestinal System

C h a p t e r 2 8

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 ( text continues on page 684 )

A

Nasopharynx

Oropharynx Soft palate Hard palate Uvula

Bolus

Esophagus

Relaxed membrane

Epiglottis

Circular muscles contract Longitudinal muscles contract Relaxed muscularis

Laryngopharynx

Tongue

Larynx

Bolus

Lower esophageal sphincter

Esophagus

Stomach

Bolus

C

B

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.