NBA2 Basic aspects of the small intestine: anatomy and physiology
34 important questions on NBA2 Basic aspects of the small intestine: anatomy and physiology
Paristalsis: the organised propulsion of material within the intestinal lumen
Gastrointestinal blood flow - splanchnic circulation
Liver: filters blood, then
the blood vessels of the gastrointestinal system are part of a more extensive system called the splanchnic circulation: receives ± 30% of cardiac output
What are the functions of the digestive system?
- Ingest the food
- break food down into small molecules that can cross plasma membranes (digestion)
- absorb these nutrient molecules (absorption)
- eliminate non-digestible wastes
- tube <-> motility
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Arterial blood supply to the intestines through the mesenteric web
What are the regions of the small intestine?
The average length/inner diamater of the small intestine averages 6m resp. 2.5 cm
- duodenum - 25 cm
- jejunum - 1m
- ileum - 2 m
How is the regulation of blood flow
- During active absorption of nutrients, blood flow in the villi and adjacent regions of the submucosa is increased as much as eightfold
- likewise, blood flow in the muscle layers of the intestinal wall increases with increased motor activity in the gut
- returns to the resting level over another 2 to 4 hours after a meal
- precise causes of the increased blood flow during increased gastrointestinal activity are still unclear, but peptide hormones (CCK, VIP), kinins (kallidin), and oxygen play a role
Our of what does the wall of the digestive tract consist?
From inside to outside
- mucosa
- submucosa
- muscularis (externa)
- circular
- longitudinal
- serosa
What is part of the wall of the small intestine?
three features contribute to increasing its surface area:
- circular folds
- villi
- microvilli
What are the cells in the small intestine
- Enterocyte
- digests and absorbs nutrients [ absorptive cells]
- goblet cell
- secretes mucus
- enter-endocrine cell
- secretes the hormones secretin, cholecystokinin, or GIP
- paneth cell
- secretes lysozyme and is capable of phagocytosis
Where are the cells of the small intestine come from?
and can make
- paneth cell
- goblet cell
- enter-endocrine cell
- enterocyte
Differentiation of cells: role of growth factors
What needs to be know about the muscular layers in the gastrointestinal tract? (+function)
- Function: motility and structure
- Smooth muscle cells
- Contractions of mammalian are preceded by changes in membrane potential differences (mainly Ca2+ in/efflux
- Depolarisation of the membrane: contract
- Hyperpolarization: opposite effect (relaxation)
- two layers
- inner circular muscle layer
- outer longitudinal muscles
- many neural innervations (plexuses)
The possible roles for diet in intestinal cancer initiation
- Normal diet
- calorie restriction
- pro-obesity high-fat diet and obesity
Where/what is the structural relationship of the intestinal musculature and the enteric nervous system
Ganglia: a complex of nerve cells
ICC: interstitial cells of Cajal
- myenteric plexus
- myenteric ICC
- submucosal plexus
Important function is fluid absorption, what is so important?
- 1-2 L each of dietary water, saliva, gastric juice, pancreatic juice, intestinal secretion, 0.5L bile
±80% absorbed in small intestine by isometric transport
- high permeability for water; osmotic equilibrium
What is part of the autonomic nervous system (CNS)
- Sympathetic - fight or flight
- Parasympathetic - rest and digest
- Enteric nerve system (ENS) - GI-tract has its own nerve system [2nd brain]
How does fluid absorption goes?
- Usually a meal is hypertonic and water initially enters the small intestine form the extracellular fluid by osmosis
- Subsequent absorption of fluid depends on the active transport of nutrients and electrolytes
- Driving force: Na+ gradient, maintained by Na/K-ATPase
What are the characteristics of the interstitial cells of Cajal (ICC) [the basic 'rhythm']
- Specialized electrical pacemaker cells associated with the smooth musculature of the GI tract
- Four different types
- Activate smooth muscle cells to contract
- Slow waves of electrical activity spread from one smooth muscle cell to another through gap junctions
- connected to NS: many locally synthesised neurotransmitters act though them
- Facilitate communication between the nervous system and the smooth muscle of the gut wall!
The frequency of ICC pacemaker activity differs in different regions of the GI tract:
- stomach
- duodenum
- ileum
- colon
[interstitial cells of Cajal: slow waves]
- stomach
- 3 per minute
- duodenum
- 11-12 per minute
- ileum
- 8-9 per minute
- colon
- 3-4 per minute
What are the central components of the ENS?
- Two networks (plexuses) of neurons (100 million)
- Embedded in wall GI-tract, from oesophagus to anus
- Myenteric (Auerbach's) plexus
- Submucosal (Meissner's) plexus
What is the structure of the exocrine pancreas
- Acinar cells produce enzymes in isotonic NaCl solution
- Duct cells secrete an isotonic NaHCO3 solution
What are the functions of the two plexuses?
- Myenteric (Auerbach's) plexus
- controls digestive tract motility (control of gut motility)
- Submucosal (Meissner's) plexus
- control absorption and secretion
How to prevent pancreatic autodigestion
- Enzymes in acing cells sequestered in vesicles
- Most enzymes = zymogens (inactive precursors)
- Activated in duodenum by (proenzyme) trypsinogen
- Pancreas produces trypsin inhibitor
How does the ENS work?
- Autonomously (using the local reflex)
- In communication with the sympathetic and parasympathetic nervous system
What is the cellular model of pancreatic duct cell secretion?
- Na+/K+ as in plasma, high HCO3-, low Cl- (1L/day)
- HCO3- secretion requires Cl- recycling: role of CFTR!
- Na+ secreted following HCO3- secretion (neutral); water follows
The autonomous function of the ENS
- ascending network of neurons
- descending network of neurons
- sensory neurons
- receives stimuli form long/ muscle
- receives stimuli form submucosa
sensory neurones --> submucosal plexus +/ myenteric plexus
--> mouvement of your bowl, independent of your own will
Autonomous function of the ENS: peristalsis [propulsion of food]
- Bolus stretches muscles --> activation sensory neuron
- food constituents --> activation (another type) sensory neuron
- interaction with ascending and descending neurons
- behind bolus: --> Asc. Neurons: activating neurotransmitters (ACh): -> contraction circle. Muscle
- Desc. Neurons: inhibiting neurotransmitters (NO): --> relaxation long. muscle
- in front of bolus: vice versa --> Desc. Nuerons: relaxation circle muscle
- asc. Neurons: contraction long. Muscle
Synthesis of bile salts
Agents that are involved in the regulation of intestinal motor activity
CCK; bombesin --> increased release of acetylcholine and substance P
Opioid peptides --> inhibit adenylate cyclase
etc
etc
etc
What are the functions of bile salts: lipid absorption
Micelle formation
What happens when there is communication of ENS with (para)sympathetic NS
- peristalsis
- blood flow
- absorption
- local secretion
Increasing: [parasympathetic]
- peristalsis
- blood flow
- absorption
- local secretion
Why is enterohepatic circulation of bile salts important?
When this goes wrong: pseudo obstruction and Hirschsprung's disease
- Characterized by symptoms of intestinal obstruction in the absence of a mechanical obstruction
- Symptoms: colicky abdominal pain, nausea and vomiting, and abdominal distension
- Cause: degenerative changes in the ENS, resulting in too many contractions
- Hirschsprung (colon): defecation difficulty or failure, resulting in enlarged proximal colon
- Cause: genetic disorder resulting in absence of enteric neurons in colon
Three types of gastrointestinal movement
- Segmentation - mixing
- movement of chyme in both directions, allowing greater mixing with the secretions of the intestines. (contraction of circular muscles)
- Tonic contration - separation (sphincter)
- sustained contraction of the muscle
- Peristalsis - propulsion
- one-way motion (contraction of mainly longitudinal muscles, but also circular muscles)
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