As we indicated earlier, the vertebrate gut is a disassembly line, and as with a standard assembly line, the control and coordination of the sequential processes of digestion are critical. Both neuronal and hormonal controls govern these processes. Once the products of digestion are absorbed, their availability to the cells of the body must also be controlled and regulated.
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Hormones control actions of the digestive tract and its accessory organs.
The liver interconverts and stores fuel molecules.
The liver controls fat metabolism through its production of lipoproteins, which move fats between storage and production sites.
The pancreas secretes hormones that help control glucose levels in the blood.
Body mass and food intake behavior are influenced by stimulatory and inhibitory hormones that activate neurons in the arcuate nucleus region of the hypothalamus in the brain.
Your salivation at the sight or smell of food is an autonomic reflex, as is swallowing. Other autonomic reflexes include the release of digestive juices and increased stomach motility stimulated by good food smells, as well as gagging at the smell of rotting meat. These reflexes are called the “cephalic phase” of control of digestive processes because they involve the CNS. Most of the control of digestive processes, however, involves neural and hormonal mechanisms that are intrinsic to the gut.
Neuronal messages travel from one region of the digestive tract to another in the enteric nervous system. One function of the gut’s nervous system is coordinating the movement of food through the gut. Of course, this intrinsic nervous system communicates information to the CNS and receives input from the CNS, but its most important role is to coordinate actions throughout the digestive tract. In spite of this marvelous intrinsic nervous system, however, much of the control and regulation of the digestive system and nutrient management involves hormonal mechanisms.