DI Processes Flashcards
Short and Long reflexes
Short reflexes: intrinsic, within GI tract, enteric path
- stimuli → receptors → plexus → effector → response
Long reflexes: extrinsic, outside GI tract, CNS path
- afferent → receptors to CNS
- efferent → autonomic motor (sympathetic and parasympathetic) of CNS → plexuses
Gastro-intestinal regulation - hormonal
Enteroendocrine cells
- one cell surface facing GI tract lumen encounters stimuli
- stimulation → opposite cell surface releasing hormone into bloodstream
- hormone through bloodstream to target cells
- responses can be excitatory in one location and/or inhibitory in another location
- target cells can be in more than one location
Gastro-intestinal regulation - Salivary centre
- Salivary centre in medulla oblongata sends parasympathetic signal mainly to submucosal plexus to create certain amount of salivary gland activity
- no specific stimuli to salivary centre means minimal parasympathetic signal and therefore, minimal salivary gland activity
- this leads to basic level of salivation (saliva production) to keep mouth moist
Gastro-intestinal regulation - Cephalic phase (hunger)
- cephalic stimuli (sight, smell, hunger, taste, even hearing like Pavlov’s dog) send signal to salivary centre
- parasympathetic stimulation increased
- goes to submucosal plexus then salivary gland activity increased
- increased salivation (more volume and composition change with more water and ENZYMES)
- cephalic phase stimulus in stomach as well since parasympathetic sent to prepare stomach for food → response of increased acid secretion and stomach motility
Gastro-intestinal regulation - Cephalic phase (not hungry)
- cephalic stimuli send signal to salivary centre
- increased sympathetic stimulation → submucosal plexus → decreased salivary gland activity → decreased salivation (saliva volume decrease, compositional changes like less water and more MUCUS, dry mouth feel)
Deglutition
- Voluntary or buccal stage - in mouth - voluntary
- bolus pushed into oral pharynx by action of tongue against palate
- Pharyngeal stage - in pharynx - involuntary
- oral pharynx receptors send signal to swallowing centre that bolus is present → swallowing centre sending signals to effectors
- structures moved to blocking positions
- uvula in nasal pharynx
- epiglottis in laryngeal pharynx
- tongue in oral pharynx
- respiratory muscles inhibited and breathing stops briefly
- upper esophageal sphincter relaxed and helps bolus enter esophagus
- structures moved to blocking positions
- bolus enter esophagus, receptors stop signal to swallowing centre and reverse effects above
- oral pharynx receptors send signal to swallowing centre that bolus is present → swallowing centre sending signals to effectors
- Esophageal stage - in esophagus - involuntary
- motility - peristalsis (series of involuntary muscle contractions to move food through gastrointestinal tract)
- circular muscles: contract to constrict above bolus
- longitudinal muscles: contract along esophageal length to push bolus along
- lower esophageal sphincter relaxes and bolus enters stomach
- motility - peristalsis (series of involuntary muscle contractions to move food through gastrointestinal tract)
*lower esophageal sphincter contracts once bolus is in stomach to block backflow into esophagus (regurgitation)
Small intestine - absorption
- lumen of small intestine through apical surface → absorptive cell
- inside of absorptive cell → basolateral surface → IF
- IF → blood capillary (cardiovascular) or a lacteal (immune)
Small intestine - absorption (carbohydrates; CHO)
***polysaccharides -> pancreatic amylase -> maltose before…
- ingested disaccharides → brush border enzymes (attached to apical membrane) → monosaccharides
- disaccharide sucrose → glucose - fructose by sucrase (brush border enzyme)
- disaccharide lactose → glucose - galactose by lactase (brush border enzyme, lactase limited = lactose intolerance)
- disaccharide maltose → glucose - glucose by maltase (brush border enzyme)
*some carbohydrates are insoluble (ex. soluble and insoluble dietary fibers) so they will go to large intestine
- monosaccharides cross apical membrane into epithelial cell
- fructose by facilitated diffusion with GLUT (glucose transporter)
- glucose by secondary active transporter involving sodium (SGLT)
- galactose by secondary active transporter involving sodium (SGLT)
- all cross basolateral membrane from inside epithelial cell by facilitated diffusion with various GLUTs (14 different types in humans) to IF
- IF → blood capillaries by simple diffusion
Small intestine - absorption (proteins; PRO)
Process!!!
- pancreatic proteases and peptidases → proteins → …
- Top path
- small peptides cross apical membrane into epithelial cell by secondary active transporter involving HYDROGEN
- small peptides then broken down into amino acids by peptidases once inside epithelial cell
- Bottom path
- single amino acids or small peptides broken into single amino acids by brush border enzymes/peptidases attached to apical membrane
- single amino acids cross apical membrane by secondary active transporter involving SODIUM
- inside epithelial cell
- single amino acids cross basolateral into IF by facilitated diffusion (involves amino acid transporters)
- once crossed basolateral membrane
- IF → blood capillary by simple diffusion (not on picture)
Points!!!
- Small peptide → single amino acids occurs in…
1. in lumen
2. cross into epithelial cell before breakdown
- breakdown location depends on amino acids that make up small peptide (around 20 different amino acids in humans)
- top path is for small peptides crossing, bottom path is for amino acids crossing
- two possibilities for bottom path, either directly amino acids from proteins or small peptides broken down into amino acids by the brush border enzymes
Small intestine - absorption (fats)
- fats not very soluble in watery intestinal juices so they react by forming fat globules to minimize surface area
Process
- emulsification is first digestive step
- small amounts of triglycerides from fat globule coated in bile salts and phospholipids to form smaller emulsification droplets
- bile salts and phospholipids have polar side out and non-polar side in (makes sense because non-polar = fat liking and it’s a fat globule basically)
- arrangement repels other emulsification droplets so decrease chances of reforming a larger fat globule
- fat globule → emulsification droplet exposes more surface area for pancreatic lipase digestion (triglycerides to monoglycerides and fatty acids)
- monoglycerides and free fatty acids can cross apical membrane from lumen to inside epithelial cell by simple diffusion
- too many being released could reform triglycerides which cannot cross apical membrane
- forming micelle is temporary measure for this
- bile salts and phospholipids surround monoglycerides and free fatty acids with polar side facing out to prevent reforming triglyerides (similar to making emulsification droplets but smaller with a core of monoglycerides and free fatty acids
- this helps to control amount of monoglycerides and free fatty acids waiting to diffuse across the apical membrane
- micelle acts like a ferry and moves near apical membrane before breaking down so monoglycerides and free fatty acids are in prime position to be absorbed
- once inside epithelial cell, monoglycerides + free fatty acids = triglycerides reformed
- triglycerides further form with phospholipids and cholesterol to form chylomicron (lipoprotein for transport)
- chylomicrons are in a vesicle
- this vesicle moves and fuses with basolateral membrane and the membrane opens up and releases the chylomicrons by exocytosis into the IF
- chylomicrons are too big for blood capillaries so they will go to lacteals
Small intestine - absorption (electrolytes)
- Sodium moves largely coupled to glucose and amino acids
- most negative charge ions like chloride and bicarbonate move by following positive sodium ions (electrical gradient)
- other ions move by concentration gradients created by water movement (water moves to even out osmotic pressure after food particles that act as solutes come in then also follow when those solutes are absorbed)
- most ions that reach small intestine will be absorbed
- 2 ions move based on nutrient needs
- calcium
- based on blood levels
- transcellular active transport involves release of parathyroid hormone to increase calcium absorption
- parathyroid hormone converts vitamin D to calcitriol
- paracellular passive transport based on concentration gradient
- iron
- based on stored levels in epithelial cells
- transcellular active transport with specific iron transporters
- women have more iron transporters than men due to menstrual cycle increasing loss of epithelial cells
- calcium
