GI tract motility

Question 1

What mechanisms are involved in the ingestion of food ?

Answer 1

1) Mastication

2) Swallowing

Question 2

Describe the process of mastication.

Answer 2

Aim: Reduces size of food particles (facilitates swallowing)

Incisor teeth- cutting
Molar teeth- grinding

Saliva: chewing allows good to be mixed with saliva to allow lubrication (mucin, a glycoprotein, acts as a lubricant) + beginning process of digestion with digestive enzymes
(carbohydrate with salivary amylase + fat with lipases)

Question 3

Describe the NS control of mastication.

Answer 3

Majority of chewing muscles innervated by 5th cranial nerve

Process controlled by brain stem nuclei

• Reticular areas for rhythmical chewing
• Additional involvement from hypothalamus/amygdala/cerebral cortex

Question 4

What are the requirements of swallowing wrt pharynx ?

Answer 4

Swallowing requires pharynx to be a tract for propulsion of food (important that respiration is not compromised).

Question 5

Is swallowing voluntary or involuntary ?

Answer 5

1) Voluntary stage: initiates the swallowing process
2) Pharyngeal stage: involuntary passage of food through the pharynx into the oesophagus
3) Oesophageal stage: involuntary transport of food from pharynx into stomach

Question 6

Describe the main stages of swallowing.

Answer 6

VOLUNTARY
1) When ready for swallowing, food is voluntarily moved posteriorly into pharynx

INVOLUNTARY

2) Trachea is closed
3) Oesophagus is opened
4) Fast peristaltic initiated by the NS of the pharynx forces the bolus of food into upper oesophagus

Question 7

How long does swallowing take ?

Answer 7

Less than 2 seconds

Question 8

Describe the nervous initiation of the pharyngeal stage of swallowing.

Answer 8

♦ Principally a reflex act
♦ Almost always initiated by a voluntary movement of food into the back of the mouth (detected in ring area around the pharyngeal opening)
♦ Excitation of involuntary pharyngeal sensory receptors to elicit the swallowing reflex
♦ Next stages automatically initiated (in orderly sequence) by neuronal areas of the reticular substance of the medulla and lower portion of the pons (i.e. deglutition center)
♦ Motor impulses from the swallowing center to the pharynx and upper oesophagus that causes swallowing are transmitted successively by the 5th, 9th, 10th, 12th cranial nerves (+ few superior cervical nerves).

Question 9

Explain the effect of the pharyngeal stage of swallowing on respiration.

Answer 9

• Pharyngeal stage lasts less than 6 seconds, so it interrupts respiration for fraction of a usual respiratory cycle
• Swallowing center specifically inhibits the respiratory center of the medulla during this time (i.e. halts respiration)

Question 10

Describe what happens during the oesophageal stage of swallowing.

Answer 10

♦ Primary peristalsis:
-Simple continuation of peristaltic wave that begins in the pharynx and spreads into the oesophagus during the pharyngeal stage of swallowing.

♦ Secondary peristalsis:

• Result from distention of the oesophagus by retained food (when some food has remained behind after primary peristalsis)
• Waves continue until all food has emptied into the stomach

Question 11

How much time does it take for the wave of primary peristalsis to go from pharynx to stomach ? How long does good typically take to reach the stomach ? Why are these two values not the same ?

Answer 11

8-10 seconds
5 seconds (because gravity helps, but we don't need it even without it it would take 8-10 seconds)

Question 12

How is secondary peristalsis initiated ?

Answer 12

By intrinsic neural circuits in myenteric NS + by reflexes that begin in pharynx

Question 13

Describe the musculature of the different parts of the oesophagus, linking it to the parts of the NS innervating each.

Answer 13

Upper 1/3, striated: peristaltic waves controlled by skeletal nerve impulses from glossopharyngeal and vagus nerves

Lower 2/3, smooth muscle: strongly controlled by vagus nerves that act through connections with the oesophageal myenteric NS.

Question 14

Explain the process of receptive relaxation of the stomach.

Answer 14

• Relaxation wave precedes perisalsis (transmitted via myenteric inhibitory neurons)
• Entire stomach gets relaxed in preparation for food arrival

Question 15

Explain the function of the lower oesophageal sphincter.

Answer 15

• Last 3 cm of oesophageal circular muscle functions as a lower oesophageal sphincter (i.e. gastroesophageal sphincter)
• This sphincter normally remains tonically constricted
• When peristaltic swallowing wave passes down oesophagus, ‘receptive relaxation’ occurs ahead of the peristaltic wave, allowing easy propulsion of food into the stomach

Question 16

Identify the motor functions of the stomach.

Answer 16

1. Storage of large quantities of food until it can be processed in the stomach, duodenum and lower intestinal tract
2. Mixing of food with gastric secretions until it forms a semifluid mixture (chyme)
3. (Slow) emptying of chyme from stomach into the small intestine at a rate suitable for proper digestion and absorption by the small intestine.

Question 17

Describe the process of storage in the stomach.

Answer 17

• Food entering the stomach forms concentric circles in orad portion of the stomach, with newest food lying closest to oesophageal opening and oldest food lying nearest the outer wall of stomach.
• Food stretches the stomach which leads to a vasovagal reflex (stomach to brainstem back to stomach) which leads to decreased tone in stomach body muscular wall, which leads to wall bulging progressively outwards, which means greater quantities of food accommodated

Question 18

What is the limit amount of food which can be stored at the stomach at one time ?

Answer 18

0.8-1.5 L

Question 19

Describe the process of mixing in the stomach.

Answer 19

• Weak peristaltic constrictor waves begin the mid/upper portions of the stomach wall, move toward antrum every 15-20 s.
• Waves become more intense as they move from the body to the antrum, forcing the antral contents under higher and higher P towards the pylorus.
• Since the pyloric sphincter is closed, most of antral contents are squeezed upstream towards body of stomach (not through pylorus)

Overall, peristaltic wave + upstream squeezing action (retropulsion) = mixing

Question 20

How is the weak peristaltic wave initiated in the stomach ?

Answer 20

Initiated by the gut wall basic electrical rhythm

Question 21

Describe the process of emptying in the stomach, notably identifying regulating factors for it.

Answer 21

♦ Promoted by intense peristaltic contractions in stomach antrum.
♦ Opposed by closed pylorus, whose degree of constriction is under the influence of nervous + hormonal signals from stomach and duodenum
♦ Regulating factors:
1) Gastric and duodenal
-Increased stomach V = Increased emptying
-Enterogastric inhibitory reflexes (inhibit emptying) occur during distention of the duodenum, presence of any irritation of the duodenal mucosa, high acidity/osmolarity of duodenal chyme, presence of certain digestion products in chyme (e.g. from proteins/fats)

2) Hormonal
- Stimulus mainly fats
- CCK potent inhibitor hormone of emptying

Question 22

Identify the main movements of the small intestine.

Answer 22

Mixing contractions- segmental contractions (also contribute to movement)
Propulsive movements- peristalsis (also have a role in mixing)

Question 23

Describe the mixing/segmental contractions of the small intestine.

Answer 23

• Chyme induced extension of GI wall → stretch → localised contractions (spaced along intestine, short lasting) which is called segmentation
• Frequency of segmentation determined by frequency of slow waves

Question 24

Where is the fastest intrinsic rate of slow waves in the GI ? What is the implication of this ?

Answer 24

Duodenum

Greatest intensity of these segmentations taking place

Question 25

What factor can result in weak mixing in the small intestine ? Explain why. Why doesn’t the mixing stop altogether ?

Answer 25

Becomes weak when excitatory activity of enteric NS is blocked by atropine. Because slow waves in smooth muscle itself that cause the segmentation contractions, are not effective without background excitation mainly from the myenteric nerve plexus.

The mixing does not stop because the small wave contractions are intrinsic.

Question 26

Describe the propulsive movement (peristalsis) in the small intestine.

Answer 26

Weak and die out after 3-5 cm (rarely more than 10 cm)

Question 27

What is the rate of movement of chyme in the small intestine ? How long does it take for the chyme to go from pylorus to ileocaecal valve ?

Answer 27

1 cm/min

3-5 hours

Question 28

Where in the small intestine does peristalsis occur ? How fast does peristalsis occur in the small intestine ? Is there anywhere in the small intestine where peristalsis is quicker/slower ?

Answer 28

Occur in a any part of the small intestine, moving towards the anus at 0.5-2 cm/min (faster in proximal SI, slower in terminal SI)

Question 29

Explain nervous/hormonal control of peristalsis in the small intestine.

Answer 29

• Peristaltic activity increases after a meal.
-As chyme enters duodenum, duodenal wall is stretched
-1) Gastroenteric reflex (from stomach distension via myenteric reflex)
-2) Gastrin, CCK, insulin, motolin, serotonin
(secretin, glucagon decrease motility)

• In some circumstances, peristaltic rush (powerful, rapid) occurs

• Initiated by nervous reflexes (autonomic/brainstem + intrinsic myenteric plexus reflexes)
• Due to mucosal irritation with infectious diarrhea

Question 30

Explain the role of ileocaecal valve.

Answer 30

Valve protruding into the lumen of the caecum, is forcefully closed when excess P builds up in caecum and tries to push caecal contents backwards against the valve lips.

Question 31

What magnitude of reverse pressure can the ileoceacal valve resist ?

Answer 31

Ileoceacal sphincter can resist reverse pressure of 50-60 cm water

Question 32

Describe the structure of the ileocaecal sphincter, and relate this to its function.

Answer 32

Wall of the ileum (several cm upstream) has thickened circular muscle (i.e. the sphincter). This remains mildly constricted and slows emptying of ileal contents into the cecum.

Question 33

Explain what happens at the ileocaecal junction after a meal.

Answer 33

Chemical irritation or pressure stimulate peristalsis in ileum (gastroileal reflex) and relax ileocaecal sphincter, which promotes emptying of ileal contents into caceum.
Then, chemical irritation or pressure in caecum inhibits peristalsis and excites sphincter.

Question 34

Explain the process of feedback control of ileocaecal sphincter.

Answer 34

• Reflexes from the caecum control:

1) Degree of contraction of ileoceacal sphincter
2) Intensity of peristalsis in terminal ileum

• Cecum-to-ileocecal sphincter and cceum-ileum reflexes are mediated via myenteric plexus and extrinsic autonomic nerves.

1) Distended caecum leads to intensification of contraction of ileocecal sphincter, and inhibition of ileal peristalsis (delays greatly emptying of addition chyme into cecum from ileum)
2) Irritants in the caceum also delays emptying

Question 35

Give an example of an irritant in the cecum delaying emptying.

Answer 35

Inflamed appendix causes intense spasm of the ileocecal sphincter and partial paralysis of the ileum, which block emptying of ileum into cecum.

Question 36

Identify the main movements occurring in the colon.

Answer 36

• Mixing movement (“Haustrations” ie bag-like bulges)

- Propulsive movement- mass movements

Question 37

Describe the mixing movements occurring in the colon.

Answer 37

(Haustrations, i.e. bag-like bulges)
Similar to small intestine segmentation (which forms ball-like components, almost solid faeces) + longitudinal muscle strips contraction

Question 38

Describe the propulsive movements occurring the colon (modified peristalsis).

Answer 38

(Mass Movements)

1. Constrictive ring occurs (usually in transverse colon) in response to distension
2. The 20+ cm of colon distal to constrictive ring lose haustrations and contract as a unit
3. Fecal material moves en masse down the colon
4. Contraction develops progressively more force about 30 s
5. Relaxation occurs during next 2-3 min
6. Another mass movement occurs, perhaps farther along the colon

A series of mass movements persists for 10-30 minutes, then cease but return (much later, i.e. hours later or following day).

Eventually, the mass movements force a mass of feces into the rectum (at which point desire for defecation is felt).

Question 39

Define gastrocolic and duodenocolic reflexes.

Answer 39

Reflexes resulting from distension of the stomach and duodenum which facilitate mass movements after meals.

Almost certainly transmitted via autonomic nervous system (occur either not at all or hardly at all when extrinsic autonomic nerves to the colon have been removed).

Question 40

Give an example of an event that can disturb the ‘normal’ pattern of mass movements in the colon.

Answer 40

Irritation in the colon can initiate intense mass movements (e.g. with ulcerative condition of the colon mucosa (ulcerative colitis) mass movements that persist almost all the time can occur.

Question 41

Describe the defecation reflex, mentioning the different nervous pathways used for it.

Answer 41

• The rectum is normally free of feces, but mass movements can force feces into rectum.
Then, internal and external sphincters prevent loss of fecal matter out of rectum.

• Subsequently, intrinsic defecation reflex is mediated by rectal enteric NS.
• Parasympathetic defecation reflex fortifies this intrinsic reflex. It involves the sacral segments of the spinal cord.