Regulation of gastrointestinal function

The digestive tract contains three types of sensory receptors that are sensitive to chemical or mechanical changes within the system. These include:

• Chemoreceptors

• Osmoreceptors

• Mechanoreceptors

Chemoreceptors respond to various chemical components within the gastrointestinal lumen. For example, in the duodenum of the small intestine, chemoreceptors are stimulated by excessive amounts of hydrogen ion secreted by the stomach. Osmoreceptors are sensitive to the osmolarity of the contents within the lumen. As the digestive process progresses, large nutrient molecules are split into their smaller components. This increases the number of molecules and therefore the osmolarity of material being processed. Excessive osmolarity may suggest that absorption is not keeping pace with digestion. Mechanoreceptors respond to stretch or distension of the gastrointestinal tract wall.

Receptor stimulation may lead to activation of any or all of the following regulatory mechanisms within the tract:

• Intrinsic nerve plexuses

• Extrinsic autonomic nerves

• Gastrointestinal hormones

Intrinsic nerve plexuses. The intrinsic nerve plexuses are interconnecting networks of nerve cells located entirely within the gastrointestinal tract and are responsible for intratract reflexes. The stimulation of a receptor in one region of the tract neurally influences activity of another region of the tract. These reflexes occur directly, independent of the central nervous system. Intratract reflexes provide a mechanism for self-regulation of the tract and help to coordinate the activity of the organs within it. An example of such a reflex is the enterogastric reflex, in which receptor stimulation in the duodenum of the small intestine elicits neural activity that regulates muscle contraction and glandular secretion in the stomach.

Extrinsic autonomic nerves. Gastrointestinal activity is also modified by extrinsic autonomic nerves. The tract is innervated by the sympathetic and parasympathetic divisions of the autonomic nervous system. The effects of these two divisions tend to oppose each other: the parasympathetic system stimulates most digestive activities while the sympathetic system inhibits them. Interestingly, the autonomic nerves to the digestive system, especially the vagus nerve of the parasympathetic system, can be discretely activated. In this way, digestive activity can be modified without affecting tissue function in other regions of the body.

Gastrointestinal hormones. A third factor contributing to regulation of digestive activity is the secretion of gastrointestinal hormones. These hormones may be released in one region of the tract, travel in the circulatory system to other regions of the tract, and influence the activity of effector cells in that region. A summary of the source, stimulus for release, and actions of several important hormones is found in Table 18.2.

Table 18.2 Digestive Hormones

Hormone

Source

Stimulus for Release

Hormone actions

Gastrin

Secretin

Cholecystokinin

Gastric inhibitory peptide

G cells in pyloric region of the stomach

Endocrine cells in mucosa of duodenum

Endocrine cells in mucosa of duodenum

Endocrine cells in mucosa of duodenum

Protein in stomach; vagal stimulation

Acid in duodenum

Breakdown products of lipid and, to a small extent, protein digestion in duodenum Lipids, acid, and hyperosmotic chyme in duodenum; distension of duodenum

Stimulates parietal cells (HC1) and chief cells (pepsinogen) in stomach; enhances gastric motility Inhibits gastric emptying and gastric secretion; stimulates secretion of bicarbonate from pancreas; stimulates secretion of bicarbonate-rich bile from liver

Inhibits gastric emptying and gastric secretion; stimulates contraction of gallbladder; stimulates secretion of digestive enzymes from pancreas Inhibits gastric emptying and gastric secretion; stimulates secretion of insulin from pancreas

Figure 18.1 Summary of the regulatory mechanisms influencing gastrointestinal function.

A summary of these three mechanisms that regulate the activity of the digestive system is illustrated in Figure 18.1. A local change in the tract may lead to stimulation of one or more of the three types of receptors present in the tract wall. Receptor stimulation may then activate any or all of the three regulatory mechanisms. These mechanisms then alter the activity of the effector tissues within the digestive system, including smooth muscle, and exocrine and endocrine glands.

The following sections will discuss each region of the digestive system separately. Where appropriate, the basic digestive processes — motility, secretion, digestion, and absorption — will be considered.

18.4 Mouth

The mouth is the region from the lips to the pharynx. The first step in the digestive process is chewing, or mastication, which is an initial mechanical breakdown of the food that facilitates its movement to the stomach. The mouth is lined with stratified squamous epithelium that provides extra protection from injury by coarse food materials. Three pairs of salivary glands secrete saliva into the oral cavity:

• Parotid glands located between the angle of the jaw and the ear

• Sublingual glands located below the tongue

• Submandibular glands located below the jaw

Saliva contains:

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