Introduction

The major functions of the GI tract are motility, secretion, and absorption. Each part of GIT is adapted to its specific functions: some to simple passage of food, such as the esophagus; others to temporary storage of food, such as the stomach; and others to digestion and absorption, such as the small intestine. The motor functions of the gut are performed by the different layers of smooth muscle - longitudinal muscle and circular muscle. In the longitudinal muscle layer, the bundles extend longitudinally down the intestinal tract;while in the circular muscle layer, they extend around the gut. Two types of movements occur in the gastrointestinal tract: (1) propulsive movements, and (2) mixing movements (segmentation). Peristalsis occurs in waves of contraction behind and relaxation ahead of the luminal bolus, and travels down the GI tract over short distances. Segmentation is a mixing pattern of contractility that is more irregular and allows for luminal contents and digestive enzymes to have adequate contact with the absorbing epithelium. The gastrointestinal tract has a nervous system all its own called the enteric nervous system. It lies entirely in the wall of the gut, beginning in the esophagus and extending all the way to the anus. The enteric nervous system is composed mainly of two plexuses, (1) an outer plexus lying between the longitudinal and circular muscle layers, called the myenteric plexus; and (2) an inner plexus, called the submucosal plexus or Meissner’s plexus, that lies in the submucosa. There are nervous connections within and between these two plexuses. The myenteric plexus controls mainly the gastrointestinal movements, and the submucosal plexus controls mainly gastrointestinal secretion and local blood flow.

As soon food is ingested, two important processes occur - masticaiton and swallowing. Mastication refers to the mechanical breakdown of food by chewing and chopping actions of the teeth. Insalivation refers to the mixing of the oral cavity contents with saliva secreted by three pairs of salivary glands. From the mouth, food passes through the pharynx and oesophagus via the action of swallowing which occur in three stages - oral, pharyngeal and esophageal.  Throughout the gastrointestinal tract, secretory glands subserve two primaryfunctions: First, mucous glands, from the mouth to the anus, provide mucus for lubrication and protection of all parts of the alimentary tract. Second, digestive enzymes and secretions (from salivary gland, gastic gland,gall bladder, pancreas) are secreted in the alimentary tract, from the mouth to the distal end of the ileum. Important for secretion and absorption of fluids, electrolytes, and solutes are the epithelial cells which differ in structure and function depending on their location in the GI tract. The stomach is a glandular organ. Gastric parietal cells in glands within the gastric body are important for the secretion of gastric acid and intrinsic factor, pepsinogen is secreted by the chief cells also within the gastric body, while hormones (gastrin, histamine)  are released from enteroendocrine cells (EEC) throughout the stomach. Proteins, fats, and carbohydrates are broken down via the action of digestive enzymes into smaller units in preparation for absorption into the network of capillaries and lymphatic vessels (lacteals) by the small intestinal epithelial cells located on the small intestinal villi. Any remaining material that is not absorbed by the small intestine passes though the ileocecal valve into the colon. The large intestinal mucosa is responsible for the absorption of water, solidification of the colonic contents into feces, and then storage of the feces prior to expulsion.

After the end of the course, students will be able to learn about

1. Enteric nervous system and its division

2. Neural control of motility of the all the organs of GIT 

3. Functional type of movements in GIT

4. Comlpete mechanism of Swallowing 

5. Motor functions of stomach, small intestine and colon

6. Defecation reflex

7. Mechanism of Saliva Secretion

8. Gastric gland secretions and gastric acid production

9. Bile and pancreatic secretions and their significance 

Further Reading:

 1. TEXTBOOK OF MEDICAL PHYSIOLOGY, GUYTON,A.C, 13TH Edition 

  2. HUMAN ANATOMY & PHYSIOLOGY (Marieb, Human Anatomy & Physiology) , 10th Edition

  3. REVIEW OF MEDICAL PHYSIOLOGY ,GANONG,W.F, 25th Edition