L13 Movement of food through GIT; motility and control

Question 1

what are phasic contractions?

Answer 1

rapid contraction and relaxation (e.g. peristalsis and segmentation)

Question 2

what are tonic contractions?

Answer 2

sustained contractions lasting minutes to hours e.g sphincters or upper stomach (fundus)

Question 3

what is BER?

Answer 3

basic electrical rhythm - set by non contractile pacemaker cells (cells of cajal - lie at interface between nerve fibres and SMC)

Question 4

how does an AP travel from interstitial cells of cajal to SMC?

Answer 4

gap junctions

Question 5

what causes depolarisation/hyperpolarisation of SMC in GIT?

Answer 5

depolarisation by stretch, parasymp (Ach)

hyper polarisation by simp, noradrenaline

Question 6

describe peristalsis

Answer 6

• propulsive
• responsible for forward moving
• adjacent segments of tract alternately contract and relax which moves food (bolus) along the tract distally
• proceeded by receptive relaxation creating an area of low resistance for bolus to move into

Question 7

descrive segmentation

Answer 7

• non-adjacent segments of intestine alternately contract and relax moving food forwards and then backwards
• results in food mixing rather than propulsion
• stationary phasic contraction
• co-ordinated by intrinsic nerves (enteric)

NB - there is a higher rate in duodenum than jejunum to give more mixing and more aboral push.

Question 8

describe motility of stomach when it is empty

Answer 8

contracts when blood glucose is low (hunger) due to activation of vagus nerve

Question 9

describe motility of stomach when it is filling

Answer 9

• expands from 50ml to 1L w/o pressure rising
• flattening of rugae
• receptive/adaptive relaxation
• plasticity of SMC

Question 10

describe motility of stomach when it is storing food

Answer 10

• not much activity in first half hour

- gentle ripples of peristalsis

Question 11

describe motility of stomach when it is mixing food with secretions

Answer 11

• peristaltic contractions building up in intensity during gastric phase after feeding
• stronger towards muscular antrum region, gives churning

Question 12

describe motility of stomach when it is emptying into duodenum

Answer 12

• small spurt into duodenum w/each peristaltic wave
• most food content returns to antrum
• pyloric sphincter closes to allow regulation of emptying/stops material refluxing from duodenum

Peristaltic waves move towards pylorus (where most vigorous peristalsis and mixing occurs). Pyloric end of stomach acts as a pump that delivers small amounts of chyme to duodenum - simultaneously forcing most of its contents back towards stomach where it undergoes further mixing.

Question 13

What stimulates gastric emptying?

Answer 13

DURING GASTRIC PHASE

• stretch (distension) of way and presence of specific food components (especially protein)
• release of gastrin and activation of CNS and local reflexes.

= increased motility and emptying of stomach

Question 14

what inhibits gastric emptying?

Answer 14

DURING INTESTINAL PHASE
- release of inhibitory hormones (secretin, CCK, gastrin inhibitory peptide) and activation of inhibitory nerve reflexes.

= decreased motility and emptying of stomach.

Question 15

Describe the motility of the small intestine.

Answer 15

spontaneous “house keeping” contractions (even when empty) - sweeps any contents (Secretions, debris, bacteria) through into colon keeping the small intestine clean.

Question 16

which part of the GIT performs “house keeping” contractions?

Answer 16

small intestine to keep it clean (spontaneous even when empty)

Question 17

what is “house keeping” contractions a result of?

Answer 17

• migrating myoelectric complex
  (slow wave activity starts in duodenum as previous wave reaches terminal ileum)

NB - it increases segmentation contractions to provide thorough mixing and brings material into contact with absorptive surfaces.

Question 18

What does the Ileocaecal sphincter control?

Answer 18

• rate of entry of substances into large intestine
• stops bacteria/pathogens getting into small intestine from large
• normally closed/opened by peristaltic wave as part of gastro-ileac reflex (and gastrin)

Question 19

structure and function of large intestine?

Answer 19

• storage and release of faecal material
• absorption of fluid and electrolytes (Na+ and Cl-)
• changes semi-liquid material to solid
• no vili thus no absorption of nutrients
• material can be retained for 18-24 hours
• main motility = segmentation (haustal contractions)
• 1-3 times a day strong peristalsis occurs to cause mass movement of material into empty descending colon -> sigmoid colon -> rectum

Question 20

describe defeacation reflex

Answer 20

1. distension/stretch of rectal walls by faeces triggers depolarisation of sensory (afferent fibres) -> spinal cord.
2. Parasymp (efferent fibres) stimulate contraction of rectal walls and relaxation of internal anal sphincter.
3. If convenient, voluntary relaxation of external anal sphincter (defecation may be delayed temporarily by conscious (cortical) controls.

Question 21

what is diarrhoea?

Answer 21

Passage of watery faeces resulting from an increase of colonic fluid volume which produces distension and activates defection reflex.

Question 22

what causes an increase in colonic fluid volume? (diarrhoea)

Answer 22

1. defective ion transport
   - active transport of Na+ is key to normal absorption, could be inhibited by bile salts, fat malabsorption, congenital defect, formation of inflammatory mediators etc.
2. Osmosis
   - fault nutrient digestion/absorption/intake of indigestible material e.g. cellulose or Mg, stays in lumen rating osmotic pressure.
3. Hypermotilty of intestine
   - reduces ability of colonic mucosa to absorb water (IBS)
4. Active secretion
   - common response to irritation by laxatives/bacterial toxins.

Question 23

what is constipation?

Answer 23

• difficulty in defeacation
• excessive dehydration of faeces in large intestine
• may result in haemorrhoids
• may over expose colonic mucosa to carcinogens
• delayed transit due to inadequate dietary fibre (lack of bulk gives inadequate stretch stimulus for peristalsis/defecation reflex)

due to: drugs, anticholinergics, female hormones, IBS, lack of exercise and bad habits.

Question 24

Drugs that enhance motility?

Answer 24

dopamine antagonists (metoclopramide, domperidone)
stimulant laxatives (senna, bisacody)

Question 25

drugs that reduce motility?

Answer 25

morphine and other opiates, diphenoxylate, loperamide, anticholinergics, peppermint oil