DI Learning Objectives Flashcards
DI 1: 6 basic processes.
- Ingestion: entry to GI tract
- Motility: mixing caused by movement/propulsion
- Secretion: water, enzymes, environmental factors contributing
- Digestion: breakdown using mechanical and physical methods
- Absorption: entry to body
- Defecation: exit to GI tract
DI 1: Neural pathways, receptors, effectors, and responses involved in enteric nervous system.
Neural pathways
- Short reflexes: stimuli → receptors → plexus → effector → response
- long reflexes: afferent (receptors to CNS) → efferent (autonomic motor, sympathetic and parasympathetic) → CNS → plexuses
- digestive system has special component in autonomic motor called enteric which is the GI tract’s own local neural control with more neurons than the spinal cord!
Enteric nerve plexuses (myenteric and submucosal)
- connections for communication (in plexus and between plexuses, receptors → plexuses or CNS to then plexuses)
- responses
- myenteric: especially to circular/longitudinal muscles
- submucosal: especially to epithelial cells
- receptors
- mechanoreceptors: distension (from food entering GI tract)
- osmoreceptors: osmotic pressure (food particles are the solutes disrupting the osmotic pressure)
- chemoreceptors: specific nutrient concentrations and acidity (ex. amino acids or fatty acids)
- effectors
- smooth muscle contraction: circular and longitudinal muscle layers
- epithelial cells (inner layer of GI tract facing lumen)
- enteroendocrine cells: hormonal secretions
- one cell surface faces GI tract lumen → stimuli encountered → opposite cell surface from GI tract lumen releases hormone into bloodstream → hormone travels to target cells → target cells in more than one body location → responses can be excitatory in one location and/or inhibitory in another location
- key hormones
1. gastrin
2. secretin
3. CCK (cholecystokinin)
4. somatostatin
- exocrine cells: enzyme and environmental factor secretions
- mucous cells: mucus secretions
DI 1: Pathways of cephalic phase
- Cephalic stimuli: sight/smell/hunger, taste (relaxed = parasympathetic)
- stimuli sends signal to salivary centre in medulla oblongata
- increases parasympathetic stimulation to submucosal plexus causing increased salivary gland activity
- increases salivation (saliva volume increasing and compositional change with more water/enzymes)
- some cephalic stimuli has no food present (ex. Pavlov’s dog)
- also cephalic phase stimulus to stomach: parasympathetic to stomach to prepare for food coming → increased acid secretion and stomach motility
- Cephalic stimuli: stress/dehydration (stressed = sympathetic)
- stimuli sends signal to salivary centre in medulla oblongata
- increased sympathetic stimulation to submucosal plexus causing decreased salivary gland activity
- decreases salivation (saliva volume decreases, compositional change with less water/more mucus, dry mouth feel)
DI 1: Components of saliva and process of mastication.
Saliva
- salivary centre in medulla oblongata
- parasympathetic signal sent to mainly submucosal plexus to create a certain amount of salivary gland activity
- no specific stimuli to salivary centre → minimal parasympathetic signal to submucosal plexus and minimal salivary gland activity
- basic level of salivation (saliva production) to keep mouth moist
Saliva composition
- 99% water to moisten food and tissues/taste/rinses mouth to help prevent bacterial growth
- mucus: lubricates food, mouth, pharynx to facilitate swallowing
- bicarbonate: help neutralize food acids (environmental: mouth slightly acidic)
- lysozymes: kill bacteria to help prevent bacterial growth
- enzymes: start of chemical digestion
- salivary amylase
- polysaccharides → disaccharide maltose
- operate best in slightly acidic environment (from mouth into fundus of stomach)
- lingual lipase
- triglycerides → monoglyceride and fatty acids
- operate best in highly acidic environment (body and antrum of stomach)
Mastication (chewing, mechanical digestion)
- involves: skeletal muscles and activity of tongue, lips, and cheeks + a combination of voluntary and involuntary mastication patterns and reflexes (rhythmic jaw movements)
- goals
- physical digestion → more SA exposed
- motility→ mix with enzymes added in mouth to start chemical digestion
- forming bolus (semisolid mass) for swallowing
DI 2: Stages of deglutition
- voluntary or buccal stage, in mouth, voluntary
- bolus pushed into oral pharynx by tongue against palate
- pharyngeal stage, in pharynx, involuntary
- oral pharynx receptors signal → swallowing centre about presence of bolus → swallowing centre sending signals to effectors
- structures → blocking positions (uvula in nasal pharynx, epiglottis in laryngeal pharynx, tongue in oral pharynx) → prevent bolus going to wrong place
- inhibit respiratory muscles → stop breathing briefly
- relax upper esophageal sphincter → help bolus enter esophagus
- bolus entering stomach → receptors stop signal to swallowing centre
- structures move back to non-blocking positions
- respiratory muscles re-engage → breathing resumes
- upper esophageal sphincter contracts → block backflow into pharynx
- oral pharynx receptors signal → swallowing centre about presence of bolus → swallowing centre sending signals to effectors
- esophageal stage, in esophagus, involuntary
- motility - peristalsis (way of moving things along)
- circular muscles: contract to constrict above bolus (push down)
- longitudinal muscles (contract along esophageal length to push bolus along)
- lower esophageal sphincter relaxes → bolus entering stomach
- motility - peristalsis (way of moving things along)
DI 5: Migrating myoelectric complex in small intestine.
- takes over for segmentation when absorption is mostly completed
What’s involved?
- peristalsis begining in stomach antrum
- peristalsis, re-establish, peristalsis, etc in distances of around 60cm
- this is like peristalsis steps and takes around 2 hours to reach ileum
- will return from ileum to stomach antrum to reset migration if chyme is still in the small intestine
- function: to “sweep” remaining material that wasn’t absorbed to minimize what’s left in small intestine to prevent bacterial growth
DI 5: Relaxation of ileocecal valve.
For movement from small to large intestine, triggered by…
- gastroileal reflex (neural): signal from stomach along nerve plexus to valve
- gastrin (hormonal) release and binding to valve
DI 5: Digestion and absorption in large intestine.
Digestion
- chemical: secretions minimal and lacking digestive enzymes, mostly mucus and water secretion for lubrication and to form feces
- bacterial: bacterial population in large intestine
- ferment some carbohydrates that were not absorbed to produce gases (farts)
- small amounts of vitamins (vitamin B complex and vitamin K) synthesized
- some undigested fiber converted to short chain fatty acids for fuel (brain especially likes these)
Outcomes
- absorption: some water and electrolytes + bacterial products for the remaining 10% of absorption in large intestine
- feces formation: mostly made of unabsorbed material, indigestible food, water, and bacteria
- storage: non-absorbed material in distal large intestine until defecation
Di 5: Mechanical processes in large intestine.
Haustral churning (large intestine equivalent of segmentation)
- motility: mixing and bringing content into contact with absorptive surfaces
- haustra filling → distension of walls → reach a certain volume → smooth muscle contraction → pushing material to adjacent haustra (back and forth like segmentation)
Peristalsis (same circular and longitudinal muscles)
- weak version
- exception is mass movements that occur around 3-4 times per day
- intense peristalsis from transverse colon to rectum to sweep material along like MMC
- triggered by:
1. gastrocolic reflex: neural, signal from stomach along nerve plexus to large intestine
2. gastrin: hormonal, release and binds to large intestine
- often stimulates defecation reflex as material is moved into rectum and causes distention
DI 5: Steps in defecation reflex.
Start: rectal muscles relaxed, anal sphincters contracted
1. mass movement moves feces into rectum → rectal wall distension
2. mechanoreceptors detect increased stretch → defecation reflex
3. defecation reflex signal sent to spinal centres
4. parasympathetic response sent back → contraction of rectum and relaxation of internal anal sphincter → feces moving into anal canal
5. higher brain centre relaxation of external anal sphincter → feces voided (helped by intra-abdominal pressure and expiration against closed epiglottis)
Delayed defecation
1. over-ride external anal sphincter relaxation → defecation prevented
2. involuntary rectal wall contraction → pressure to move feces back into sigmoid colon
3. temporarily stop defecation reflex since rectum no longer distended
4. contents come back into rectum (probably more material now) → restarting defecation reflex
DI 5: Metabolism – metabolic rate / heat generation.
Metabolic rate: amount of energy expended over given period of time
- anabolism: building reactions (ex. anabolic steroids for building muscle)
- catabolism: breakdown reactions
Basal metabolic rate: energy needed for most essential physiological activities (keeping you alive)
Total metabolic rate: total daily energy expenditure composed of:
- BMR: 60%
- physical activity: 30-35%
- food induced thermogenesis (digestion, absorption, storage): 5-10%
Heat generation: homeostasis around a core temperature
- core temperature decreases → produce or conserve heat
- vasoconstriction skin blood vessels especially in extremities (blood diverted from surface to core)
- epinephrine, norepinephrine, thyroid hormones released and stimulate metabolic rate
- shivering to cause muscular activity
- behaviour: clothing choices, hot fluids, increased activity
- core temperature increases → lose heat
- vasodilation skin blood vessels especially to extremities to bring blood to surface
- enhanced sweating for greater evaporative loss
- behavioural: clothing choices, cold fluids or location, reduced activity
Di 5: Basics of absorptive and postabsorptive states.
Carbohydrates: glucose
-Absorptive state: fueld stored as glycogen in muscle and liver, excess is converted and stored as triglycerides
-post absorptive: glycogen breakdown in liver and muscle to glucose, lactic acid converted to glucose
Fat: triglycerides
-absorptive state: form is cell membranes/insulation/protection, fuel, stored as triglycerides
post-absorptive: glycerol converted to glucose, free fatty acids for fuel and also converted to ketones for fuel
Protein: amino acids
-absorptive: form is structural proteins for tissues and functional proteins for enzymes, antibodies, and hemoglobin, fuel under certain conditions, excess is converted to triglycerides
-post-absorptive: amino acids converted to glucose
