Lecture 24 Flashcards
Motility and mechanical digestion
What is the overall function of the GIT?
To bring nutrients and water into the internal environment to be used by the body
What are the four major functions of the GIT?
Motility, secretion, digestion, absorption
What is the motility function of the GIT?
The movement of food to chyme within the GIT caused by contraction and relaxation of smooth and skeletal muscle
What is the secretion function of the GIT?
To release fluid, mucus, acid, bicarbonate and digestive enzymes from epithelial cells or accessory organs into the lumen of the GIT
What is the digestion function of the GIT?
To break food and nutrient into smaller parts
What are the two types of digestion?
Mechanical and chemical
What is mechanical digestion?
Physically breaking food into smaller pieces
What is the form of mechanical digestion in the mouth
Chewing
What is the form of mechanical digestion in the stomach?
Retropulsion
What is chemical digestion?
Breaking nutrients into smaller molecules through bond breaking
How does chemical digestion occur?
Using acid and digestive enzymes
What is the absorption function of the GIT?
The 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
What are the regulators of the GIT?
The receptors in the walls on the GIT
What are the two receptors in the GIT?
Mechanoreceptors and chemoreceptors
What activates the mechanoreceptors?
The stretch from food/chyme in the lumen
What activates the chemoreceptors?
A change in the composition of the food - pH, osmolarity, amino acids, sugars, fat
What do the receptors activate?
The regulatory system
What are the two regulatory systems?
Nervous system and endocrine system (hormones)
What is the nervous systems response?
Fast responses and contains two reflex pathways for neural regulation
What are the two reflex pathways?
Local/short and CNS with the long reflex pathway
What is the component of the local/short reflex pathway?
The enteric nervous system only
What is the component of the CNS / long reflex pathwy?
CNS → ENS / ENS → CNS → ENS respectively
What is the ENS?
The nervous system of the GIT, totally self-contained and can function independently of the CNS. Contains submucosal plexus and myenteric plexuses
Where is the submucosal plexus and what does it control?
Just under the mucosa, to regulate secretion
Where is the myenteric plexus and what does it control?
Between layers of smooth muscle, to regulate motility
What are the two components of the CNS?
The parasympathetic and sympathetic nervous systems
What does the PNS control?
The ‘rest and digest’ aspect. This increased the overall activity of the GIT by activating the ENS. It stimulates motility and secretion
What does the SNS control?
The ‘fight, flight or freeze’ aspect. This decreases the overall activity of the GIT by inhibiting the ENS. It inhibits motility and secretion
What is the endocrine systems response?
Slower, but longer lasting responses and can affect multiple pasts of the GIT at once
What is the special name for the endocrine cells in the ENS?
Enteroendocrine cells
What are the four important GIT hormones?
Gastrin, secretin, CCK and GIP
What effectors do these regulatory systems change the activity of in the GIT?
Smooth muscle cells and gland / secretory cells
What is the function of the local and short reflex pathway?
To return the lumen of the GIT back to its resting state
What is the stimulus of the local and short reflex pathway?
A change in the contents of the GIT lumen due to the arrival or departure of food/chyme
What is the path of response of the local / sort reflex pathway?
- Change in the contents of the GIT lumen
- Detected by receptors on GIT epithelial cells
- Response coordinated by the ENS
- Myenteric plexus (a)
- Submucosal plexus (b) - Change in motility of GIT smooth muscle (a)
- Change in secretion &/or absorption of GIT epithelium and glands (b)
- Returns the contents of the GIT lumen to resting state (empty)
What is the function of the CNS and long reflex pathway?
To integrate the response of the GIT to external stimuli (smell) and/or changes in the rest of the body (stress/danger) and to return the lumen of the GIT back to its resting state (long reflex pathway)
What is a tonic contraction?
Sustained contractions for minutes to hours e.g. sphincters
What is a phasic contraction?
Waves of contraction and relaxation, each lasting seconds e.g. peristalsis, segmentation
What is a motility pattern?
Any pattern of contraction or relaxation of GIT smooth muscle (including relaxation)
What is propulsion?
Motility patterns that specifically move contents along the GIT
Where do phasic contractions occur?
Smooth muscle
What are phasic contractions controlled by?
Pacemaker cells that spontaneously contract and relax without external input
What is the frequency of contraction in the stomach, duodenum and ileum respectively?
3, 12 and 9 contractions per minute
These do not change!
How can contractions in the GIT be controled?
Through the strength of the contraction, regulated by nervous and hormonal input
What is the feed motility pattern?
Chewing, Peristalsis, Relaxation
Segmentation, Contraction or relaxation of sphincters
What is the fasting motility pattern controlled by?
The migration motor complex (MMC)
Where is the MMC?
Beings in the stomach and travels to the SI
What is the function of the MMC?
Remove residual secretions, Remove undigested material, Promote epithelial turn over
When does the MMC activate?
4 hours after a meal
What is the MMC motility pattern?
45-60 minutes of inactivity
30 minutes of intermittent uncoordinated activity
5-15 minutes of intense coordinated peristaltic contractions
Repeats every 1.5-2 hours until food is consumed
What is the motility pattern in the mouth?
Chewing (mastication). This reduces the size of food particles and allows swallowing. Is voluntary action controlled by skeletal muscles and reflex control (involuntary) of the strength and frequency. This mixes food with saliva for taste, lubrication and protection.
What is the motility pattern in the oesophagus?
Swallowing - rapid transfer of material from the mouth to the stomach which is initially under voluntary control but continues via reflexes. After swallowing, the upper and lower oesophageal sphincters open and peristalsis occurs
What prevents the flow of food into the nasopharynx?
The soft palate and uvula rise
What prevents food from going into the trachea?
The folding of the epiglottis
What is the motility pattern of the stomach?
Relaxation, retropulsion and contractions and relaxations of the pyloric sphincter
Why does the stomach need to relax?
Allows the stomach to increase in volume from fasting volume (~50mL) to hold a meal with minimal change in pressure within the lumen. Also prevents reflux
Where does relaxation of the stomach occur?
At the fundus and body of the stomach
What are the two forms of stomach relaxation?
Receptive relaxation and accommodation relaxation
What is receptive relaxation?
Part of swallowing, the stomach is about to receive food and relaxes
What is accommodation relaxation?
When food is in the stomach and the stomach needs to find space/accommodation for it
What is retropulsion?
The combination of peristalsis and the contraction of the pyloric sphincter
Where does retropulsion of the stomach occur?
Between the body of the stomach and the pyloric sphincter
What is the function of retropulsion of the stomach?
To complete mechanical digestion and mixing food with stomach acid and enzymes
How does mechanical digestion occur in the stomach from retropulsion?
The waves of peristaltic contractions grind the flood particles against the close pyloric sphincter, the chyme is then pushed backwards from the pyloric atrium to the body of the stomach
Where does peristalsis of the stomach occur?
Contractions begin in the body of the stomach and spread to antrum
How often does peristalsis of the stomach occur?
3 contractions per minute. The first 60 minute following meal are gentle and 60 - 300 min after eating are more intensely active.
What is the function of the pyloric sphincter?
Gastric emptying, a controlled movement of fully mechanically digested chyme from the stomach into the duodenum
What adjusts the rate of gastric emptying?
Using feedback from the stomach and the duodenum. To match the digestive capacity of the intestines and at the correct rate to match with the release of bicarbonate, fluid, bile and digestive enzymes
What factors affect the rate of gastric emptying from the stomach?
Size of a mean and composition of a meal
What factors (from the duodenum) affect the rate of gastric emptying?
Chemoreceptors, osmoreceptors and endocrine cells
What happens when the chemoreceptors and osmoreceptors are activated in the duodenum?
Triggers the short and long enterogastric reflexes
What happens when the enteroendocrine cells in the duodenum are activated?
They secrete hormones which decreases the force of peristalsis in the stomach and slows gastric emptying so the small intestine and accessory organs can keep up.
What are the accessory organs?
Liver, gallbladder and pancreas
What occurs in the small intestine during digestion?
Segmentation, peristalsis and contraction & relaxation of the ileocecal sphincter
What does segmentation do in the SI?
Mixes chyme with secretions from the pancreas, biliary system and SI mucus to increase the chemical digestion of nutrients, expose it to absorptive surfaces, and also produce some movement along the SI
What is the billary system?
Liver and gallbladder
What is the function of peristalsis in the SI?
To move chyme along the SI through some weak peristalsis during absorption and stronger, main peristalsis after most of the nutrients has been absorbed
What is the function of the contraction and relaxation of ileocecal sphincter
To deliver chyme from the SI to the LI
What forms of digestion occurs in the large intestine?
Segmentation, relaxation and peristalsis
What is the function of segmentation in the LI?
To expose the chyme to absorptive surfaces. Some absorption occurs of water and sodium ions from chyme turning chyme into semi-solid faeces
Where does relaxation occur in the LI?
Descending and sigmoid colon
What is the function of relaxation in the LI?
To store the faeces
What is the function of peristalsis in the LI?
To make a mass movement of faeces a day following meals driving faeces into the rectum and initiating defecation
How often does peristalsis occur in the LI?
1-2 times a day
