control of gut motility Flashcards
aim of G.I. tract control
to maximise absorption of nutrients, whilst minimising damage from toxins
how is control of GI tract achieved
-achieved by regulating motility
- controlling secretion of digestive juices
senses the lumen contains
the intestinal epithelium is the largest sensory organ in the body
senses the lumen contents:
- distention = mechanoreceptors
- osmolality = osmoreceptors
- acidity = chemoreceptors
- digestive products = chemoreceptors
receptor activation causes
nerves: a) long reflexes (brain), b) short reflexes (enteric NS)
hormones
paracrine transmission
what regulates movement of food through the G.I tract
smooth muscle (and sphincters) regulate movement of food
- circular muscle - narrows
- longitudinal muscle - shortens
mucosa of the gut wall
epithelium
lamina propria
muscularis mucosae
serosa of gut wall
- areolar connective tissue
- epithelium
muscularis of gut wall
- circular muscle
- longitudinal muscle
extrinsic nerves
parasympathetic (preganglionic) -> increased motility, increased secretion (integrates with ENS many transmitters released)
sympathetic (postganglionic) -> decreased motility (decreased blood flow)
G.I smooth muscle
- 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
slow waves of depolarisation
- 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
modulation of slow waves
- 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
G.I hormones
- 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
enteroendocrine cells
- microvilli on lumen side - sense lumen contents or movement
- 90% of EE cells - enterochromaffin cells
- remaining 10% = hormones
enterochromaffin cells
stimulation -> release of serotonin (5-HT)
released serotonin stimulates afferent neurons via 5-HT3 receptors
SERT
removes serotonin to terminate signal
PIEZO 2 channel
mechanosensor
paracrine transmission
- 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
the mouth
- chewing (mastication)
- decrease size of food
- increase surface area
- increase exposure to saliva
oral stage of swallowing
voluntary
- food pushed towards pharynx by tongue
pharyngeal stage of swallowing
swallowing reflex - involuntary
- elevation of soft palate
- contraction of pharyngeal muscles
(inhibition of respiration and closure of glottis)
- opening of upper oesophageal sphincter
oesophageal stage of swallowing
involuntary
- peristalsis in oesophagus
- opening of lower oesophageal sphincter
stomach function
- storage
adaptive and receptive relaxation - peristalsis
nerves and hormones (modulate force)
- 3 contractions/ min
-churning action - pyloric S. closed by wave of contraction
- limits emptying (3ml/min)
-retains particles over 2 mm in diameter
regulation of emptying
- increased distention of the stomach detected by gastrin/nerves, causes increased emptying
- 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
small intestine
transit = 2-4 hours
functions:
1. mixing (increased digestion and absorption
2. propulsion (minor role)
main types of movement in the small intestine
- segmentation (mixing and propulsion
- peristalsis (propulsion)
segmentation
- contractions of circular muscle
- contractions occur at different sites -> mixing
- basal electrical rhythm and local distension -> myenteric plexus = local contractions
- gradually moves chyme forward
BER in duodenum and ileum
12 min-1 in duodenum
ileum = 8 min -1
peristalsis
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
gastro-ileal reflex in small intestine
increased gastric distension -> increased ileal motility and ileocaecal valve relaxes
ileogastric reflex in small intestine
- distention of ileum -> decreased gastric motility
ileocaecal valve
between small and large intestine
- normally closed
opens when:
- peristaltic wave reaches it
- as part of gastro ileal reflex
large intestine functions
absorption of water and sodium
storage of feacal matter
transmit time 33h - range 5-45 hour
mixing in large intestine
- 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
propulsion in colon
ileocaecal valve -> transverse colon
gradual forward movement by HAUSTRAL contractions
propulsion in colon
descending colon -> rectum
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
rectum
- usually empty - segmental contraction impedes filling
- mass movements - faecal matter into rectum
- distends rectum -> rectospincteric reflex
anal sphincters
internal sphincter - smooth muscle - 80-85% resting anal sphincter tone
(involuntary
external sphincter - skeletal muscle (voluntary)
defaecation reflex - short reflex
- peristalsis in colon and rectum
- relaxation of internal anal sphincters
- propels faeces through anus
defaecation reflex - long reflex
-strengthens peristalsis
- increased intra abdominal pressure
- pelvic floor descends
defaecation
puborectalis contracts to choke rectum or relaxes to straighten pathway to anus
inter digestive motility
presence of food -> increased nerves and hormones = motility
peristaltic waves housekeeping functions
- empties stomach of large particles
- removes sloughed off epithelial cells
- prevents bacteria in colon moving into small intestine
