General principles of gastrointestinal system function

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General principles of gastrointestinal system
function
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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.

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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)

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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
• 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
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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.

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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)

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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

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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

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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.

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Neural Regulation of the GIT functions

• Enteric Nervous system
• Autonomic Nervous System (Extrinsic GIT
  Innervations)

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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.

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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.

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• Parasympathetic
• The dorsal motor nucleus of the vagus supplies
  efferent vagus nerve fibers toLower 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.

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4. Hormonal regulation
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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

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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

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Summary of the pathway controlling digestive
system activities
Mechanoreceptors Chemoreceptors Osmoreceptors