McKenna's Pharmacology for Nursing, 2e

893

C H A P T E R 5 6 Introduction to the gastrointestinal system

• Peritoneointestinal reflex : Irritation of the peritoneum as a result of inflammation or injury leads to a cessation of GI activity, preventing continued movement of the GI tract and thus further irritation of the peritoneum. • Renointestinal reflex : Irritation or swelling of the renal capsule causes a cessation of movement in the GI tract, again to prevent further irritation to the capsule. • Vesicointestinal reflex : Irritation or overstretching of the bladder can cause a reflex cessation of movement in the GI tract, again to prevent further irritation to the bladder from the GI movement. Many people with cystitis or overstretched bladders from occupational constraints or neurological problems complain of constipation, which can be attributable to this reflex. • Somatointestinal reflex : Taut stretching of the skin and muscles over the abdomen irritates the nerve plexus and causes a slowing or cessation of GI activity to prevent further irritation. During the era when tight girdles were commonly worn, this reflex was often seen among women, and constipation was a serious problem for many women who wore such constraining garments. Tight-fitting clothing (e.g. jeans) can have the same effect. People who complain Central reflexes Two centrally mediated reflexes—swallowing and vomiting—are very important to the functioning of the GI tract. Swallowing The swallowing reflex is stimulated whenever a food bolus stimulates pressure receptors in the back of the throat and pharynx. These receptors send impulses to the medulla, which stimulates a series of nerves that cause the following actions: the soft palate elevates and seals off the nasal cavity; respirations cease in order to protect the lungs; the larynx rises and the glottis closes to seal off the airway; and the pharyngeal constrictor muscles contract and force the food bolus into the top of the oesophagus, where pairs of muscles contract in turn to move the bolus down the oesophagus into the stomach. This reflex is complex, involving more than 25 pairs of muscles. This reflex can be facilitated in a number of ways if swallowing (food or medication) is a problem. Icing the tongue by sucking on ice cube blocks external nerve impulses and allows this more basic reflex to respond. Icing the sternal notch or the back of the neck, although not as appealing, has also proved effective in stimu- lating the swallowing reflex. In addition, keeping the head straight (not turned to one side) allows the muscle pairs to work together and helps the process. Providing of chronic constipation may be suffering from overactivity of the somatointestinal reflex.

stimulation of the receptors in the mouth through tem- perature variations and textured foods helps to initiate the reflex. People who do not produce their own saliva can be given artificial saliva to increase digestion and to lubricate the food bolus, which also helps the swallow-

ing reflex. Vomiting

The vomiting reflex is another basic reflex that is cen- trally mediated and important in protecting the system from unwanted irritants. The vomiting reflex is stimu- lated by two centres in the medulla. The more primitive centre is called the emetic zone. When stimulated, it ini- tiates a projectile vomiting. This type of intense reaction is seen in young children and whenever increased pressure in the brain or brain damage allows the more primitive centre to override the more mature chemo­ receptor trigger zone (CTZ). The CTZ is stimulated in several ways: • Tactile stimulation of the back of the throat, a reflex to get rid of something that is too big or too irritating to be swallowed • Excessive stomach distension • Increasing intracranial pressure by direct stimulation • Stimulation of the vestibular receptors in the inner ear (a reaction often seen with dizziness after “wild” rides in amusement parks) • Stimulation of stretch receptors in the uterus and bladder (a possible explanation for vomiting in early pregnancy and before delivery) • Intense pain fibre stimulation • Direct stimulation by various chemicals, including fumes, certain drugs and debris from cellular death (a reason for vomiting after chemotherapy or radiation therapy that results in cell death) Once the CTZ is stimulated, a series of reflexes occurs. Salivation increases, and there is a large increase in the production of mucus in the upper GI tract, which is accompanied by a decrease in gastric acid production. This action protects the lining of the GI tract from poten- tial damage by the acidic stomach contents. (Nauseated people who start swallowing repeatedly or complain about secretions in their throat are in the process of preparing for vomiting.) The sympathetic system is stim- ulated, with a resultant increase in sweating, increased heart rate, deeper respirations and nausea. This prepares the body for fight or flight and the insult of vomiting. The oesophagus then relaxes and becomes distended and the gastric sphincter relaxes. The person takes one deep res- piration; the glottis closes and the palate rises, trapping the air in the lungs and sealing off entry to the lungs. The abdominal and thoracic muscles contract, increas- ing intra-abdominal pressure. The stomach then relaxes, and the lower section of the stomach contracts in waves, approximately six times per minute. With nothing in the