20-01-23 – GI Tract Motility Flashcards
Learning outcomes
- List the main functions of Gastrointestinal Tract motility
- Summarise the physiological mechanism of swallowing
- Describe the motor functions of the stomach and the processes by which they occur
- Describe the normal movements of the small and large intestines and the processes by which they occur
- Describe the defecation reflex
What is the alimentary canal?
What is GI tract motility?
What are the 4 main functions of GIT motility?
What is timing of food movement important for?
How do requirements for mixing and propulsion differ at each stage?
- The alimentary canal is the whole passage along which food passes through the body from mouth to anus during digestion.
- GI tract motility is the ability of the alimentary canal (tract) to contract and propel ingested substances along its length
- 4 main functions of GIT motility:
1) Propel ingested food along the entire length of the GIT
2) Mixing and grinding of the contents of the GIT
3) Aid absorption of nutrients and water
4) Clear the stomach and intestines of luminal contents - Timing of food movement important for optimum processing
- Requirements for mixing and propulsion are quite different at each stage of processing
What is mastication also known as?
What are the 2 types of teeth and what are they responsible for?
What are a majority of chewing muscles innervated by?
Which area of the brain controls chewing?
What are 3 reasons for chewing?
- Mastication is also known as chewing
- 2 types of teeth:
1) Incisors - for cutting
2) Molars – for grinding - Majority of chewing muscles innervated by cranial nerve V (CN V - trigeminal nerve)
- The process of chewing is controlled by brain stem nuclei:
1) Reticular areas for rhythmical chewing
2) Additional involvement from hypothalamus/amygdala/cerebral cortex - 3 reasons for chewing:
1) Mixes food with saliva
* Lubrication (mucin – glycoprotein acts as lubricant)
2) Reduces size of food particles
* Facilitates swallowing
3) Mixes food components with digestive enzymes
* Carbohydrate with salivary amylase for carbohydrate digestion
* Fat with lipases for lipid digestion
What is swallowing also known as?
Why does swallowing require the pharynx?
What factor shouldn’t be compromised during the process of swallowing?
What are the 3 stages of deglutition?
- Swallowing is also known as deglutition
- Swallowing required the pharynx to be a tract for propulsion of food
- It is Important that respiration is not compromised during this process
- 3 stages of deglutition:
1) a voluntary (oral) stage: initiates the swallowing process
2) a pharyngeal stage: involuntary passage of food through the pharynx into the oesophagus
3) an oesophageal stage: involuntary transport of food from the pharynx to the stomach
What are the 4 stages of the oral/voluntary phase of deglutition?
How fast is this phase?
- 4 stages of the oral/voluntary phase of deglutition:
1) Food voluntarily moved posteriorly into the oropharynx by tongue
2) The trachea is closed
3) The oesophagus is opened
4) A fast peristaltic wave initiated by the nervous system of the pharynx forces the bolus of food into the upper oesophagus
- This phase takes place in <2 seconds
What type of action is the pharyngeal phase of deglutition?
How is it initiated?
Describe the 4 events in the pharyngeal phase of deglutition.
Which receptors are excited during this process?
How are the next stages initiated?
What motor impulses are transmitted and where do they go to?
- The pharyngeal phase of deglutition is a reflex act (automatic)
- This phase is initiated by voluntary movement of food into the back of the mouth, which is detected in ring area around the pharyngeal opening
- 4 events in the pharyngeal phase of deglutition:
1) Nasopharynx closes
2) Pharynx wall contracts and the epiglottis is pushed by the bolus to meet the larynx which moves upwards
3) The epiglottis contracts and closes the trachea
4) The upper oesophageal sphincter opens
- During this phase, there is the excitation of involuntary pharyngeal sensory receptors to elicit the swallowing reflex
- Next stages automatically initiated (in orderly sequence) by neuronal areas of the medulla and lower pons (swallowing centre)
- The motor impulses from the swallowing centre to the pharynx and upper oesophagus that cause swallowing are transmitted successively by CN V, CN IX, CN X, CN XII (+ few superior cervical nerves)
How long does the pharyngeal stage occur?
How is respiration affected during this process?
How does the swallowing centre affect the respiratory centre?
- The pharyngeal stage occurs in < 6 seconds
- Interruption of respiration is for a fraction of usual respiratory cycle during this phase
- The swallowing centre specifically inhibits the respiratory centre of the medulla during this time – respiration arrested very briefly
What are the 2 types of peristalsis in the oesophageal phase of deglutition?
How long does each type continue for?
What 2 things initiate 2nd degree peristalsis?
- 2 types of peristalsis in the oesophageal phase of deglutition:
1) 1st degree peristalsis
* Continuation of peristaltic wave that begins in the pharynx and spreads into the oesophagus during the pharyngeal stage of swallowing
* Continuous wave passes from pharynx to stomach in about 8-10s
2) 2nd degree peristalsis
* Result from distention of the oesophagus by retained food
* Waves continue until all food has emptied into the stomach
- 2 things initiate 2nd degree peristalsis:
1) Intrinsic neural circuits in myenteric nervous system (mostly controlled by myenteric plexus)
2) Reflexes that begin in pharynx
How is peristaltic wave generation controlled in the upper/striated region of the oesophagus?
How is this controlled in the lower 2/3rds of the oesophagus?
What would happen if the vagus nerve to the oesophagus is cut?
- Peristaltic waves generated in the upper/striated region of the oesophagus are controlled by skeletal nerve impulses from the glossopharyngeal (CN IX) and vagus nerves (CN X)
- In the lower 2/3rds of the oesophagus consisting of smooth muscle, peristaltic waves generation is strongly controlled by the vagus nerves (CN X) that act through connections with the oesophageal myenteric nervous plexus
- When vagus nerves to oesophagus are cut, myenteric nerve plexus of oesophagus becomes excitable enough after several days to cause strong secondary peristaltic waves even without support from vagal reflexes
- So even after paralysis of the brain stem swallowing reflex, food fed by tube or in some other way into oesophagus still passes readily into stomach
Receptive Relaxation of Stomach and Function of Lower Oesophageal Sphincter.
Where is the wave of relaxation positioned in peristalsis?
How is this transmitted?
When it the stomach relaxed?
What makes up the lower oesophageal sphincter?
What is it also known as?
When it constricted?
Where does receptive relaxation occur in peristalsis?
What is the purpose of this?
- The relaxation wave precedes peristalsis, which is transmitted via myenteric inhibitory neurones
- The stomach becomes relaxed in preparation for food arrival
- Last 3cm oesophageal circular muscle functions as a lower oesophageal sphincter (gastroesophageal sphincter)
- It is normally tonically constricted
- When peristaltic swallowing wave passes down oesophagus, “receptive relaxation” occurs ahead of the peristaltic wave
- This allows easy propulsion of food into the stomach
Motor Functions of the Stomach – Storage.
What can be stored in the stomach?
What does food entering the stomach form?
How is the stomach able to accommodate more food?
- The stomach can be the storage of large quantities of food until it can be processed in the stomach, duodenum and ileum
- Food entering stomach forms concentric circles in stomach
1) Newest food lying closest to oesophageal opening
2) Oldest food lying nearest the outer wall of stomach - Food stretches stomach, which triggers the “vagovagal reflex” (stomach->brain stem->back to the stomach)
- This leads to decreased tone in stomach body muscular wall, causing the stomach to bulge forward and accommodate greater quantities food (up to a limit - 0.8-1.5 L)
Motor Functions of the Stomach – Mixing.
What is mixed in the stomach?
Describe the 3 steps in the contents being mixed in the stomach.
What is an important mixing mechanism in the stomach?
- In the stomach there is fixing of food with gastric secretions which forms a semifluid mixture (chyme)
- 3 steps in the contents being mixed in the stomach:
1) Mixing waves - weak peristaltic constrictor waves - begin in the mid/upper portions of the stomach wall, move toward antrum every 15-20s
* Initiated by gut wall Interstial Cells Cajal (basic electrical rhythm / slow waves)
2) Waves become more intense (body to the antrum)
3) Antral contents squeezed upstream through peristaltic ring towards body of stomach, not through the pylorus
- Peristaltic constrictive ring + upstream squeezing action (retropulsion) important mixing mechanism in the stomach
What occurs during emptying of the stomach?
When will chyme pas into the small intestine?
What promotes stomach emptying?
What is stomach emptying opposed by?
- Emptying of the stomach is slow and involves the emptying of chyme from stomach into small intestine at a rate suitable for proper digestion and absorption by the small intestine
- Chyme is passed into the small intestine when it is of the right consistency, which can take hours of mixing in the stomach
- Stomach emptying is promoted by intense peristaltic contractions in stomach antrum
- Stomach emptying is opposed by the pylorus - constriction under influence of nervous + hormonal signals from stomach + duodenum
What are 2 gastric and duodenal regulating factors of stomach emptying?
How are local myenteric reflexes activated?
What are 2 hormonal regulating factors of stomach emptying?
- 2 gastric and duodenal regulating factors of stomach emptying:
1) Increased stomach volume, increased gastric emptying
2) Enterogastric inhibitory reflex triggered by:
1)) Distention of duodenum
2) Irritation of the duodenal mucosa
3) Acidity/osmolality of duodenal chyme
4) Presence of digestion products in chyme e.g., proteins/fats
- Stretching of stomach wall activates local myenteric reflexes, and it is not due to increased pressure
- 2 hormonal regulating factors of stomach emptying:
1) Stimulus - mainly fats
2) CCK most potent hormone
Movements of the Small Intestine Mixing Contractions – Segmentation Contraction.
Where do segmentation contractions occur?
How is segmentation in the lower intestine initiated?
How is frequency of segmentation determined?
When do segmentation contractions become weak?
What causes the segmentation contractions?
What else is needed for these segmentation contractions to be effective?
- Movements of the Small Intestine Mixing Contractions – Segmentation Contraction
- Segmentation contractions occur in the small intestine
- Chyme induced extension of GI wall➔stretch➔localised concentric contractions (spaced along intestine, short lasting)➔segmentation
- The frequency of segmentation determined by frequency of slow waves
- Segmentation contractions become weak when the excitatory activity of the enteric nervous system is blocked by atropine
- Slow waves in smooth muscle cause the segmentation contractions
- However, these segmentation contractions are not effective without background excitation from myenteric nerve plexus
Movements of the Small Intestine Propulsive movements – Peristalsis.
Describe the muscular contractions during peristalsis in the small intestine.
Where does peristalsis occur in the small intestine?
How fast do peristaltic waves move towards the anus?
When do they die out?
What is the rate of movement of chyme?
How long does it take for chyme to move from the pylorus to ileocecal valve?
When does peristalsis activity increase?
What are 2 reasons for this?
What are 5 factors that increase motility?
What are 2 factors that decrease motility?
- During muscular contraction in peristalsis of the small intestine, the longitudinal muscle relaxes and circular muscle contracts behind the bolus, and vice versa Infront of the bolus
- Peristalsis in the small intestine occurs in any part of small intestine
- Peristaltic waves move towards anus at 0.5-2cm/s
- They become weak and die out after 3-5cm. Rarely >10cm
- Net movement of chyme is slow: 1cm/min
- Chyme movement from pylorus to ileocecal valve takes about 3-5 hours
- Peristalsis activity increases after a meal:
1) Chyme entry into duodenum leads to duodenal wall stretch
2) Gastroenteric reflex (from stomach distension via myenteric plexus)
- 5 factors that increase motility:
1) Gastrin
2) CCK
3) Insulin
4) Motilin
5) Serotonin - 2 factors that decrease motility:
1) Secretin
2) Glucagon
Where is the illeocecal valve (sphincter) found?
What Is the illeocecal valve made from?
What is its function?
What is the cacum?
What does it connect?
Where does the illeocecal valve protrude into?
When is it forcefully closed?
How does it exist normally?
What reflexes intensify in the ileum immediately after a meal?
What mediates Cecum-to-ileocecal sphincter and cecum-ileum reflexes?
How much chyme empties into the cecum each day?
- The ileocecal valve is a sphincter muscle situated at the junction of the ileum (last portion of your small intestine) and the caecum
- The illeocecal valve is made from thickened circular muscle in the terminal ileum
- Its function is to allow digested food materials to pass from the small intestine into your large intestine
- The cecum is a pouch that forms the first part of the large intestine.
- It connects the small intestine to the colon, which is part of the large intestine
- The illeocecal valve protrudes into the cecum
- It is forcefully closed when excess pressure builds up in cecum
- The illeocecal valve normally remains mildly constricted and slows emptying of ileal contents into the cecum
- Immediately after a meal, gastroileal reflex intensifies peristalsis in ileum, which leads to emptying of ileal contents into cecum
- Cecum-to-ileocecal sphincter and cecum-ileum reflexes are mediated via the myenteric plexus and extrinsic autonomic nerves
- Normally, only 1.5L to 2L of chyme empty into the cecum each day
What are haustrations?
What are haustrations characteristic of?
What is this similar to?
How long do these haustrations last?
What are the 2 functions of the colon?
- Haustra are saccules in the colon (longest part of large intestine) that give it its segmented appearance
- Haustrations are characteristics that you see in the large intestine as a results of segmentation contractions occurring in both the circular and longitudinal smooth muscle
- These are similar to segmentation movements in small intestine and allow for mixing movements
- Haustrations generates characteristic bag-like sacs (30 secs peak, then 60 secs disappear)
- 2 functions of the colon:
1) Absorption of water and electrolytes from the chyme to form solid feces
2) Storage of fecal matter until it can be expelled
What is mass movement compared to peristalsis?
What are 3 characteristics of mass movement in the colon?
How long does contraction and relaxation occur?
How long do series of mass movements last for?
When does the sensation of the need to defecate occur?
- Mass movement can be seen as a special form of peristalsis in the colon.
- 3 characteristics of mass movement in the colon:
1) Constrictive ring occurs (usually in transverse colon) in response to distension
2) 20cm of colon distal to constrictive ring lose haustrations and contract as a unit
3) Fecal material moves en masse down the colon
- Contraction develops progressively over 30secs
- Relaxation occurs over next 2-3mins
- Another mass movement then occurs
- Series of mass movements persists for 10-30min, then cease. Returns at variable time points (e.g 12 hours later)
- Sensation of need to defecation felt when feces moved into rectum
What is much of the movement in the colon due to?
When can mass movements take over?
When do mass movements occur?
- Much movement in the colon is due to slow but persistent haustral contractions
- From cecum to sigmoid colon, mass movements can take over propulsive role
- These movements occur only 1-3 times a day – in particular 15 minutes during the first hour after eating breakfast
What do Gastrocolic and Duodenocolic Reflexes facilitate?
How are these reflexes triggered?
What is the evidence this reflex exists?
What else can also trigger intense mass movements?
- Gastrocolic and Duodenocolic Reflexes facilitate mass movements after meals
- These reflexes are triggered due to distention of stomach and duodenum - Reflex from GIT to prevertebral sympathetic ganglia and back to GIT
- Evidence this reflex exists – reflex inhibited when extrinsic autonomic nerves to the colon removed
- Irritation in the colon can also initiate intense mass movements – e.g ulcerated condition of colon mucosa (ulcerative colitis), mass movements persist almost all the time
What is the rectum usually empty of?
What is the anal canal closed by?
What does gas/faeces in the rectum stimulate?
What is the rectosphinteric reflex?
Is defecation a reflex activity?
- The rectum is usually empty of faeces
- The anal canal is tightly closed via contraction of anal sphincters
- Gas or faeces in the rectum stimulates stretch receptors in its wall initiating the rectosphincteric reflex - reflex relaxation of the smooth muscle of the internal anal sphincter and contraction of the striated muscle of the external anal sphincter
- Defecation is a reflex activity but is subject to conscious control
