Introduction to Gastrointestinal Physiology, Motility and Mechanical Digestion Flashcards
Why do we need the GIT?
We can only absorb small molecules
Mechanical digestion: breaks the food into smaller and smaller pieces (mouth: chewing, stomach: retropulsion)
Chemical digestion: breaks the large nutrient molecules into smaller and smaller pieces which can be absorbed (acid and digestive enzymes)
What are the major functions of the gastrointestinal tract (GIT)?
to bring nutrients and water into the internal environment to be used by the body
- motility: movement of food of chyme within the GIT caused by contraction and relaxation of smooth and skeletal muscle
- secretion: release of fluid, mucus, acid, bicarbonate and digestive enzymes from epithelial cells or accessory organs into the lumen of the GIT
- Digestion: breaking food and nutrients into smaller parts (mechanical digestion: breaking food into smaller particles. chemical digestion: breaking nutrients into smaller molecules)
- absorption: net movement of substances from the lumen of the GIT, across the epithelial lining of the intestine, into the interstitial fluid and then into the blood or lymph
describe the regulation of the GIT
Receptors in the wall of the GIT respond to:
- mechanoreceptors are activated by stretch: food/chyme in the lumen
- chemoreceptors are activated by changes in composition: pH, osmolarity, amino acids, sugars, fats
Receptors signal the GIT regulatory systems to control to conditions in the intestinal lumen:
- nervous system: enteric, parasympathetic and sympathetic nervous systems
- hormones
Regulatory systems change the activity of the GIT effectors:
- smooth muscle: change in motility
- glands/secretory cells: change in amount or type of secretion
describe how the nervous system regulates the GIT
Enteric nervous system (ENS):
Nervous system of the GIT:
- totally self contained
- can function independently of the CNS
Submucosal plexus:
- regulation of secretion
Myenteric plexus:
- regulation of motility
Central nervous system:
Parasympathetic NS: “rest and digest”
- increases the overall activity of the GIT but activating the ENS
- stimulates motility and secretion
Sympathetic NS: “fight, flight or freeze”
- decreases the overall activity of GIT by inhibits the ENS
- inhibits motility and section
Coordinates ENS activity over long distances
Two reflex pathways for neural regulation:
- local/short reflex pathway: ENS only
- CNS and Long reflex pathway: CNS to ENS and ENS to CNS
describe how the nervous system and hormones regulate the GIT
Nervous system regulation:
- generates fast responses
- Enteric nervous system (ENS) and central nervous system (CNS - sympathetic and parasympathetic)
Hormonal regulation:
- slower, but longer lasting and can affect multiple parts of the GIT at once
- Endocrine cells: enteroendocrine cells
- epithelial cells with receptors
- release hormones
- Important GIT hormones: Gastrin, Secretin, Cholecystokinin (CCK), Gastric inhibitory peptide (GIP)
Combination of neural and hormonal:
- responses the start quickly but last longer
- can coordinate responses across different parts of the GIT
Describe the local/short reflex pathway that regulates the GIT
Function: to return the human of the GIT back to its resting state
Stimulus: a change in the contents of the GIT lumen die to the arrival or departure of food/chyme
Receptors: detect change in local conditions, stretch and chemical composition, of the GIT
ENS response: occurs vis short LOCAL reflex pathways. acts on GI tract smooth muscle GI tract and epithelia/glands
- Change in the contents of the GIT lumen
- The arrival o food/chyme changes the: stretch, pH, osmolarity, presence of nutrient: proteins, lipids, carbohydrates - Detected by reports on GIT epithelial cells
- chemo-, osmo- and mechanoreceptors - Response coordinated by the enteric nervous system (ENS)
- myenteric plexus: change in motility
- submucosal plexus: change in secretion and/or absorption - returns the contents of the GIT lumen to resting state (empty)
- digestion/absorbtion complete in (this) part of GIT
- chyme moves to the next part of the GIT
describe the CNS and long reflex pathway that regulates the GIT
- sight, smell or tasting food and emotional responses
Function 1: to integrate the response of the GIT to external stimuli (smell) and/or changes in the rest of the body
Function 2: to return the lumen of the GIT back to its resting state
Sympathetic NS: reflexes that generally inhibit GI function
Parasympathetic NS: reflexes that generally stimulate GI function
- change in the contents of the GIT lumen
- detected by receptors on GIT epithelial cells
- activates either CNS or ENS directly - Central nervous system activates either the:
- sympathetic NS: fight or flight, decreased GIT activity
- parasympathetic NS: rest and digest, increased GIT activity - either of those systems send signals to the ENS which coordinates a response
- submucosal plexus: change in secretion and/or absorption
- myenteric plexus: change in motility - GIT lumen
define the four motility in the GIT definitions
Tonic contractions: sustained contractions for minutes to hours
- eg. sphyncters
Phasic contractions: waves of contraction and relaxation, each wave lasting seconds
- eg. peristalsis, segmentation
Motility pattern: any pattern of contraction or relaxation of GI tract smooth muscle
- some motility patterns are a combination of two or more sphincter patterns (eg. restropulsion: peristalsis and contraction of the pyloric sphincter)
- relaxation is also a motility pattern (eg. storage in the stomach: receptive relaxation and accomodation)
Movement/propulsion: motility patterns that specifically move contents along the GIT
Describe phasic contractions
- smooth muscle performs these contractions
- controlled by pacemaker cells (spontaneously contracts and relaxes without external output)
- frequency of contraction is a property of the region of the GIT
- stomach 3 contractions per min
- duodenum 12 contractions mer pin
- ileum 9 contractions per min
- DOES NOT CHANGE
- strength of contraction is regulated by nervous and hormonal input
Describe the motility patterns of motility in the GIT when fed
When Fed:
- chewing: mechanical digestion of food
- peristalsis: movement of food/chyme along the GIT
- relaxation: storage of food/chyme by increasing the volume without an increase in pressure
- segmentation: mixing of chyme with secretions and exposure to absorptive surfaces
- contraction or relocation of sphyncters: prevents backwards movements and controls rate of forward movement of food/chyme
describe the motility patterns of motility in the GIT when fasting
Fasting
Migrating motor complex (MMC):
- location: begins in the stomach and travels to the small intestine
- functions: housekeeping
- removes residual secretions
- removes undigested material
- promotes epithelial cell turnover
Occurs 4 hours after a meal
- 45-60 min of inactivity
- 30 min intermittent uncoordinated activity (to help loosen everything up)
- 5-15 min intense coordinated peristaltic contractions
- repeats every 1.5-2 hours until food is consumed
describe motility in the mouth
Chewing (mastication):
Initial step of mechanical digestion
- reduces the size of food particles, to allow ingestion/swallowing
- voluntary: skeletal muscle but reflex control of strength and frequency/rhythm
- mixes food with saliva: taste, lubrication and protection
describe motility in the esophagus
Swallowing:
- rapid transfer of material from mouth to stomach
- initiated under voluntary control
- continues via reflexes
Prevention of food going into the wrong place:
- into the nasopharynx: soft palate and uvula rise
- into the trachea: folding of the epiglottis
Relaxation of sphincters:
- location: upper and lower oesophageal sphyncters open
- function: allows food to move freely down esophagus into the stomach
Peristalsis:
- location: esophagus
- function: wave of peristalsis moves food down esophagus to the stomach
describe the gastric/stomach patterns of motility
Relaxation:
- location: fundus and body of stomach
- function: storage
Retropulsion: peristalsis and contraction of pyloric sphincter
- locations: body to pyloric antrum and pyloric sphyncter
Contraction and relaxation of pyloric sphincter:
- location: pyloric sphincter between stomach and SI
- functions: gastric emptying and controlled delivery of chyme to the duodenum
describe relaxation as part of the gastric/stomach patterns in more detail
Relaxation
location: fundus and body of stomach
function:
- storage: of food while it is undergoing digestion in the stomach
- allows the stomach to increase in volume from fasting volume to hold a meal with minimal change in the pressure within the lumen
- prevents reflux of food/chyme into the esophagus
Two types of relaxation:
Receptive relaxation:
- part of swallowing, the stomach is about to receive food and relaxes
Accomodation:
- food is in the stomach (has arrived) and the stomach needs to find space/accomodation for it
describe retropulsion as part of the gastric/stomach patterns in more detail
Retropulsion: peristalsis and contraction of pyloric sphincter:
locations: pyloric antrum and pyloric sphincter
functions:
- completion of mechanical digestion: physical break down of food particles into smaller particles, resulting in liquid ‘pureed’ chyme
- mixing: of food/chyme with stomach acid enzymes to help chemical digestion
- the waves of peristaltic contractions grind the food particles against the closed pyloric sphincter
- the chyme is pushed backwards (retropulsion) from the pyloric antrum to the body of the stomach
propulsion: forwards
retropulsion: backwards
describe peristalsis and contraction of the pyloric sphyncter as part of gastric/stomach motility patterns
peristalsis:
- contractions begin in the body of the stomach and spread to antrum
- 3 contractions per min
- first 60 min following meal gentle, 60-300 min more intense activity
contraction of pyloric sphincter:
- closed during gentle contractions
- during intense contractions starts to partially relax, allowing small spurts of liquid chyme into the duodenum
describe contraction and relaxation of the pyloric sphyncter as part of gastric/stomach motility patterns
contractions and relaxation of the pyloric sphincter
location: pyloric sphyncter
functions:
- gastric emptying: controlled movement to fully mechanically digest chyme form the stomach into the duodenum
- the rate of gastric empyting is adjusted to match the digestive capacity of intestines
- at the correct rate to match with release of bicarbonate, fluid, bile and digestive enzymes
The rate of gastric emptying can be sped up or slowed down, using feedback from the stomach and the duodenum (main mechanism)
describe how the stomach feeds back as part of gastric/stomach motility patterns
stomach: factors affecting the rate of gastric emptying
- size of meal: the larger the meal, the greater the stretch/distension, and faster the emptying
- composition of meal: fluids faster than solids as solids require reduction in the size of for particles to increase the surface area of the food, mechanical digestion of food takes longer
describe how to duodenum feeds back as part of gastric/stomach motility patterns
duodenum: factors affecting the rate of gastric emptying
Fats, acid or a change in the osmolarity of the duodenum:
- detected by chemoreceptors osmoreceptors and enteroendocrine cells
- indicates that chyme from the stomach has arrived in the small intestine
- fats: beacuse they take longer to digest and require bile
- acid: need to neutralise by releasing the correct amount of bicarbonate (HCO3-)
Activated:
- chemo- and osmoreceptors trigger the short and long enterogastric reflexes
- enteroendocrine cells in the duodenum secrete hormones (CCK and secretin)
- decreases the force of peristalsis in the stomach
- slows gastric emptying so the small intestine and accessory organs (liver, gall bladder and pancreas can keep up
describe the motility patterns in the small intestine
segmentation:
- location: throughout small intestine
- functions: mixing, exposure to absorptive surfaces, some movement
peristalsis (propulsion):
- location: throughout small intestine
- function: movement of chyme
Contraction and relaxation of ileicecal sphincter:
- location: ileocecal sphincter between small and large intestines
- functions: delivery of chyme form the small to large intestine
describe segmentation and peristalsis of the small intestine in more detail
Segmentation (main pattern):
location: throughout the small intestine
function:
- exposure to absorbitive surfaces to increase absorption of water, Na+ and nutrients from chyme
- some movement along small intestine
- mixing of chyme with secretions form the pancreas, biliary system (liver and gall bladder) and small intestine mucosa to increase chemical digestion of nutrients
Peristalsis (propulsion):
location: throughout small intestine
functions: movement of chyme along small intestine
- some weak peristalsis during absorption
- main peristalsis occurs after most nutrients have been absorbed
describe the colonic/large intestine motility patterns
large periods of inactivity
segmentation:
- location: throughout the large intestine
- function: exposure to absorptive surfaces (absorption of water and Na+ from chyme turning liquid chyme into semi-solid faeces)
Relaxation:
- location: descending and sigmoid colon
- function: storage of faeces
Peristalsis (propulsion):
- peristaltic wave
- mass movement of faeces 1-2 times a day following meals
- drives faeces into rectum and initiates defecation
