This section is from the book "Practical Dietetics With Special Reference To Diet In Disease", by William Gilman Thompson. Also available from Amazon: Practical Dietetics with Special Reference to Diet in Disease.
The influence of the nervous system on digestion is very complex. In a general way, the peripheral nerves may affect the digestive process (a) through the circulation, (b) through motion, (c) through glandular action. The nerves chiefly concerned in these processes are the branches of the sympathetic system and the vagus. The latter, through its association with the cardiac, vasomotor, and respiratory centres in the medulla oblongata, places the vital functions of the body in very intimate connection with food stimulation acting through the branches of the nerve in the alimentary canal.
The nerves influence digestion through the circulation by their vasomotor control, regulating the calibre of the vessels and quantity of blood supplied to the walls of the alimentary canal, the local blood pressure, and the consequent rate of absorption.
The nerves influence the movements of the entire alimentary canal, either accelerating or inhibiting them, thus controlling the propulsion of the food, its admixture with secretions, and its contact with absorbing surfaces.
The nerve supply of the digestive glands is distributed to their blood vessels, and also probably to some extent to the cells of the gland parenchyma. This latter distribution is not always demonstrable histologically in man, but the influence is unquestionable.
Under normal conditions, the nerves act mainly in connection with the digestion through reflex stimulation produced by mechanical irritation of food and by the chemical irritation of its different ingredients as they undergo absorption. But, in addition, nerve currents from the central nervous system or from a remote peripheral origin may interfere with the normal nerve functions. Every one is familiar with examples of acute indigestion produced by fatigue of the nervous system, undue mental excitement, emotion, etc. For any given phase of digestion, disturbance of normal nerve function will retard the process more in its earlier stages by checking or altering gland secretion. In its later stages the effect of the nervous system will be more pronounced in controlling or inhibiting absorption. Overstimulation of the local nerves of the alimentary tract may excite an increased watery secretion and exaggerate peristaltic movement of the intestines, thereby hastening the passage of the food through them before there is time for digestion or absorption, and giving rise to diarrhoea.