Nervous and humoral control

Parasympathetic supply to the colon is provided by the vagi to the proximal colon and pelvic nerves to the distal colon, whereas sympathetic supply is via the splanchnic and lumbar colonic nerves which supply the proximal and distal colon respectively. Axons from both branches of the autonomic nervous system impinge on the neurons of the myenteric plexus. Vagal stimulation initiates segmental contractions in the proximal colon, whereas pelvic nerve stimulation causes tonic propulsive contractions in the distal colon. Stimulation of either the splanchnic or lumbar sympathetic nerves causes the colonic muscles to relax.

Plexuses of Auerbach, involved with colonic motility, and Meissner, associated with mucus secretion, have an integrated system enabling stimuli to be detected and the appropriate response produced. The muscle of the large intestine can be stimulated by stretching, and a number of chemical substances such as acetylcholine and histamine can cause contraction or enhance motility. Adrenaline and noradrenaline can depress smooth muscle activity.

The central nervous system (CNS) modifies many functions of the gastrointestinal tract. In cats, stimulation of the cerebrum, midbrain and hypothalamus increases colonic motility. The colon appears to receive impulses from the CNS and upper gastrointestinal tract via the spinal cord. This is supported by the association of thoracic spinal cord injury with intractable constipation6. If the proximal region of the colon is distended, contraction distally is inhibited by impulses passing along intermesenteric neurons between pre- and para- vertebral ganglia and then via splanchnic or lumbar colonic nerves. With extrinsic regulation the parasympathetic neurons appear to be excitatory and the sympathetics inhibitory in the musculature of the colon.

A number of hormones influence colonic motility. Gastrin can intensify contractions and may decrease the evacuation time of the colon. Cholecystokinin has been proposed as a mediator of the lipid generated increase in rectosigmoid motor activity. However infusion of cholecystokinin at physiological concentrations does not reproduce the complex milieu of events that occur postprandially. Neurotensin is present in the myenteric plexus and mucosa of the colon and may also play a part in the increase in colonic motility seen on ingestion of fat as the serum levels rise. Substance P appears to stimulate the circular smooth muscle, unlike progesterone, which acts an inhibitor of muscle contraction, thus perhaps accounting for some of the changes seen in colonic motility during the menstrual cycle and pregnancy. Secretin and glucagon also inhibit colonic motility. Despite the influence of

Table 7.1 Comparison of the environment in different parts of the gastrointestinal tract

Region

Length (m)

Surface Area (m2)

pH

Residence Time

Micro-organisms

Oesophagus

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Essentials of Human Physiology

Essentials of Human Physiology

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