Factors Controlling Digestive
The activities of the digestive system are controlled by nerves,
hormones, and local mechanisms. Sensory nerves present in the walls of the gut
can be stimulated by food material in the lumen and by stretching of the
walls. By their communication with the nerve that supplies the smooth muscles,
contraction of smooth muscle and secretion of glands in the local area can be
affected. In addition, communication of these sensory nerves with the central
nervous system can alter activities in various remote parts of the gut.
The gastrointestinal tract is an endocrine factory in itself.
Eighteen or more secreted hormones have been identified, with more hormones
being identified con-tinually. The hormones secreted in the gastrointesti-nal
tract are transported by the blood and have their effect on other regions of
the gut. For example, gas-trin, a hormone secreted by cells in the stomach,
stim-ulates gastric motility and secretion. Similarly, hor-mones (e.g.,
secretin, cholecystokinin) from the upper part of the intestine cause an
increase in the secretion from the pancreas and relaxation of the sphincter be-tween
the small and large intestines.
In addition to hormones, chemicals released locally regulate the
activities of the cells. For example, hista-mine released by cells in the
stomach stimulates the ad-jacent acid-secreting cells. Local mechanisms are im-portant
when small areas of the gut must be regulated.
Although the gut appears autonomous, with its own nerve supply,
hormones, and local mechanisms, its activities can be modified by other
factors. The central nervous system (i.e., the brain and the spinal cord)
hormones secreted by other endocrine glands and even changes in the electrolyte
content of the blood can modify its activities. That is how stress can cause
changes in bowel habits. Individuals with hy-perthyroidism tend to have
diarrhea, and individuals with hypothyroid, constipation. Many drugs taken for
other ailments can also affect the normal func-tioning of the gut.