NROSCI 1070-2070. November 10, 2014. Gastrointestinal 1. Function of the GI System. The digestive system has two primary roles:
November 10, 2014
• The organs involved in digestion and absorption include:
1. Salivary glands 5. Liver
2. Esophagus 6. Pancreas
3. Stomach 7. Large intestine
4. Small intestine
• In addition, 7 sphincters control the movement of material and secretions between the organs.
• The total length of the GI tract is about 15 feet, of which 13 feet are comprised of intestine.
• The processed material within the GI tract is referred to as chyme.
• The outer serosa forms the wall of the GI tract. It is an extension of the peritoneal membrane that lines the abdominal cavity. Sheets of mesentery connect with the serosa to hold the intestines in place.
• Three types of electrical activity are important in control of contraction of gastrointestinal smooth muscle:
1. Slow waves
3. Resting membrane potential
• Most gastrointestinal contractions occur rhythmically, and the slow waves play a key role in controlling these contractions.
• Slow waves are slow oscillations that occur at different frequencies at different points in the gut (3/minute in the body of the stomach to 12/minute in the duodenum). These slow waves are presumably due to cycling changes in activity in the Na+-K+ pump. The slow waves mainly reflect the entry of sodium into the smooth muscle cell, and do not cause muscle contraction (calcium triggers smooth muscle contraction).
• In other words, control of resting membrane potential is all important in determining whether gastrointestinal smooth muscle cells will contract.
1) Tonic, sustained contractions (mainly in sphincters).
2) Peristaltic contractions (move food forward in GI tract)
3) Segmental contractions (responsible for mixing food with GI secretions and pushing it against the wall of the mucosal layer)
• The musculature of the upper third if the esophagus, like that of the pharynx, is striated muscle, which is controlled bythe vagus nerve.
• The lower two-thirds of the esophagus contains smooth muscle, whose activity is mainly controlled by the enteric nervous system.
• Thus, even if the vagus nerve is sectioned, swallowing can occur if food reaches the lower part of the esophagus.