control of gut motility

Question 1

aim of G.I. tract control

Answer 1

to maximise absorption of nutrients, whilst minimising damage from toxins

Question 2

how is control of GI tract achieved

Answer 2

-achieved by regulating motility
- controlling secretion of digestive juices

Question 3

senses the lumen contains

Answer 3

the intestinal epithelium is the largest sensory organ in the body

senses the lumen contents:
- distention = mechanoreceptors
- osmolality = osmoreceptors
- acidity = chemoreceptors
- digestive products = chemoreceptors

Question 4

receptor activation causes

Answer 4

nerves: a) long reflexes (brain), b) short reflexes (enteric NS)

hormones

paracrine transmission

Question 5

what regulates movement of food through the G.I tract

Answer 5

smooth muscle (and sphincters) regulate movement of food
- circular muscle - narrows
- longitudinal muscle - shortens

Question 6

mucosa of the gut wall

Answer 6

epithelium
lamina propria
muscularis mucosae

Question 7

serosa of gut wall

Answer 7

• areolar connective tissue
• epithelium

Question 8

muscularis of gut wall

Answer 8

• circular muscle
• longitudinal muscle

Question 9

extrinsic nerves

Answer 9

parasympathetic (preganglionic) -> increased motility, increased secretion (integrates with ENS many transmitters released)

sympathetic (postganglionic) -> decreased motility (decreased blood flow)

Question 10

G.I smooth muscle

Answer 10

• irregular arrangement of actin/ myosin -> still generates tension when stretched
• cells grouped into bundles of 5000 cells
• cells are electrically coupled (gap junctions) = functional units
• unstable membrane potential (5-15 mV) = slow waves
• slow waves determines frequency of contraction

Question 11

slow waves of depolarisation

Answer 11

• resting Vm = -40 to -60 mV
• slow waves superimposed on resting Vm
• size of slow wave modulated by nerves and hormones
• contraction only if slow wave reaches threshold Vm

Question 12

modulation of slow waves

Answer 12

• food stimulates nerve and hormonal activity: increases or decreases the size of the maximum depolarisation
• nerves (intrinsic and extrinsic)
  • ACh -> depolarisation (increased contraction)
  • NO, VIP, NA, Ade -> hyper pol ( decreased contraction

hormones
- gastrin, motilin -> depolarisation
- secretin, GIP -> hyperpolarisation

Question 13

G.I hormones

Answer 13

• all short chain peptides
• secreted by the enteroendocrine cells (found in the mucosa) into the blood
• target various regions of G.I. and glanfs
• may have effects on CNS

Question 14

enteroendocrine cells

Answer 14

• microvilli on lumen side - sense lumen contents or movement
• 90% of EE cells - enterochromaffin cells
• remaining 10% = hormones

Question 15

enterochromaffin cells

Answer 15

stimulation -> release of serotonin (5-HT)

released serotonin stimulates afferent neurons via 5-HT3 receptors

Question 16

SERT

Answer 16

removes serotonin to terminate signal

Question 17

PIEZO 2 channel

Answer 17

mechanosensor

Question 18

paracrine transmission

Answer 18

• short hormones do not enter blood; diffuse through interstitial fluid
• histamine best example (gastric secretion)
• prostaglandins, bradykinin ( released in response to irritation of gut wall

Question 19

the mouth

Answer 19

• chewing (mastication)
• decrease size of food
• increase surface area
• increase exposure to saliva

Question 20

oral stage of swallowing

Answer 20

voluntary
- food pushed towards pharynx by tongue

Question 21

pharyngeal stage of swallowing

Answer 21

swallowing reflex - involuntary
- elevation of soft palate
- contraction of pharyngeal muscles
(inhibition of respiration and closure of glottis)
- opening of upper oesophageal sphincter

Question 22

oesophageal stage of swallowing

Answer 22

involuntary
- peristalsis in oesophagus
- opening of lower oesophageal sphincter

Question 23

stomach function

Answer 23

1. storage
   adaptive and receptive relaxation
2. peristalsis
   nerves and hormones (modulate force)
   - 3 contractions/ min
   -churning action
3. pyloric S. closed by wave of contraction
   - limits emptying (3ml/min)
   -retains particles over 2 mm in diameter

Question 24

regulation of emptying

Answer 24

1. increased distention of the stomach detected by gastrin/nerves, causes increased emptying
2. products of digestion in duodenum -> decreased emptying
   a) enterogastric reflex: peptides and HCl in duodenum -> decreased mixing and decreased emptying

b) hormones e.g. CCK, G.I.P and secretin
food in duodenum -> decreased motility

Question 25

small intestine

Answer 25

transit = 2-4 hours functions: 1. mixing (increased digestion and absorption 2. propulsion (minor role)

Question 26

main types of movement in the small intestine

Answer 26

- segmentation (mixing and propulsion - peristalsis (propulsion)

Question 27

segmentation

Answer 27

- contractions of circular muscle - contractions occur at different sites -> mixing - basal electrical rhythm and local distension -> myenteric plexus = local contractions - gradually moves chyme forward

Question 28

BER in duodenum and ileum

Answer 28

12 min-1 in duodenum ileum = 8 min -1

Question 29

peristalsis

Answer 29

ring of contraction forms behind chyme -> pushes chyme forward - occurs at any point in small intestine - initiated by distension and coordinated by myenteric plexus - travels on a few cm - influenced by neural/ hormonal factors - peristaltic rush -> diarrhoea

Question 30

gastro-ileal reflex in small intestine

Answer 30

increased gastric distension -> increased ileal motility and ileocaecal valve relaxes

Question 31

ileogastric reflex in small intestine

Answer 31

- distention of ileum -> decreased gastric motility

Question 32

ileocaecal valve

Answer 32

between small and large intestine - normally closed opens when: - peristaltic wave reaches it - as part of gastro ileal reflex

Question 33

large intestine functions

Answer 33

absorption of water and sodium storage of feacal matter transmit time 33h - range 5-45 hour

Question 34

mixing in large intestine

Answer 34

- haustration = segmentation - constriction of circular muscle - longitudinal muscle arranged in 3 strips (taeniae coli and does not prevent bulging of wall -> ovoid segments = haustra) - contractions occur at different sites -> gradual 'digging over' of faecal material

Question 35

propulsion in colon ileocaecal valve -> transverse colon

Answer 35

gradual forward movement by HAUSTRAL contractions

Question 36

propulsion in colon descending colon -> rectum

Answer 36

mass movements (gastrocolic/ duodenocolic reflexes) - occur 3 or 4 times per day for 10-30 min - initiated by distension - ring of contraction propel chyme 30-80% length of the colon - often after meals

Question 37

rectum

Answer 37

- usually empty - segmental contraction impedes filling - mass movements - faecal matter into rectum - distends rectum -> rectospincteric reflex

Question 38

anal sphincters

Answer 38

internal sphincter - smooth muscle - 80-85% resting anal sphincter tone (involuntary external sphincter - skeletal muscle (voluntary)

Question 39

defaecation reflex - short reflex

Answer 39

- peristalsis in colon and rectum - relaxation of internal anal sphincters - propels faeces through anus

Question 40

defaecation reflex - long reflex

Answer 40

-strengthens peristalsis - increased intra abdominal pressure - pelvic floor descends

Question 41

defaecation

Answer 41

puborectalis contracts to choke rectum or relaxes to straighten pathway to anus

Question 42

inter digestive motility

Answer 42

presence of food -> increased nerves and hormones = motility

Question 43

peristaltic waves housekeeping functions

Answer 43

- empties stomach of large particles - removes sloughed off epithelial cells - prevents bacteria in colon moving into small intestine