Gastrointestinal Tract 1 - general intro Flashcards
Digestion
- Chemical alteration of food into absorbable molecules
- GI motility, pH changes, biological detergents and enzymes.
Absorption
Movement of digested food from the intestine into the blood or lymphatic system
Does the GI tract have an immune system?
YES
Components of the GIT
— Mouth
- Pharynx
- Esophagus
- Stomach
Small intestine
- Duodenum
- jejunum
- Ileum
Large intestine
Accessory organs
- Pancreas
- Liver
- Gall bladder
Structure of the GIT
- Long muscular tube stretching from mouth to anus.
- Composition is similar from mid-esophagus to anus
Layers of the GIT
- Mucosa
- Submucosa
- Muscularis externa
- Serosa
Layers of mucosa
a) Epithelium
b) Lamina propria
c) Muscularis mucosa
Layers of mucosa
a) Epithelial layer
- layer of cells that lines all body cavities and surfaces.
“Polarized” = different at one surface compared to the other
- Basolateral and apical arrangement, each with different transport proteins
- Tight junctions confine transport proteins to specific membrane regions
Which layer of the mucosa contains the villi and the crypts? And if so what do they do?
Epithelium layer.
Increase SA, stem cells from crypts divide and migrate upwards.
Selective Transport of Nutrients Across the Epithelium
Paracellular pathway is limited by tight junction seal
- Water and small ions can diffuse through tight junctions
Transcellular pathway is a two step process and requires a transport protein on the apical and basolateral surface of the cell
Layers of the mucosa
b) Lamina propria
- connectivetissue
- smallbloodvessels
- nervefibres(slide16)
- lymphaticvessels
- immune and inflammatory cells
Layers of the mucosa
C) Muscularis mucosa
Thin layer of smooth muscle
- Not involved in contraction of the GIT
- May be important in villi movement
- Submucosa
- Plexus of nerve cell bodies
- Relay information to and away from the mucosa
- Also composed of connective tissue, blood and lymphatic vessels
- Muscularis Externa
Thick inner layer of circular muscle
- Fibres oriented to cause narrowing of lumen
Myenteric nerve plexus
- Network of nerve cells, regulate muscle function
Thinner outerlayer of longitudinal muscle
- Fibres oriented to shorten tube
- Serosa
Connective tissue, encases intestine and forms connection point to the abdominal wall
Blood supply to the GIT
Blood perfuses intestine and then flows to liver via the portal vein
Portal circulation
- The portal vein drains blood from the digestive tract and empties directly into the liver.
Purpose: allow the liver to:
- Remove harmful substances (filter)
- process nutrients
circulation of nutrient-rich blood between the gut and liver is called the
portal circulation
Liver is an unusual organ because it receives blood from both
venous (portal) and arterial circulation
Liver receives less
oxygenated blood, more nutrient rich blood than other organs.
Regulation of GIT is governed by
volume and composition of lumen contents
Reflexes in the lumen initiated by
- Distension of wall by volume of luminal contents
- Osmolarity of contents
- pH of contents
- Concentrations of specific digestion contents
(Monosaccharides, fatty acids, peptides, and amino acids)
The reflexes in the GIT are reproduced by
- Mechanoreceptors-activated by mechanical stimuli such as pressure and stretch
- Osmoreceptors -activated by changes in osmolarity
- Chemorecpetors - activated by binding of certain chemicals
Intrinsic Neural Regulation of GI Processes via
Enteric nervous system
Enteric nervous system
Controls the activity of the secretomotor neurons
(motility and secretary funct.)
- contained within walls of GIT
- independent of CNS
The two nerve networks that compose the Enteric nervous system, and what they each do?
- Myenteric plexus-influences smooth muscle
- Submucosal plexus- influences secretion
Extrinsic Neuronal Regulation of GI Processes
Regulation is through the autonomic nervous system
- Parasympathetic
- Sympathetic
Influences the motility and secretion of the GIT
- Hunger
- Sight/smell of food
- Emotional state
Parasympathetic does what to the GIT?
- stimulates peristalsis and secretion in stomach
- stimulates release of bile in liver
- stimulates saliva flow
sympathetic does what to the GIT?
- inhibits peristalsis and secretion in stomach
- conversion of glycogen to glucose in liver
- stimulates saliva flow
Four categories of chemical messengers
- Endocrine: chemical messenger passes from cell which produced it into blood and is carried by blood to its (relatively distant) target
- Neurocrine: chemical messenger is released from a nerve, travels across a synapse and acts on a post-synaptic target cell
- Paracrine: chemical messenger diffuses through intestinal fluid to nearby cells
- Autocrine: chemical messenger acts on the cell which produced it
Endocrine cells are scattered throughout the
epithelium of the stomach and small intestine.
One surface of each endocrine cell exposed to the GI lumen, meaning ?
- Chemical substances in lumen stimulate cell to release hormones across opposite surface of cell into blood vessels in lamina propria
- Hormones travel through blood to target cells
Four best understood GI hormones are
- secretin
- cholecystokinin (CCK)
- gastrin
- glucose dependent insulinotropic peptide (GIP)
(ALL PEPTIDES)
- secretin
- cholecystokinin (CCK)
- gastrin
- glucose dependent insulinotropic peptide (GIP)
Do what?
- Each participates in a feedback control system that regulates some aspect of the GI lumen
- Most GI hormones affect more than one type of target cell
Fatty acids and amino acids in the small intestine triggers
CCK
secretion from cells in small intestine into blood.
Circulating CCK stimulates:
- pancreas to increase digestive enzyme secretion
- gall bladder contraction
(release bile acids = fat breakdown
Fats and amino acids are absorbed and stimulation of CCK release is stopped-due to their removal, therefore it is a
Negative feedback control system
Gastrin
- Stomach antrum (G cells)
- Stimuli for release: Peptides/amino acids in stomach, parasymp nerves
- HCl increases
- motility increases (stomach, ileum, lrg intestine)
Secretin
- small intestine (S cells)
- Stimuli for release: acid in small intestine (pH<4.5)
- HCl decreases
- motility decreases (stomach, ileum, lrg intestine)
- HCO3 / H20 increase from pancreas and in bile
CCK *
- small intestine (I cells)
- Stimuli for release: digested fat/protein in small intestine
- HCl decreases
- motility decreases (stomach, ileum, lrg intestine)
- enzyme from pancreas increase
- bile expulsion increase
GIP*
- Stomach antrum (K cells)
- Stimuli for release: glucose or fat in small intestine
- increase insulin
What allows for the movement of contents form one site to another in the GIT
Contraction and relaxation of the two outer smooth muscle layers
Peristalsis (propulsion)
- Circular muscle contracts on the oral side of a bolus of food (longitudinal
layer relaxes). - Circular muscle contracted moves toward the anus, propelling the contents of the lumen in that direction,
- as the ring moves, the circular muscle on the other side of the distended area relaxes (longitudinal muscle contracts), facilitating smooth passage of the bolus
Segmentation (mixing)
- Contraction and relaxation of intestinal segments (not towards LI)
- Mostly in small intestine
- Allows mixing with digestive enzymes
- Slows transit time to allow absorption of nutrients and water
GIT has pacemaker cells throughout
smooth muscle cells
The pacemaker cells are Constantly undergoing spontaneous depolarization-repolarization cycles, these cycles are called
slow waves: basic electrical rhythm
Slow waves propagated through the
circular and longitudinal muscle layer through gap junctions
In the absence of neural/hormonal input spontaneous slow waves
don’t result in significant contractions
- Excitatory hormones/neurotransmitters further depolarize the membrane and bring it close to threshold
number of AP and the force of contraction relationship
- # of action potentials fired = to force of contraction
- frequency of contraction maintained by the basic electrical rhythm
- force of contraction mediated by neuronal and hormonal input
