gastrointestinal system Flashcards
what is the GI tract
-a muscular tube, differentiated along its length for sequential food processing
-intestines suspended and anchored within abdominal cavity by mesenteries (membranes)
-hollow organs separated by sphincters
-accessory glands/organs secrete into lumen
what are the functions of the GI tract
-motility propels ingested food
-secretions from associated glands
-digestion and hydrolysis into absorbable molecules
-absorption into blood stream of nutrients, electrolytes and water
what is the structure of the GI wall
-there can be variations on basic structure related to function :
=muscosal layer for epithelial cells, lamina propria and thin muscular muscoae
=submuscoal layer for connective tissue, gland and large blood vessels
=circular and longitudinal smooth muscle layers (enteric neurones between)
-serosa (squamous epithelia)
what role does the mouth play in digestion
-mechanical disruption if food (mastication)
-multi-parted exocrine glands- lingual lipase and amylase initiates lipids and starch hydrolysis
-saliva lubricates bolus, antimicrobial so it resists bacteria, buffers (neutralises acid) and dissolves food
-sensory regulated relayed to brainstem (CNS) from mechanoreceptors and chemoreceptors
-swallowing moves bolus into the oesophagus
what role does the oesophagus play in digestion
-lumen lined with squamous stratified epithelia
-swallowing reflexes- closes upper oesophageal sphincter and initiates primary peristaltic wave, coordinated sequential contractions
-continued distension initiates secondary peristaltic wave by ENS
-vagovagal reflex controls lower oesophageal sphincter (LOS) tone
-PNS vagus nerve controls LOS opening by releasing VIP neurotransmitter
-sensory feedback to brain
what does ENS stand for
-enteric nervous system
what are 3 divisions of the ANS which regulates the GI tract
-extrinsic: sympathetic and parasympathetic innervation
-intrinsic: ENS which is the primary mechanism
-can direct all function of GI tract even in absence of extrinsic innervation
where is ENS
-solely within GI tract but modified by the brain
-two main plexuses- ganglia within submucosal and myenteric plexuses
-neurochemically very complex
what’s neural control: parasympathetic
-parasympathetic ganglia within plexuses coordinate information received from PNS and relay to smooth muscle, endocrine and secretory cells
-postganglionic neurons are either cholinergic (release ACh) or peptidergic (release peptides)
what’s neural control: sympathetic
-sympathetic postganglionic nerve fibres are adrenergic (release norepinephrine)
-nerve fibres are mixed afferent and efferent e.g. sensory and motor information is relayed between GI tract and CNS, coordinated by the plexuses
what role does the stomach play in digestion: what are the 3 phases of motility in the stomach
1) receptive relaxation in thin-walled orad stomach to receive food
2) 3 muscular layers of caudad region contract to mix food with gastric juice from mucosal glands (ANS controlled) forming chyme
HCL, pepsinogen( pepsin which is a protease), intrinsic factor, mucus
3)gastric emptying through pyloric sphincter into small intestine- fat content and H+ which slows emptying
what’s motility
-subthreshold slow waves produce weak contraction (tonic)
-action potentials on top (phasic contractions)
-low pressure organs separated by sphincters- specialised circular muscles with positive resting pressure
-regulate antegrade and retrograde movement
what is the GI tract motility like
-most contractile tissue is unitary smooth muscle- cells electrically coupled via gap junctions leading to rapid spread of action potentials leading to coordinated contraction
-contractions can be tonic or phasic
-slow waves- sub threshold membrane depolarisation (inward Ca2+ through channel), repolarisation (outward K+)
-extension to threshold generates a true action potential
-contraction is preceded by electrical activity
what’s phasic contraction
-periodic contraction and relaxation
what’s Tonic contraction
-constant level of contraction and tone
what role does the small intestine play in digestion
-digestion and absorption of nutrients- carbs, AA, fats and water and ions
-chyme is mixed with digestive enzymes and pancreatic secretions
-many hydrolytic enzymes embedded in membrane of brush border
-3 sections: duodenum-> jenjunum-> ileum
-SA increased on 3 levels: plicae> villi>microvilli
-peristalic contractions propel the chyme
-segmentation contractions split and expose chyme to secretions through coordinated actions
-material not absorbed passes through lleocaecal sphincter into caecum of large intestine
what role does the pancreas play in digestion
-1L of exocrine pancreatic secretion into the duodenum per day:
. solution rich is HCO3- secreted by centroacinar and ductal cells to neutralise H+ delivered from stomach
.enzymes secreted by acinar cells
-PNS stimulates secretion, SNS inhibits (‘rest and digest’)
-secretion phases: cephalic, gastric and intestinal
what role do the liver and gall bladder play in digestion
-liver secretes biles in which the gall bladder stores, concentrates and ejects
-bile:
=water
=amphipathic bile salts for lipid emulsification which aids fat digestion and absorption
=bilirubin for waste products and RBC breakdown (excretion)
=cholesterol- excretion and recirculation
=phospholipids emulsify fats
=electrolytes such as Na+, K=, HCO3-, neutralise the acidic chyme
what are GI regulatory substances
-hormones- from GI endocrine cells
-paracrines- from endocrine cells, they act locally
-neurocrines- released from neurones following action potential
what are the 3 primary functions of the large intestine
1) absorbs water and electrolytes: Na+, Cl-, HCO3- (aldosterone causes and increase in sodium absorption)
2)makes and absorbs vitamins K and B
3) forming and propelling faeces (ferments indigestible food and bacteria) to be excreted
what role does the large intestine have in digestion
-surface columnar epithelial cells (absorptive) interspersed with crypts e.g. glands secrete
-taenia coli - 3 bands longitudinal muscle and haustra
-motilitiy: segmented contraction- caeca, and proximal colon, mix conetnts- haustra
-mass movement, 1-3 /day over large distance called gastrocolic reflex
