Stimulation of the Digestive System

FIGURE 2.8 Distribution of hormone production over the gastrointestinal tract. 

The cephalic phase of digestion stimulates only a fraction of the maximum possible levels of gastric and pancreatic secretions. This phase does not seem to produce a rise in the levels of gastrin and CCK. Cephalic stimulation of the pancreas, as mediated by the vagus nerve, provokes release of pancreatic enzymes into the small intestine. Cephalic stimulation of the parietal cells, as mediated by the vagus nerve, provokes release of gastric acid into the lumen of the stomach- In humans, the cephalic phase does not seem to result in release of bicarbonate into the lumen of the small intestine. 

The following experiment describes the release of gastric acid during the cephalic phase. The study involved human subjects who volunteered to have a tube inserted into a nostril and pushed down into the stomach, a standard procedure for collecting stomach juices. The subjects were instructed to be silent, to discuss sports, or to discuss food. Samples were removed at 15-minute inter als during the course of the experiment. The concentration of hydrogen ions (H ) in the samples was used to calculate the rate of acid release. 

FIGURE 2.9 Ca hric add release during an hour of silence followed by half-hour discus-sions of sports (O) or food ( . The rate of secretion during the silent period is used as the basal rate that is, the rate occurring with no stimulation. The black bar indicates the 30-inin period during which the discussions occurred. Redrawn with permission from Feldman and Richaidson, 19S6.) 

Note that gastrin injection produced the maximal rate of add output, followed by sham feeding and by talking about food. If the latter responses are lo be calculated as percentages of this maximum, the basal rate most be subtracted from all three values first. This manipulation is a simple example of the preliminary corrections that must be applied to experimental data before more complicated questions can be answered correctly. 

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Stimulation of the Digestive System Cephalic Phase Chemical Phase... 

Control mechanisms for the food intake of farm animals can be envisaged as operating at three levels. At the metabolic level, concentrations of nutrients, metabolites or hormones may stimulate the nervous system to cause the animal to start or stop feeding. At the level of the digestive system, the quantities of digesta may determine whether or not the animal ingests more food. Finally, external influences such as climatic variables and the ease with which food can be ingested will also influence food intake. In... 

Prominent effects within the digestive tract include stimulation of salivation and acid secretion, increased intestinal tone and peristaltic activity, and relaxation of most sphincters. Bronchoconstriction and stimulation of secretions are prominent effects in the respiratory system. Muscarinic agonists can also evoke secretion from nasopharyngeal glands. Urination is promoted by stimulation of the detrusor muscle of the bladder and is facilitated by relaxation of the trigone and external sphincter muscles. 

The more classical Itinehoii of parathyroid hormone is concerned with its control of the maintenance of constant circulating calcium levels. Its action is on 11) Ihe kidney, where it increases the phosphate in the urine. (2) the skeletal system, where it causes calcium resorplion from bone, and t3l the digestive system, where it accelerates (stimulates) calcium absorption into the hitskI The hormone and gland exhibit characteristics of feedback control when the concentration of calcium tons in the blood falls, the secretion of the hormone increases, and when their concentration rises, the secretion of hormone decreases... 

Other drugs may increase the effects of dextroamphetamine. For example, bicarbonate and other alkalin-izing agents increase the amount of amphetamines absorbed in the digestive system. Thiazides (potassium-depleting diuretics) decrease the amount of amphetamines that leave the body in urine. Also, other central nervous system stimulants, such as cocaine and nicotine, can amplify the stimulating effects of dextroamphetamines. 

In general, the sympathetic nervous system promotes the fight or flight response by shutting down the body s housekeeping roles (digestion, defecation, urination, etc.), and stimulating the heart. The stimulation of the adrenal medulla releases the hormone adrenaline which reinforces the action of noradrenaline. 

Arnold Krochmal, Ph.D., in his hook Medicinal Plants of the United States, briefly mentions Gulf Coast Indians who made an infusion of the leaves and roots for dysentery and for relieving stomach pain. From the inner trunk bark, they also made poultices for insect and snake bites and skin ulcers. The Houma of Louisiana used a decoction of the root for sore eyes, high blood pressure, and kidney problems. The fruits of closely related species of palms have been used as medicine tWughout the Caribbean and the Yucatan in Mexico. Sabal japa fruit bears a close resemblance to the fruit of saw palmetto an extract of that fruit is used as a sedative and digestive stimulant. It was also used for respiratory problems and for weakness of the reproductive system. 

The influence of ACh and parasympathetic innervation on various organs and tissues is discussed in detail in Chapter 6. ACh and its analogs stimulate secretion by all glands that receive parasympathetic innervation, including the lacrimal, tracheobronchial, salivary, and digestive glands. The effects on the respiratory system, in addition to increased tracheobronchial secretion, include hronchoconstriction and stimulation of the chemoreceptors of the carotid and aortic bodies. When instilled into the eye, muscarinic agonists produce miosis (see Chapter 63). 

Ill-health can reduce the intake of both ruminants and non-ruminants. Contrary to popular opinion, infestations of gastrointestinal parasites tend to reduce intake, presumably because the interference with digestive function overrides any metabolic stimulus arising from a reduction in the absorption of nutrients. There is also evidence that stimulation of the animal s immune system, as happens with parasite infestations, may be responsible for a reduction in food intake. Infestations with external parasites, such as ticks, also reduce intake. 

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