Lecture 4: Gastromotility Flashcards
describe the action of chewing
voluntary activity involving skeletal muscle aided by chewing reflex
what are the functions of chewing
- prevents airway obstructions
- lubrication and digestion
- ^ SA of food exposed to secretions
describe the 3 stages of chewing
voluntary (oral) stage:
- food bolus squeezed into pharynx by tongue
pharyngeal stage:
- food into oesophagus whilst protecting resp tract
oesophageal stage:
- transport of food from pharynx into stomach
outline the process of the pharyngeal stage of swallowing
- pharyngeal mechanoreceptors around opening of pharynx
- food touches/stimulates mechanic receptors
- activates swallowing centre in medulla oblongata
- initiates efferent motor responses via somatic (glossopharyngeal) and parasympathetic (vagus) nerves
- automatic pharyngeal contraction;
- -> soft palate prevents nasal reflux
- -> palatopharyngeal contraction
- -> vocal cord approximation
- -> larynx upwards
- -> closes epiglottis
- upper oesophageal sphincter relaxes
- pharyngeal peristalsis
(pharyngeal stage takes less than 6 seconds)
describe the oesophageal stage of swallowing
motility:
- food bolus in oesophagus
- -> primary peristalsis
- oesophageal distension
- -> secondary peristalsis
musculature:
- upper 1/3 of oesophagus is striated muscle
- lower 2/3 of oesophagus is smooth muscle
- innervated by glossopharyngeal and vagus nerves
food reception by stomach:
- lower oesophageal peristalsis
- -> stomach relaxation
- -> opening of oesophageal sphincter
outline the functions of the stomach
storage:
- vagovagal reflex
- -> receptive relaxation up to 1.5L mainly in fundus and body
mixing:
- slow spontaneous rhythmic contractions progressively ^ from body –> antrum –> mix w/ gastric gland secretions –> chyme
emptying:
- intense peristaltic contractions in antrum versus constriction of pyloric sphincter
what might be seen in a clinical perspective when the bottom oesophageal sphincter isn’t working
px w/ hiatus hernia
what is hiatus hernia
- incompetent oesophageal valve
- reflux of gastrointestinal contents back up the oesophagus
- causing irritation and heart burn due to acidity
what condition might be seen if the bottom oesophageal sphincter doesn’t open (works too well)
achalasia
how might achalasia be treated
- surgery
- CCBs
describe the electrical activity of the GIT smooth muscle
- food in stomach activates spontaneous (peristaltic) constrictor waves, 3-4min
- two types:
- slow waves
- -> mediated by interstitial cells of Cajal
- spike potentials
- -> RMP >-40mV
- -> ^ freq and potency by stretch of GIT and activation of parasympathetic NS
- -> can be reduced by norepinephrine and sympathetic NS
(PNS (ACh) = excitatory, SNS (norepinephrine) = suppressive)
describe the regulation of gastric emptying
- balance between intense antral contractions and degree of pyloric resistance
- spike potentials –> tight ring like constrictions –> pyloric pump
- chyme enters duodenum not > rate at which constituents can be processed
where is the vomiting centre in the brain
medulla oblongata
what is the effect of vomiting
- loss of NaCl, H2O, H+
- leads to dehydration and metabolic alkalosis
what factors can stimulate the vomiting centre
- emotion
- motion
- circulating chemicals (chemoreceptor trigger zone)
- pharyngeal stimulation
- gastric/duodenal irritation/distention
what are some response after stimulation of vomiting centre
- pyloric sphincter and gastric relaxation
- duodenal contraction
- glottis closes and soft palate raised
- forced inspiration
- abdominal/diaphragm contraction
- relaxation of LES and UES
describe the movements of the small intestine
- spontaneous activity of GI smooth muscle –> contraction/movement:
- segmentation contractions
- -> mixing of intestinal contents
- peristalsis
- -> propulsive movement of chyme
- migratory motility complex
- -> mass clearance of small intestine
typical chyme transit time is 2-4hrs
describe segmentation contractions of small intestine
- dominant form of contracting in small intestine
- stretch
- -> localised concentric contraction
- -> segmentation and mixing of chyme
- 12min in duodenum, 8-9min in ileum
due to slow waves –> spike potentials
- intrinsic, but excitability influenced by ANS and hormones
describe peristalsis of small intestine
- stimuli:
- -> distension
- -> chemical/physical irritation
- -> parasympathetic activation
- concentric contraction proximal to point of stimulation –> peristaltic wave
- weak, localised –> slow forward movement, distribution of chyme
- unidirectional transmission aided by receptive relaxation
- mediated by myenteric plexus, ^ by gastroenteric reflux and GI hormones
describe migratory motility complex of small intestine
- fasted state
- -> bursts of intense electrical activity, stomach to ileum
- -> migratory motility (myoelectric) complex
- motilin from M cells –> ^ vagal impulses, repeated every 75-90mins, suppressed by eating
- propulsion
- -> sweep remaining small intestine contents towards colon
- -> inhibit migration of colonic bacteria, limit bacterial growth
outline emptying of small intestine
- determined by ileocecal valve –> prevents fecal backflow
- resistance to emptying –> <2L chyme/day
- opening promoted by local peristalsis and via gastroifleal reflex
- regulated by pressure, chemical irritation
outline emptying of large intestine
- movements slower than small intestine
- mixing movements
- -> sustained segmentation contractions –> haustrations
- propulsive movements
- -> mass movement esp after eating
- defecation reflex
- -> intrinsic w/ parasympathetic fortification
what are the functions of large intestine
- absorption of water and electrolytes
- storage of fecal matter
describe movements of the large intestine
MIXING MOVEMENTS:
segmentation contraction:
- circular muscle and teniae coli
- -> haustrations
- last 1-2 mins, provide minor forward propulsion
- main function is mixing
- -> <200ml faeces per day
- parasympathetic stimulation ^ contraction
MASS MOVEMENTS:
- 1-3 times per day, esp after eating, <30 sec over ~ 30 mins
- distension
- -> distal relaxation, loss of haustrations
- -> coordinated contraction
- gastrocolic reflex
- -> ^ mass movements, mediated by ANS, gastrin, CCK
describe the movements of sigmoid colon and defecation
mass movements in sigmoid colon:
- filling of rectum
- relaxation of internal anal sphincter
- immediate urger to defecate
defecation reflex:
- intrinsic component - distention of rectum
- -> ^ afferent signals in myenteric plexus
- -> peristalsis in colon and sphincter relaxation
- parasympathetic activation
- -> fortifies intrinsic reflex involves sacral segments of spinal cord
- reflex initiation
- -> forced inspiration, closure of glottis abdominal contraction and conscious effort
what factors can stimulate diarrhoea and how do they affect GIT
factors:
- enteritis/colitis
- bacterial toxins
- emotion
how affect GIT:
- ^ intestinal motility
- ^ intestinal secretion
- malabsorption
what are the results of diarrhoea
- dehydration
- hypokalaemia
- Metabolic Acidosis
what are the treatments of diarrhoea
- cessation of eating
- drugs to decrease gut motility
- oral/IV rehydration w/ H2O NaCl and sucrose
