1. General principles of gastrointestinal system function

2. 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.

3. 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)

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

5. 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

6. 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.

7. 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)

8. 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

9. 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

10. 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.

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

12. 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.

14. 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.

15. 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.

16. 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)

17. 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

18. 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

19. Summary of the pathway controlling digestive system activities
Mechanoreceptors
Chemoreceptors
Osmoreceptors