GIT Motility

Question 1

Main functions of GIT Motility

Answer 1

• Propel food along GIT
• Mixing and Grinding of contents of GIT
• Aid absorption of nutrients and water.
• Clear it of contents

Question 2

Mastication - Innervation? Which teeth? Function?

Answer 2

• Majority of mastication muscles are innervated by the trigeminal nerve. Rhythmical chewing controlled by brain stem nuclei in reticular areas.
• Incisors cut, molars grind.
• Mixes food with saliva for lubrication (mucin), reduces food particle size and mixes food with digestive enzymes.

Question 3

Deglutition - Voluntary/Oral Stage

Answer 3

Food voluntarily moves posteriorly into pharynx by tongue. Trachea is closed by epiglottis. Oesophagus is opened. A fast peristaltic wave initiated by the nervous system of the pharynx forces thebolus of food into the upper oesophagus. Under 2 seconds.

Question 4

Deglutition - Pharyngeal Stage

Answer 4

Reflex act. Initiated by voluntary movement of food into the back of the month, detected in the ring around pharyngeal opening. Excitation of involuntary pharyngeal sensory receptors to elicit the swallowing reflex. Next stages automatically inititiated by neuronal areas of the medulla and lower pons. The motor impulses from the swallowing centre to the pharynx and upper oesophagus that cause swallowing are transmitted successively by the trigeminal, glossopharyngeal, vagus and hypoglossal nerves plus a few superior cervical nerves. Under 2 seconds.

Question 5

Deglutition - Oesophageal Stage

Answer 5

• Primary 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.
• Secondary Peristalsis - Happens if food gets stuck and oesophagus is distended by it. Waves continue until food has all emptied into the stomach. Initiated by intrinsic neural circuits in myenteric nervous system and reflexes that begin in pharynx.

Question 6

Effect of Swallowing on Respiration

Answer 6

Swallowing centre specifically inhibits the respiratory centre of the medulla during this time, respiration is briefly arrested.

Question 7

Upper oesophagus: Type of muscle? Innervation?

Answer 7

Striated skeletal muscle. Glossopharyngeal and Vagus nerves.

Question 8

Lower oesophagus: Type of muscle? Innervation?

Answer 8

Smooth muscle. Vagus and oesophageal myenteric plexus.

Question 9

Receptive Relaxation

Answer 9

A relaxation wave precedes peristalsis, it is transmitted by the myenteric inhibitory neurons. It prepares the stomach for food arrival and loosens the lower oesophageal sphincter which is normally tonically constricted.

Question 10

What does the stomach do in order to create more room for storage?

Answer 10

Food stretches stomach and activates the vagovagal reflex. Decreases tone in stomach body muscular wall, the wall bulges outward and greater quantities of food are accommodated (between 0.8-1.5L).

Question 11

How does the stomach mix food?

Answer 11

It mixes the food with gastric secretions to form chyme (semifluid mixture). It does this using mixing waves (weak peristaltic constrictor waves) which begin in the mid/upper portions of the stomach wall, move toward antrum every 15-20s. They are initiated by the Interstitial Cells Cajal in the gut wall. Waves become more intense as the go towards the antrum to force the antral contents towards the pylorus. The antral contents are squeezed upstream through the peristaltic ring back to the body (instead of through the pylorus).

Peristaltic constrictive ring and upstream squeezing action (retropulsion) are the important mixing mechanism in stomach.

Question 12

How does the stomach empty itself?

Answer 12

• Slow emptying of chyme from stomach to duodenum at a suitable rate for proper digestion and absorption by small intestine.
• Promoted by intense peristaltic contraction in antrum of stomach.
• Opposed by pylorus which is constricted under the nervous and hormonal signals from stomach and duodenum.
• Increased stomach volume, increased gastric emptying.
• Enterogastric Inhibitory Reflex - Inhibits emptying due to: distension of duodenum, irritation of duodenal mucosa, acidity/osmolality of duodenal chyme, and presence if digestion products in chyme (proteins/fats).
• Hormonal regulating factors: stimulus mainly fats and cholecystokinin.

Question 13

Small Intestine Segmentation

Answer 13

Chyme induces extension of the GI wall, it stretches, localised concentric contactions start and mix the food. Frequency of segmentation determined by frequency of slow waves. However slow waves are not effective without background excitation from the myenteric plexus.

Can become weak when excitatory activity blocked by atropine.

Question 14

Small Intestine Peristalsis

Answer 14

Occurs in any part of small intestine and move towards anus at 0.5-2cm/s. It is weak and dies out after 3-5cm, barely ever exceeds 10cm. Net movement is slow; 1cm/min. Chyme movement takes around 3-5 hours from pylorus to ileocaecal valve.

Activity increases after a meal:

• chyme enters into duodenum and the wall stretches,
• gastroenteric reflex
• gastrin, cholecystokinin, insulin, motilin and serotonin increase motility
• secretin and glucagon decrease motility

Question 15

Ileocaecal Valve

Answer 15

Protrudes into lumen of caecum and is forcefully closed when excess pressure builds up in caecum. Immediately after a meal, gastroileal reflex intensifies peristalsis in ileum causing emptying of ileal contents into caecum. It is mediated via myenteric plexus and extrinsic autonomic nerves.

Question 16

Haustrations

Answer 16

Combined contraction of circular muscle and longitudinal smooth muscle which generates characteristic bag-like sacs (30 sec peak, then 60 sec disappear).

Proximal half absorbs water and electrolytes and distal half stores the now solid faeces.

Question 17

Poor motility causes…

Answer 17

Greater absorption and harder faeces resulting in constipation.

Question 18

Excess motility causes…

Answer 18

Less absorption resulting in looser faeces or diarrhoea.

Question 19

Mass Movements

Answer 19

Constrictive ring occurs (usually in transverse colon) in response to distension. 20cm of colon distal to constrictive ring lose haustrations and contract as a single unit. Faecal material moves altogether down the colon. Contraction develops progressively over 30s and relaxation occurs over the next 2-3mins until the next mass movement. This goes on for 10-30mins and then stops. It mreturns around 12 hours later.

Question 20

Gastrocolic and Duodenocolic Reflexes

Answer 20

Facilitate mass movements after meals due to distension of stomach and/or duodenum. Reflex goes from GIT -> prevertebral sympathetic ganglia -> GIT.

Irritation in colon can also activate mass movements. An example includes ulcerative colitis where mass movements persist all the time due to ulceration of colon mucosa.

Question 21

Defecation

Answer 21

Gas or faeces in the rectum stimulates stretch receptors in its wall initiating the rectosphincteric reflex (relaxation of smooth muscle of internal anal sphincter and contraction of striated muscle of external anal sphincter). Defecation is a reflex activity but is also subject to conscious control.