gastrointestinal system

Question 1

what is the GI tract

Answer 1

-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

Question 2

what are the functions of the GI tract

Answer 2

-motility propels ingested food
-secretions from associated glands
-digestion and hydrolysis into absorbable molecules
-absorption into blood stream of nutrients, electrolytes and water

Question 3

what is the structure of the GI wall

Answer 3

-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)

Question 4

what role does the mouth play in digestion

Answer 4

-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

Question 5

what role does the oesophagus play in digestion

Answer 5

-lumen lined with squamous stratified epithelia
-swallowing reflexes- closes upper oesophageal sphincter and unities 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

Question 6

what does ENS stand for

Answer 6

-enteric nervous system

Question 7

what are 3 divisions of the ANS which regulates the GI tract

Answer 7

-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

Question 8

where is ENS

Answer 8

-solely within GI tract but modified by the brain
-two main plexuses- ganglia within submucosal and myenteric plexuses
-neurochemically very complex

Question 9

what’s neural control: parasympathetic

Answer 9

-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)

Question 10

what’s neural control: sympathetic

Answer 10

-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

Question 11

what role does the stomach play in digestion: what are the 3 phases of motility in the stomach

Answer 11

1) receptive relaxation in thin-walled road 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

Question 12

what’s motility

Answer 12

-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

Question 13

what is the GI tract motility like

Answer 13

-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

Question 14

what’s phasic contraction

Answer 14

-periodic contraction and relaxation

Question 15

what’s Tonic contraction

Answer 15

-constant level of contraction and tone

Question 16

what role does the small intestine play in digestion

Answer 16

-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: place> 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

Question 17

what role does the pancreas play in digestion

Answer 17

-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

Question 18

what role do the liver and gall bladder play in digestion

Answer 18

-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

Question 19

what are GI regulatory substances

Answer 19

-hormones- from GI endocrine cells
-paracrines- from endocrine cells, they act locally
-neurocrines- released from neurones following action potential

Question 20

what are the 3 primary functions of the large intestine

Answer 20

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

Question 21

what role does the large intestine have in digestion

Answer 21

-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