General principles of gastrointestinal system function

Objectives List the main functions of the digestive system. Define the terms: ingestion, digestion, absorption, motility, secretion. Outline the basic processes performed by the digestive system. Describe the role of autonomic nervous system on GIT function. Describe the innervations and the neural control of the gastrointestinal tract. Describe the role of enteric nervous system in control of gastrointestinal function. List the types of motor activity of the gastrointestinal tract. List the main functions of the GI hormones. List factors affecting gastrointestinal secretion and motility.

Gastrointestinal System Gastrointestinal tract ( continues tube that extends from the mouth to the anus) Accessory digestive organs ( teeth, tongue, salivary glands, liver, gallbladder, and pancreas)

Functions of Gastrointestinal System Absorption of nutrients, water and electrolytes Excretion of waste products Endocrine functions Immune functions

General Principles of Gastrointestinal System Supply the body with water, electrolytes and nutrients. To achieve this it: Moves food through the alimentary tract (motility). Secretes digestive juice to digest food (Secretion and digestion) Absorbs the digestive products (absorption) ALL these functions are under nervous and hormonal regulation

Functions of the digestive system Ingestion Taking food and liquids into the mouth Secretion Cells within the walls of the GI tract and accessory digestive organs secrete around 7 liters of water, acid, buffers and enzymes into the lumen of the tract.

Functions of the digestive system cont… Mixing and propulsion Alternating contraction and relaxation of smooth muscles in the wall of the GI tract. It mixes the food with secretions and propel it towards the anus (motility)

Functions of the digestive system cont… Digestion Mechanical: Teeth= breaking down the food into small parts before it is swallowed Smooth muscles = churning of food Chemical ( enzymes) : large carbohydrate, lipid, protein and nucleic acids in the food are split into smaller molecules

Functions of the digestive system cont… Absorption: The entrance of ingested and secreted fluids, ions, and small molecules into the epithelial cells lining the lumen of the GI tract. The absorbed substances pass into blood or lymph and circulate to cells throughout the body

Functions of the digestive system cont… Defecation: Elimination of wastes, indigestible substances, bacteria, cells sloughed from the lining of the GI tract, and digested materials that were not absorbed through the anus.

Neural Regulation of the GIT functions Enteric Nervous system Autonomic Nervous System (Extrinsic GIT Innervations)

1. Enteric Nervous system The intrinsic nerves make up a very dense network embedded in the walls of the esophagus, stomach, small intestine, and colon.

Extrinsic GIT Innervations Sympathetic: Arises from thoracic and upper lumber spinal cord (T5-L2). Pre-ganglionic fibers synapse outside GI tract in pre-vertebral ganglia Post-ganglionic adrenergic fibers innervate the cells of the myenteric and submucosal plexus Elements from the two plexuses innervate the smooth muscle, secretory and endocrine cells. The postganglionic neurotransmitter is mainly Epinephrine (adrenaline). Increase of sympathetic nerve activity → inhibitory effects, e.g. ↓secretion, ↓ motility and ↓ blood flow.

Parasympathetic: The dorsal motor nucleus of the vagus supplies efferent vagus nerve fibers to:Lower esophagus, Stomach, Small intestine and Proximal colon The sacral outflow arises from neurones in the sacral spinal code (S2-S4)→ pelvic nerve → innervation of the distal colon, rectum and anal canal. Pre-ganglionic fibers synapse with ganglionic cells located in the enteric nervous system. The preganglionic neurotransmitter is mainly Acetylcholine. Increase of parasympathetic nerve activity → Stimulatory effects, e.g. ↑gastric, pancreatic and small intestinal secretion, ↑ blood flow and muscle contraction.

4. Hormonal regulation Gastrin Cholesystokinin (CCK) Secretin GIP (Gastric Inhibitory Peptide) Motilin Somatostatin Histamine VIP (Vasoactive Intestinal Peptide) GRP (Gastrin Releasing Peptide) NO (Nitric Oxide)

GIT Hormones Gastrin (from antrum): increase acid secretion Cholesystokinin (CCK, Duodenum): causes gallbaldder contraction and pancreatic secretion Secretin: inhibits acid secretion and stimulates pancreatic HCO3- secretion GIP (Gastric Inhibitory Peptide): reduces gastric motility and stimulates insulin secretion Motilin: increases GI motility & stimulate the production of pepsin

Neurocrines VIP (Vasoactive Intestinal Peptide): relaxes GI smooth muscle cells GRP (Gastrin Releasing Peptide, also called Bombesin): mediates release of hormone Gastrin NO (Nitric Oxide): relaxes smooth muscle cells Enkephalins: contract sphincters

Summary of the pathway controlling digestive system activities Mechanoreceptors Chemoreceptors Osmoreceptors