Medical Physiology/Gastrointestinal Physiology/Principles of GI function

Neural Control of GI Function
The gut is controlled by its own nervous system the Enteric nervous system, as well as the parasympathetic and the sympathetic systems

Enteric Nervous System
The GI system has its own nervous system, the enteric nervous system with about 100 million cells, the same number as the spinal cord! This nervous system lies entirely within the gut, and can function quite independently of the central and autonomic nervous system. A bowel removed from any contact with an animal is capable of rhythmical contractions. It starts in the oesophagus and runs the entire length of the gut down to the anus.

It consists of two plexuses, the submucosal, which runs by the muscularis mucosa, and the myenteric plexus that runs between the circular and longitudinal muscle layers. The two plexuses are connected to each other.

The myenteric plexus is mainly organised as longitudinal chains of neurons. when stimulated this plexus increases the tone of the gut as well as the velocity and intensity of contractions. This plexus is concerned with motility throughout the whole gut. Inhibition of the myenteric system helps to relax sphincters.

The submucosal plexus is more involved with local conditions and controls local secretion, absorption, as well as local muscle movements.

The mucosa and epithelium also have sensory nerve endings that feed signals to the both layers of the enteric plexus as well as sending information back to the sympathetic pre-vertebral ganglia, the spinal cord and to the brain stem. This arrangement is shown in the following diagram.

Numerous transmitters seem to be involved, the more important of which are acetylcholine and norepinephrine. The former excites gut activity, the latter inhibits it.

Parasympathetic Innervation
The effect of parasympathetic stimulation is to increase activity in the entire enteric nervous system The proximal half of the nervous system is innervated from the cranial parasympathetic nerve fibers via the Vagal Nerve. The distal half is innervate via the Sacral Parasympathetic nerves. The latter gives a rich supply to the Sigmoid colon, rectum and anus, and are important in controlling defecation.

Sympathetic Innervation
The fibers originate in the sympathetic ganglia of T-5 to L-2 and terminate mainly on the enteric plexuses, but also a few nerves terminate in the mucosa it self. Stimulation of the enteric nerves by the sympathetic system inhibits GI activity. It does this in a minor way with the direct effect of its secreted norepinephrine, and in a major way by inhibiting action in the enteric plexuses.

Afferent Sensory Innervation
Numerous afferent sensory fibers innervate the gut. Some have their cell bodies in the enteric plexus, and some in the Spinal cord. As well as sending information concerning irritation and over distension, they can also pick up the presence of chemical signals in the gut. 80% of the fibers in the vagus nerve are afferent, and these send signals all the way to the medulla for processing.

Gastrointestinal Reflexes
GI reflexes can be considered: Local reflexes are processed entirely within the enteric system and control secretion, local motility, mixing contractions etc.
 * 1) Local
 * 2) Regional
 * 3) Systemic

Regional reflexes go back to the sympathetic ganglia, and are important for reflexes at a distant, such as the gastro-colic reflex causing evacuation of the colon, and messages from the intestine to the stomach to inhibit emptying, the entero-gastric reflex, or the colono-ilial reflex that inhibits emptying of the ilial contents into the colon.

Systemic reflexes are processed in the spinal cord or brainstem and will control overall activity of the GI system, for example pain reflexes that will inhibit the entire GI system.

Hormonal Control of Gastrointestinal Function
Here is a brief overview of the hormones affecting Gastrointestinal mobility. They will be further reviewed in the sections on motility and digestion.

Gastrin
Secreted by the 'G' cells of the antrum of the stomach in response to an ingestion of a meal, Gastrin stimulates Gastric acid secretion and the growth of Gastric mucosa

Cholecystokinin
Secreted by the 'I' cells of the duodenum and Jejunum in response to the digestive products of fatty acids, Cholecystokinin causes contraction of the gall bladder. To a lesser extent it also decreases emptying of the stomach.

Secretin
The first GI hormone to be discovered, Secretin is secreted by the 'S' cells of the duodenum in response to the presence of acidic chyme. It acts to increase pancreatic secretion of bicarbonate to neutralize the acid chyme.

Gastric Inhibitory peptide
Secreted in the mucosa of the upper small intestine in response to the products of digestion. It inhibits motility in the stomach, thus delaying stomach emptying.

Motilin
Secreted by the duodenum during fasting, it increases gastrointestinal motility causing hunger pangs.

Gastro-intestinal blood flow
Blood supply is via. the Splanchnic circulation (see Anatomy Chapter above).

Local Factors affecting Blood flow
Normally blood supply to the gut is controlled by local factors which match supply with activity. During absorption blood supply to the mucosa will be increased; during increased motor activity blood supply to the muscles will be increased. this is accomplished by several local factors:
 * 1) Vasodilator substances released from the mucosa during digestion which include cholesystokinin, secretin, and gastrin.
 * 2) Kinin release including kallidin and bradykinin which are powerful vasodilators. These are released by some glands when they secrete into the lumen.
 * 3) decreased O2 tension due to metabolic activity will also cause increased blood flow.

Nervous control of GI Blood flow
Parasympathetic stimulation will increase overall blood flow to the gut as well as increasing secretions and general gut activity.

Sympathetic stimulation will have the opposite effect. The effect of sympathetic stimulation often only lasts a few minutes, then an autoregulatory effect takes over restoring the blood supply to normal. However this shutting off of the blood supply acts to increase flow to other areas of the body in time of stress. Sympathetic stimulation can also cause contraction of the Splanchnic veins, making as much as half a liter of volume available to the general circulation.