General principles of General principles of gastrointestinal gastrointestinal system functionsystem function

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ObjectivesObjectives List the main functions of the digestive system.List the main functions of the digestive system. Define the terms: ingestion, digestion, absorption, Define the terms: ingestion, digestion, absorption,

motility, secretion. motility, secretion. Outline the basic processes performed by the Outline the basic processes performed by the

digestive system.digestive system. Describe the role of autonomic nervous system on GIT Describe the role of autonomic nervous system on GIT

function. function. Describe the innervations and the neural control of the Describe the innervations and the neural control of the

gastrointestinal tract.gastrointestinal tract. Describe the role of enteric nervous system in control Describe the role of enteric nervous system in control

of gastrointestinal function.of gastrointestinal function. List the types of motor activity of the gastrointestinal List the types of motor activity of the gastrointestinal

tract.tract. List the main functions of the GI hormones.List the main functions of the GI hormones. List factors affecting gastrointestinal secretion and List factors affecting gastrointestinal secretion and

motility.motility.22

Gastrointestinal tract Gastrointestinal tract

( continues tube that ( continues tube that extends from the mouth to extends from the mouth to the anus)the anus)

Accessory digestive Accessory digestive organs organs

( teeth, tongue, salivary ( teeth, tongue, salivary glands, liver, gallbladder, glands, liver, gallbladder, and pancreas) and pancreas)

Gastrointestinal System

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Functions of Gastrointestinal System

Absorption of nutrients, water and electrolytes

Excretion of waste products

Endocrine functions

Immune functions

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General Principles of Gastrointestinal System

Supply the body with water, electrolytes and nutrients.

To achieve this it:

1. Moves food through the alimentary tract (motility).

2. Secretes digestive juice to digest food (Secretion and digestion)

3. Absorbs the digestive products (absorption)

ALL these functions are under nervous and hormonal regulation

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Functions of Functions of thethe digestive system digestive system

IngestionIngestionTaking food and liquids into the Taking food and liquids into the

mouthmouth

SecretionSecretionCells within the walls of the GI tract and Cells within the walls of the GI tract and

accessory digestive organs secrete accessory digestive organs secrete around 7 liters of water, acid, buffers around 7 liters of water, acid, buffers and enzymes into the lumen of the and enzymes into the lumen of the tract.tract.

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Functions of the digestive systemFunctions of the digestive system cont…cont…

Mixing and propulsionMixing and propulsionAlternating contraction and relaxation of smooth muscles in the Alternating contraction and relaxation of smooth muscles in the

wall of the GI tract. It mixes the food with secretions and propel it wall of the GI tract. It mixes the food with secretions and propel it towards the anus (towards the anus (motilitymotility))

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Functions of the digestive systemFunctions of the digestive system cont…cont…

DigestionDigestion MechanicalMechanical: : Teeth= breaking down the food into small parts Teeth= breaking down the food into small parts

before it is swallowed before it is swallowed Smooth muscles = churning of foodSmooth muscles = churning of foodChemicalChemical ( enzymes) : ( enzymes) : large carbohydrate, lipid, protein and nucleic large carbohydrate, lipid, protein and nucleic

acids in the food are split into smaller acids in the food are split into smaller moleculesmolecules

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Functions of the digestive systemFunctions of the digestive system cont…cont…

Absorption:Absorption:The entrance of ingested and secreted fluids, ions, and The entrance of ingested and secreted fluids, ions, and

small molecules into the epithelial cells lining the small molecules into the epithelial cells lining the lumen of the GI tract. The absorbed substances pass lumen of the GI tract. The absorbed substances pass into blood or lymph and circulate to cells throughout into blood or lymph and circulate to cells throughout the bodythe body

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Functions of the digestive systemFunctions of the digestive system cont…cont…

Defecation:Defecation:Elimination of wastes, indigestible substances, bacteria, Elimination of wastes, indigestible substances, bacteria,

cells sloughed from the lining of the GI tract, and cells sloughed from the lining of the GI tract, and digested materials that were not absorbed through the digested materials that were not absorbed through the anus.anus.

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Neural Regulation of the GIT functions

1.Enteric Nervous system

2.Autonomic Nervous System (Extrinsic GIT Innervations)

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

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Extrinsic GIT InnervationsExtrinsic GIT InnervationsSympathetic:– 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. 1414

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. 1515

4. Hormonal regulation

SomatostatinHistamine

GastrinCholesystokinin (CCK)SecretinGIP (Gastric Inhibitory Peptide)

Motilin

VIP (Vasoactive Intestinal Peptide)GRP (Gastrin Releasing Peptide)NO (Nitric Oxide)

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• 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

GIT Hormones

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• 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

Neurocrines

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Summary of the pathway controlling digestive system activities

MechanoreceptorsChemoreceptorsOsmoreceptors

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