Motility in the GIT

Question 1

Size of the small intestine and what organs is it located between

Answer 1

9in - extend from stomach to colon

Question 2

Function of small intestine?

Answer 2

Almost all digestion and absorption. Secretion and motility.

Question 3

What is the motility of the small intestine used for?

Answer 3

Mixining - chyme from the stomach with secretions of pancreas, liver and intestines.
Propulsion - moving the food to correct sites for further digestion, absorption and make way for further material emptied from the stomach.

Question 4

How does segmentation mechanism cause motility in the SI?

Answer 4

Rings of circular muscle at intervals contract and relax. Then adjacent rings contract and relax. Overall results in mixing.

Question 5

How does peristalsis mechanism cause motility in the SI?

Answer 5

Sequential contractions of rings of circular muscle followed by sequential relaxation. Combination of activity between muscular layers, with sequential activity in each of them. Overall it propels chyme along.

Question 6

Describe the migrating motor complex

Answer 6

MMC aids clearing of residual content in the fasting state. These additional contractions are regulated by motilin and are known as MMC. They are synchronized, rhythmic changes in both electrical and motor activity. Initiated in the stomach and continue through the SI at 60-120 minutes intervals.
Role of the MMC is to propel particles greater than 2mm in diameter from the stomach into the duodenum.

Question 7

What are the 4 stages of MMC in humans?

Answer 7

1. Prolonged quiescent period
2. Increased action-potential frequency and contractility
3. Peak of electrical and mechanical activity (few minutes).
4. Declining activity that merges into the next quiescent period.

Question 8

How are MMCs terminated?

Answer 8

Feeding terminated MMCs and initiates the appearance of the “fed motor pattern-segmentation and peristalsis”.

Question 9

What regulates MMC

Answer 9

ENS, humoral factors and extrinsic innervations.

Question 10

What is motilin what is its effect on the MMC?

Answer 10

Motilin is a 22-amino acid peptide, synthesized in the duodenal mucosa and released just before the electrical and mechanical activity of the MMC starts.

Question 11

What neurotransmitters and hormones control smooth muscle contractility?

Answer 11

Neurotransmitter - Acetylcholine
Hormones - motilin and somatostatin stimulates smooth muscle contraction.
Stretch causes a myogenic reflex

Question 12

Explain motility in the ileum

Answer 12

Ileogastric reflex = ileal distension (dilation) leads to decrease gastric motility.
Gastrioileal reflex = increase gastric activity causes increased ileal motility and increased movement of chyme through the ileoceacal vlave.
Mediated through the enteric nervous system.

Question 13

What is the Ileocaecal sphincter and how is it controlled?

Answer 13

Separates the ileum from the colon. Stimuli proximal to the sphincter causes sphincter relaxation. Whereas as stimuli distal to the sphincter causes sphincter contraction.
Distension of the ileum = sphincter relaxation
Distension of the proximal colon = sphincter contraction
Thus ileal flow into the colon is regulated by luminal contents and pressure, both proximal and distal to the sphincter. Under autonomic and ENS.

Question 14

Size of the colon and features

Answer 14

2.5in diameter, last 4 ft of the GIT. The surface area is 1/30th of the small intestine because it contains no villi.
Longitudinal muscle layers thicken to form 3 muscular bands running the length of the colon - taeniae colon.

Question 15

What is the colon composed of?

Answer 15

Cecum; ascending, transverse, descending, and sigmoid colon; rectum and anus.

Question 16

Explain movement in the colon

Answer 16

Segmentation, peristalsis and mass movement contractions. The muscles extend and contract and gather up the colon forming haustra (multiple rounded pillow like pouches that give the colon its unique puckered appearance.

Question 17

Functions of the colon?

Answer 17

Absorbs large quantities of fluid and electrolytes, converting the ileocaecal fluid material into solid.
Avidly absorb SCFA
Storage/reservoir function
Regulated release of faecal material
Provides environment for beneficial bacteria which synthesis vitamins (K, B complex)
Secrete mucus and ions

Proximal colon = site of absorption and bacterial fermentation
Distal colon = reservoir/ storage function.

Question 18

Explain colonic rhythmic phasic contractions.

Answer 18

Short duration RPC (2-3 seconds) provide no propulsion, and their amplitudes vary considerably.
Long duration RPC (15-20 seconds) may propagate over short distances. Longer duration allow turn over and propel the semi-solid to solid contents more effectively.
As water is absorbed fluid contents of the ascending colon become semi-solid then to sold in the sigmoidal colon.

Question 19

Explain colonic giant migrating contractions

Answer 19

GMC are very large amplitude lumen-occluding contractions that propagate very rapidly (1cm/sec) in the distal direction to produce mass movement.
Spontaneous GMCs occur randomly about 2 to 10 times a day in the fasted state.

Question 20

How does bulk increase motility in the colon?

Answer 20

Bulk causes distension. Indigestible material passes through quickly and activates pressure receptors. Eg. cellulose, lignins, pectins. Mediated via the myenteric plexus.

Question 21

Classify the different laxatives

Answer 21

Stimulant laxatives - increase motility through chemoreceptors activation and the myenteric nerve plexus.
Bulk laxative - Dietary fibre and products based on fibre eg. methyl cellulose (safest laxative)
Saline (osmotic laxatives) - Draw water out of the bowel through osmosis due to an increase in osmotic particals.
Emollient (faecal softener) laxatives - non absorbable laxatives.

Question 22

How is motility controlled in the colon?

Answer 22

Autonomic nervous control
Sympathetic inhibition
Parasympathetic increases motility - innervation of the cecum, and the ascending and transverse colon via branches of the vagus nerve. Vagal stimulation causes RPCs of the proximal part of the colon.
Pelvic nerve innervates the descending and sigmoid colon, rectum and anal canal. Pelvic stimulation causes GMC of the distal colon and sustained contraction of some segments.

Question 23

Explain defecation

Answer 23

1. Internal anal sphincter-circular involuntary smooth muscle and external anal sphincter-striated voluntary smooth muscle.
2. Faeces enter rectum and causes distension due to mass peristalsis.
3. Intrinsic defecation reflex - myenteric plexus
4. Peristalsis and relaxation of internal anal-sphincter requires reinforcement as it is weak. This comes from parasympathetic defaecation reflex.
5. This is mediated by via the sacral segments of the spinal cord. Results in amplification or peristaltic waves and relaxation of the internal anal sphincter.
6. Defaecation can be inhibited by conscious control over the external anal sphincter.