Gastrointestinal tract Flashcards
What organs are involved in the gastrointestinal tract?
- mouth
- oesophagus
- stomach
- small intestine
- large intestine
What is the GI tract?
a tube, differentiated along its length for sequential processing of food
What anchors the suspended intestines within the abdominal cavity?
mesenteries (membranes)
What are immunologic omentum?
immunological organ that is a source if inflammatory cells & stem cells that play a role in the control of infection through wound-healing & regeneration
What separates the different hollow organs?
sphincters & secretory accessory glands/organs
What are sphincters?
muscular rings - control the progression of the food from 1 section to the next. They also prevent the backflow of material from one section to the previous one.
What is the function of GI tract?
- motility propels ingested food
- secretions from associated glands
- digestion/hydrolysis into absorbable molecules
- absorption into the bloodstream of nutrients, electrolytes, water
What are the 2 layers of the GI tract?
- mucosal layer
- submucosal layer
What is in the mucosal layer?
in touch with contents of the lumen - contains epithelial cells.
- either absorbable or secretory cells
- lamina propria (connective tissues with blood & lymph vessels)
- thin muscularis mucosae
What is in the submucosal layer?
connective tissue (collagen, elastin), glands larger blood vessels (that transport molecules, as well as removing & metabolites
What shape are the 2 smooth muscle layers (enteric neurones between)?
circular + longitudinal
What is serosa?
squamous epithelia
What is the function of the mouth?
- mechanical disruption of food
- (mastication) aims to increase the surface area to attack by enzymes & chemicals later in the GI tract.
- also lubricate the food with the secretions of these 3 glands.
What can be found in the multi-parted exocrine glands?
ligual lipase + amylase initiates lipid + starch
What are the 3 sections of the salivary glands?
- parotid gland
- submandibular gland
- sublingual gland
What are the roles of saliva?
- lubricate food (bolus)
- antimicrobial - resists bacterial infection
- buffering - neutralises acid
- dissolves food - detection by taste buds
What controls the level of mastication & swallowing?
the brainstem (CNS) will control the level of mastication & swallowing
What receptors relay sensory information to the CNS?
mechanoreceptors (found in mouth & teeth) & chemoreceptors relay sensory information to brainstem
How do later parts of the GI tract prepare for the arrival of food?
they start to secrete, in preparation for the arrival of the bolus
What does stimulation of the central nervous system enable?
enables later sections of the GI tract to get ready for the arrival of food?
What does swallowing do?
move the bolus into the oesophagus
What is the oesophagus lined with?
lumen lined with stratified squamous epithelia
What is the benefit of having stratified squamous epithelia on the lumen of the oesophagus?
loose cells at the surface allow cells to be lost without causing damage to the underlying tissue, as they are in contact with the bolus
How does the swallowing reflex work?
- closes upper oesophageal sphincter.
- initiates primary peristaltic wave, coordinates sequential contractions
- continued distension (food in the oesophagus) initiates secondary peristaltic wave (by ENS - enteric nervous system - the nervous system within the GI tract)
What initiates the second peristaltic wave?
continued distension - food in the oesophagus - with activates the ENS (enteric nervous system)
What is the ENS (enteric nervous system)?
nervous system within the GI tract
What does the vasovagal reflex control?
lower oesophageal sphincter (LOS) tone
How does the PNS (parasympathetic nervous system) vagus nerve control the LOS (lower oesophageal sphincter) opening?
by releasing VIP neurotransmitter
What is the autonomic nervous system made up of?
- sympathetic nervous system
- parasympathetic nervous system
- enteric nervous system
What are the extrinsic part of the nervous system that regulate the GI tract?
sympathetic & parasympathetic
What are the intrinsic part of the nervous system that regulate the GI tract?
enteric nervous system
What modifies the enteric nervous system?
the brain - 2 MAIN plexuses
What are the 2 main plexuses in the brain?
- ganglia within submucosal
- myenteric plexuses (including pancreases + biliary system)
How does neural control of the GI tract work?
- parasympathetic ganglia within plexuses coordinates information received from PNS and relay to smooth muscle, endocrine & secretory cells
- postganglionic neurones are either cholinergic (release ACh) or peptidergic (release peptides)
What do sympathetic postganglionic nerve fibres release?
they are adrenergic (release norepinephrine)
What are nerve fibres a mix of?
mixed afferent & efferent i.e sensory & motor information is relayed between GI tract & CNS, coordinated by the plexuses
What are the 3 phases to stomach motility?
- receptive relaxation in thin-walled Orad stomach to receive food.
- 3 muscular layers of Caudad region contract to mix food with gastric huice from mucosal glands (ANS control) forming chyme
- Gastric emptying through pyloric sphincter into small intestine (SI) - fat content & H+ slows emptying
What is chyme a mixture of?
- HCI (hydrochloric acid)
- Pepsinogen - pepsin (protease)
- Intrinsic factor
- Mucus
Why is there mucus in chyme?
to prevent the acid & proteins damaging the linin
What is low level, weak contraction below the threshold called?
tone
What are contractions called when there are action potentials on top of tone contractions?
phasic contractions
How many sphincters are there in the Oddi?
6+
What type of pressure is found in sphincters?
positive resting pressure
What do sphincters regulate?
antegrade (forward) & retrograde (backward) movement of food
What is the most contractile tissue?
unitary smooth muscle
Why is unitary smooth muscle the most contractile tissue?
cells electrically coupled via gap junctions –> rapid spread of action potentials leading to coordinated contraction
What are 2 types of contractions?
tonic - constant level of contraction/tone
phasic - periodic contraction + relaxation (3-12/min)
What cells are the gastrointestinal pacemaker cells?
interstitial cells of Cajal (ICC) in stomach, SI, LI
What is contraction always preceded by?
electrical activity
What is the function of the small intestine?
- digestion + absorption of nutrients - carbs, amino acids, fats, water & ions
- chyme is mixed with digestive enzymes + pancreatic secretions
- many hydrolytic enzymes embedded in membrane of brush border
What are the 3 sections of the small intestine?
- duodenum
- jejunum
- ileum
How does surface area increase?
plicae –> villi –> microvilli
How much of exocrine secretion occurs into the duodenum per day?
1 litre
Why is the exocrine solution rich in bicarbonate?
to neutralise the H+ delivered from the stomach
From which cells is bicarbonate released from?
centroacinar & ductal cells
How is pancreas secretion controlled by the autonomic nervous system?
PNS - stimulates secretion
SNS - inhibits the ‘rest & digest’ response
What do liver cells (hepatocytes) secrete?
bile
When is CCK hormone ejected from the small intestine?
when chyme enters - following relaxation of Oddi
What is bile made up of?
- water
- amphipathic bile salts (lipid emulsification)
- bilirubin - involved in erythrocyte breakdown
- cholesterol - excretion & recirculation
- phospholipids - emulsify fats
- electrolytes - neutralise the acidic chyme
How does 95% of bile acids get recirculated to the liver?
enterohepatic circulation
What type of contractions are seen in the small intestine?
peristaltic contractions propel the chyme
What do enterochromaffin cells release?
serotonin (neurocrine) - peristaltic reflex
What happens to the material not absorbed in the small intestine?
passes through lleocaecal sphincter into the caecum of the large intestine
What are the 3 types of GI peptides classified as?
- hormones
- paracrines
- neurocrines
Where do GI pepetide hormones come from?
GI endocrine cells - e.g. GIP
Where do GI pepetide paracrines come from?
from endocrine cells, act locally, e.g. somatostatin
Where do GI peptide neurocrines come from?
released from neurones, following action potential
What are the 3 main functions of the large intestine?
- absorbs water & electrolytes: Na+, Cl-, HCO3, K+.
- Make & absorb vitamins K+B.
- Forming + propelling faeces (ferments indigestible food - bacteria), to be excreted.
What increases Na+ absorption?
aldosterone
What are the 2 parts of the large intestine?
- surface columnar epithelial cells (absorptive)
- taenia coli - 3 bands of longitudinal muscle that is shorter than the large intestine caused bunching, forming small sacs - haustra
How does food move through the large intestine?
- segmented contraction - caecum + proximal colon, mic contents - haustra.
What reflex moves contents through the large intestine?
gastrocolic reflex
