GI wrap up Flashcards
How is chewing controlled?
Chewing is controlled by somatic nerves to skeletal muscles of the mouth and jaw.
Pressure of food activates mechanoreceptors –> reflexive inhibition of muscles holding the jaw shut.
5 muscles/groups of muscles involved in swallowing
- Buccinator
- Suprahyoids
- Muscles of the floor of the mouth
- Infrahyoids
- Pharyngeal constrictor muscles
Role of the buccinator and suprahyoid muscles in swallowing
Manipulate food during chewing and flatten the floor of the mouth during the first phase.
Suprahyoids act to elevate the hyoid bone.
Role of the muscles of the floor of the mouth in swallowing
Depress mandible if hyoid is fixed, or elevate hyoid bone
and larynx if mandible is fixed, during second phase
Role of the infrahyoid muscles in swallowing
Pull the larynx down during the third phase
Rolf of pharyngeal constrictor muscles in swallowing
Innervation?
Contract sequentially to push food down the throat.
Innervation = pharyngeal plexus (CN X and XI)
Name the 4 infrahyoid muscles
Thyrohyoid
Omohyoid
Sternohyoid
Sternothyroid
Why is it important that the swallowing mechanism is quick?
So as not to interfere with breathing - epiglottis blocks off the trachea to prevent aspiration.
Describe stage 1 of swallowing
Motor and sensory innervation.
Voluntary/Involuntary?
Swallow preparation.
Voluntary.
Food is compressed against the roof of the mouth and pushed towards the oropharynx by the tongue.
Hypoglossal nerve = motor of tongue.
Lingual nerve (branch of mandibular division of the trigeminal nerve (CN V3)) supplies sensory innervation to anterior 2/3 tongue.
Describe stage 2 of swallowing
Voluntary/Involuntary?
Passage from mouth to pharynx.
Involuntary.
Soft palate contracts and elevates, closing off the nasopharynx.
Suprahyoids contract, lifting the hyoid bone and widening the pharynx.
Epiglottis covers trachea.
Describe stage 3 of swallowing
Voluntary/Involuntary?
Passage down the oesophagus.
Involuntary - sequential contraction of the pharyngeal muscles moves the bolus through the oesophagus towards the stomach.
Infrahyoids depress the hyoid.
What is the gag reflex?
Reflex elevation of the pharynx caused by irritation of the oropharynx.
Controlled by a reflex arc between IX (afferent) X (efferent) nerves.
3 major salivary glabds
parotid, sublingual, submandibular
What type of secretions do parotid/sublingual/submandibular glands have?
Parotid = serous Sublingual = mixed but mainly mucous Submandibular = mixed
Is salivary secretion PSNS or SNS?
PSNS stimulates secretion, SNS inhibits
3 functions of saliva
- lubricant for mastication, swallowing and speech
- oral hygiene - antibacterial, buffer
- aqueous solvent for taste
What enzyme is contained in serous secretions?
Amylase for starch digestion
Which salivary gland is not continuously active?
Which gland is the main source of saliva when stimulated vs unstimulated?
Parotid - but when stimulated they are the main source of saliva.
Submandibular = main source when unstimulated
Describe the anatomy of the parotid gland.
Important structure passing through it?
Innervation?
Superficial triangular gland.
Duct crosses masseter, pierces buccinators and enters oral cavity.
External carotid artery and facial nerve pass through it.
Innervated by CN 9, glossopharyngeal.
Describe the anatomy of the submandibular gland.
Where does it empty?
2 lobes separated by the mylohyoid muscle.
Empties into the oral cavity at the sublingual papillae.
Describe the anatomy of the sublingual gland.
Where does it drain?
Located in the floow of the mouth beneath the tongue.
Doesn’t have its own duct - drains into the submandibular duct.
List the layers of the GI tract from lumen out
Lumen
- mucosa
- submucosa with Meissner’s plexus
- muscularis propria with Auerbach’s plexis between inner circular myscle and outer longitudinal
- serosa
List some functions of the stomach
- store and mix food that has been broken down by the teeth
- breaks food down by churning
- Secretes proteases for protein digestion
- regulates emptying of contents into the duodenum
- kills microbes, sterilises thanks to HCl
- secretes pepsinogen
- secretes intrinsic factor needed for vitamin B12 absorption
Describe the anatomy of the stomach from oesophagus –> duodenum
Oesophageal sphincter Cardia Fundus Body - greater and lesser curvature Antrum Pylorus Pyloric sphincter
What 4 things make up the lower oesophageal sphincter?
- circular muscle of the gastroesophageal wall
- cardiac notch
- motility pattern due to innervation from vagus nerve branches from oesophagus to stomach
- diaphragm
What are some factors that increase pressure on abdominal contents, and are therefore risk factors for gastroesophageal reflux?
- large meal
- obesity
- pregnancy
Name the cells of the gastric mucosa and what they secrete
- mucous neck cells secrete mucous
- parietal cells secrete HCl and intrinsic factor
- chief cells secrete pepsinogen
- ECL cells secrete histamine
- G cells secrete gastrin
- D cells secrete somatostatin
Gastrin:
- site of production
- site of action
- action
Produced by G cells in the stomach.
Acts on ECL cells stimulating histamine secretion.
Acts on parietal cells stimulating their proliferation –> increased HCl secretion.
ACh:
- site of production
- site of action
- action
- produced in neurones
- stimulates gastrin secretion
- Acts on ECL cells and parietal cells, stimulating histamine and HCl secretion.
Somatostatin:
- site of production
- site of action
- action
- produces in D cells
- acts on parietal cells to inhibit HCl secretion
Secretin:
- site of production
- site of action
- action
- S cells of the small intestine
- acts on G cells to inhibit gastrin secretion, and promotes somatostatin release
CCK:
- site of production
- site of action
- action
Produced in I cells of the small intestine.
Acts on parietal cells and cells of the pancreas.
Inhibits HCl secretion
Promotes flow of digestive enzymes from the pancreas and bile from the gall bladder.
Control of HCl secretion is regulated by…
- 1 neurotransmitter
- 1 hormone
- 2 paracrine factors
- 2 enterogasterones
- 1 neurotransmitter: ACh
- 1 hormone: gastrin
- 2 paracrine factors: histamine, somatostatin
- 2 enterogasterones: secretin and CCK
Gastric mucosal defense mechanisms
- alkaline mucus on luminal surface
- tight junctions between epithelial cells
- rapid replacement of damaged cells by stem cells present in the base of pits
- feedback loops for regulation of gastric acid secretion
Which cells of the stomach secrete pepsinogen? What stimulates them to do so?
Chief cells.
Triggered by the same stimuli as parietal cells - parasympathetics, low luminal pH of stomach
How is pepsinogen activated?
Released by chief cells.
Activated by HCl in stomach lumen to form pepsin.
How does positive feedback aid the production of protease?
Pepsinogen activated by HCl to form pepsin.
Pepsin itself cleaves pepsinogen. - positive feedback loop.
Cleavage to pepsin is most efficient when pH < 2
How is pepsin inactivated?
Irreversibly inactivated by HCO3- in small intestine
Luminal volume of empty stomach?
What can this be increased to?
50mL
increases to 1.5L
Gastric motility - what process means that the stomach is sensitive to small increases in luminal pressure?
Smooth muscle in the body and fundus undergoes receptive relaxation
How is gastric motility/receptive relaxation mediated?
- PSNS (enteric nerve plexuses)
- afferent input from stomach –> brain via vagus nerve
- nitric oxide and serotonin released by enteric nerves, mediating relaxation
Where in the stomach do the most powerful peristaltic contractions occur? Why is this important?
- most powerful contractions occur in the gastric antrum
- this mixes luminal contents and closes the pyloric sphincter
- mechanically break down food into chyme by churning
How is contraction initiated in gastric peristalsis?
Impulse to trigger contraction is fired 3 times per minute (every 20 seconds – initiated by the interstitial cells of Cajal)
How is chyme forced back towards the antrum for further mixing in gastric peristalsis?
The closing of the pyloric sphincter means only a little chyme enters the duodenum with each wave, so antral contents are forced back towards the body to mix
How are gastric secretions and gastric emptying turned off?
- gastric contents enter the duodenum through the pyloris, decreasing the duodenal pH due to HCl
- duodenal distension also occurs
This causes increased secretion of enterogasterones (CCK and secretin) and stimulates neural receptors
How does the duodenum connect to the liver?
- superiorly, by the hepatoduodenal ligament
what is the major duodenal papilla, visible inside the duodenum?
An opening of the common bile duct and pancreatic duct into the duodenum through which bile and pancreatic secretions enter.
What is the name of the point at which the duodenum joins the jejenum?
duodenojejunal flexure
Compare the plicae circularis in jejenum vs ileum
More pronounced in the jejenum
Main location of the jejenum
mainly upper left quadrant
Main location of the ileum?
Mainly lower right quadrant
Compare the thickness of the intestinal wall in the jejenum vs ileum
Thick in jejenum, thin in ileum
Compare the arteries supplying the jejenum and ileum
Jejenum: - vasa rectae (straight arteries) longer - less arcades (arterial loops) Ileum - vasa rectae (straight arteries) shorter - more arcades (arterial loops)
Where are Peyer’s patches found? what are they?
what do they do?
- In the ileum - small masses of lymphatic tissue
- monitor intestinal bacteria populations and prevent the growth of pathogenic bacteria
What is Merckel’s diverticulum? What is it a remnant of?
Only present in 2% of the population due to incomplete obliteration.
Forms a slight bulge in the small intestine (ileum)
remnant of the vitelline duct, (joins the yolk sac to the midgut lumen of the developing fetus)
colour of the jejunum vs ileum
jejenum is red, ileum pink
Which organs are retroperitoneal?
SADPUCKER Suprarenal glands Aorta/IVC Duodenum Pancreas (head neck and body) Ureters Colon (asc and dec) Kidneys E(oe)sophagus Rectum (part)
Which organs are intraperitoneal?
SALTDSPRSS Stomach Appendix Liver Transverse Duodenal cap Small intestines Pancreas - tail only Rectum (upper 1/3) Sigmoid colon Spleen
Where is the lesser omentum?
Connects liver and stomach
Where is the greater omentom
attaches to stomach and transverse colon, covers structures beneath - apron.
What is the foramen of Winslow?
Foramen between the greater sac (general cavity of the abdomen), and the lesser sac (posterior to stomach and lesser omentum)
Branches off the coeliac trunk
- L gastric artery, supplies the lesser curvature of the stomach
- splenic artery
- common hepatic artery
Branches from the common hepartic artery (a brach off the coeliac trunk)
- Gastroduodenal artery
- Right gastric artery (supplying the lesser curvature of the
stomach and anastomoses with the left gastric artery) - Proper hepatic artery
What is the greater curvature of the stomach supplied by?
- short gastric arteries which branch off the splenic artery
- Left gastroepiploic artery which branches off the splenic artery
- Right gastroepiploic artery which branches off the
gastroduodenal artery
branches of the superior mesenteric artery - what do they supply?
- iInferior pancreaticoduodenal artery – supplies the head
of the pancreas and the duodenum
Jejunal and ileal arteries
Middle and right colic arteries – Middle supplies the
transverse colon and right supplies the ascending
colon.
Ileocolic artery – gives rise to branches to the
ascending colon, appendix, caecum and ileum.
branches of the inferior mesenteric artery - what do they supply?
Left colic artery – supplies the distal 1/3rd of the
transverse colon and the descending colon.
Sigmoid arteries – supplies the descending colon and
sigmoid colon.
Superior rectal artery – supplies the rectum and anal
canal.
Marginal artery of Drummond – forms a continuous
circle along the inner border of the colon. Formed from
anastomoses of branches from the IMA and a branch of
the middle colic artery.
What does the small intestine epithelial secrete into the lumen?
Na+, Cl- and HCO3- into the lumen, water follows
Describe the absorption that takes place in the small intestine?
- almost all fluid secreted by the small intestine is absorbed back into the blood
- salivary, gastric, hepatic and pancreatic secretions absorbed, so there is no net water absorptin
describe the motility of the small intestine
- stationary contraction and relaxation of intestinal segments - known as segmentation
- these contractions have little apparent net movement towards the large intestine
compare the surface area of the large vs small intestine
large intestine has a greater diameter, but its epithelial surface area is smaller because it is shorter, not convoluted and has no villi
primary function of the large intestine
store and concentrate faecal matter before defecation
secretions of the large intestine
mostly mucous and fluid containing K+ and HCO3-
What is the primary absorptive process of the large intestine?
active transport of NA+ from lumen, water follows by osmosis.
where does starch digestion begin?
In the mouth - alpha-amylase enzyme in salive, optimal pH 6.7
Where is 95% of starch digestion completed in the body?
In small intestine by pancreatic a-amylase.
Most ingested carbohydrates are digested and absorbed within the first 20% of
small intestine.
What does starch digestion by a-amylase produce?
Maltose (disaccharide) and a mixture of other chains
What happens to maltose and other chains produced by starch digestion in the small intestine?
Broken down into monosaccharides by oligosaccharides and disaccharidase enzymes of the luminal membranes of small intestine epithelial cells which can then be absorbed into the blood.
Name 3 monosaccharides
- Glucose
- Fructose
- Galactose
How are glucose and galactose taken up into the enterocytes?
via membrane transporter proteins:
secondary active transport coupled to Na+ via SGLT1
How is fructose taken up into enterocytes?
By GLUT5 carrier - absorption independent of luminal Na+
How do monosaccharides leave epithelial cells and enter interstitial fluid from the small intestine?
Facilitated diffusion via GLUT proteins in the epithelial cell basolateral membrane.
Then can then diffuse into the blood via capillary pores
Describe protein digestion in the stomach
Proteins partially broken down to peptide fragments in the stomach by pepsin
Describe protein digestion in the small intestine
Peptides from the stomach are further fragmented by proteolytic pancreatic enzymes - trypsin and chymotrypsin.
These fragments can now either be absorbed, or further digested to amino acids by:
- carboxypeptidases from the pancreas
- amino peptidases located on the luminal membrane of the small intestine epithelial cells
Name 2 proteolytic pancreatic enzymes
trypsin and chymotrypsin
Protein absorption -
how are free amino acids absorbed into intestinal epithelial cells?
By secondary active transport coupled to Na+.
How are chains of 2/3 amino acids absorbed into the small intestine?
By secondary active transport coupled to the H+ gradient.
What happens to dipeptides and tripeptides once within the cytosol of intestinal epithelial cells?
- they are hydrolysed to free amino acids
- they are now able to leave the cel and enter the interstitial fluid through facilitated diffusion transporter in the basolateral membrane
- they then diffuse passively into portal blood
where in the gut does protein digestion and absorption largely occur?
in the upper portion of the small intestine
what is the role of pancreatic lipase?
In the small intestine - hydrolyses the bonds of triglycerides, forming monoglycerides and free fatty acids - large lipid droplets
Describe emulsification in the small intestine
large lipid drops from tryglyceride hydrolysis are divided into many very small lipid droplets to increase their SA:V ratio and lipase accessibility.
This speeds up digestion.
WHat happens to the products of emulsification in the small intestine?
- absorption of the water insoluble products would be too slow
- SO lipids and bile salts interact to form micelles containing FFAs, monoglycerides and cholesterol
- micelles transport the poorly soluble monoglycerides and fatty acids to the surface of the enterocyte where they can be absorbed. They are small enough to fit between microvilli.
Why are micelles constantly breaking down and reforming?
As the concentration of free lipids decreases due to their diffusion across the membrane, micelles release more lipids as they breakdown to replenish the free lipids that have been absorbed, to maintain a concentration gradient.
Micelles themselves cannot pass through the cell membrane but once transported to the brush border they release their contents which are absorbed by passive diffusion.
Once in the epithelial cells of the small intestine, what happens to fatty acids and monoglycerides?
Why does this process occur?
- they are resynthesised into triglycerides in the sER
- this maintains a diffusion gradient for these molecules to enter the cell from the lumen
How are chylomicrons form?
Triglycerides combine with cholesterol, phospholipids and apolipoproteins in the epithelial cells of the small intestine
Describe the passage of chylomicrons
- formed in the small intestine epithelial cells
- pass into lacteals - lymphatic vessels in intestinal villi
- lacteals have large pores which allow chylomicrons to pass into lymph
- from lymph chylomicrons can pass into systemic veins
How are chylomicrons in systemic veins enabled to enter cells(adipocytes and myocytes)?
Lipoprotein lipase converts triglycerides to fatty acids and glycerol, able to enter adipocytes and myocytes.
What happens to fatty acids once in body cells?
oxidised as fuel or reesterified for storage
Which vitamins are fat soluble?
How are they absorbed?
ADEK
Absorbed in micelles, in the same way as fats
How are water soluble vitamins absorbed?
what is the exception?
Diffusion or mediated transport.
Except B12, which must bind to intrinsic factor to be absorbed
How is iron absorbed for use by the body?
Iron ions actively transported into intestinal epithelial cells, where most are incorporated into ferritin or transferrin.
Summary of absorption in the small intestine - how do the following enter epithelial cells of villi?
- Glucose and galactose
- Fructuse
- Amino acids
- Dipeptides and tripeptides
- Short chain fatty acids
- Micelles containing long-chain fatty acids and monoglycerides
- Glucose and galactose: secondary active transport with Na+
- Fructose: facilitated diffusion
- Amino acids: active/secondary active transport with Na+
- Dipeptides and tripeptides: secondary active transport with H+
- Short chain fatty acids: simple diffusion
- Micelles containing long-chain fatty acids and monoglycerides: micelles release contents at brush border which can then be absorbed by simple diffusion
Sequence of events following formation of micelle to entering cell
- contents released at brush border of intestinal epithelial cell
- FFAs and monoglycerides enter cell by passive diffusion
- form triglycerides in the cell to maintain conc gradient
- incorporated into chylomicrons
- enter lacteal of a villus
- enter lymph
- enter systemic bv
- lipase breaks down triglycerides into ffa and monoglycerides, able to enter cell
which tissues use fatty acids as fuel?
renal cortex
skeletal muscle
cardiac muscle
liver
Cell type: oral cavity
SSNKE
cell type: lips
SSNKE, keratinised at its margin
cell type: tongue
dorsal keratinised, ventral non-keratinised
cell type: oesophagus
SSNKE
cell type: stomach
simple columnar epithelium, glandular
cell type: small intestine
simple columnar epithelium (with crypts and villi)
cell type: colon and rectum
simple columnar epithelium (no villi)
cell type: anal canal
stratified squamous epithelium, becomes keratinised at distal end
Describe the formation of the primitive gut in week 4
- formation of foregut and hindgut
- connection to the yolk sac
Cranio-claudal and lateral folding cause the opening of the gut
tube to the yolk sac to draw closed forming two pockets: anterior
and posterior intestinal portions (foregut and hindgut).
The remaining part of the yolk sac and allantois remains outside
the embryo
The yolk sac and primitive gut are connected by the Vitelline duct
Foregut extends from where to where?
Supplied by?
From the mouth to the first part of the duodenum.
Supplied by the coeliac artery.
In the embryo the gut extends from where to where?
Oropharyngeal membrane –> cloacal membrane
What do the oropharyngeal and cloacal membrane consist of?
Endoderm and ectoderm, no mesoderm.
Borders of the midgut
Blood supply?
From the first part of the duodenum to 2/3 of the transverse colon.
Supplied by the superior mesenteric artery
Borders of the hindgut?
Blood supply?
From the distal 1/3 of the transverse colon to the anal canal.
Supplied by the inferior mesenteric artery.
How are the lower part of the foregut, the midgut and major part of the hindgut suspended from the posterior abdominal wall by week 5?
- suspended by dorsal mesentery
Ventral mesentary is only present at the region of the foregut.
During rotation of the stomach after week 5 in the embryo what happens to the dorsal mesentery?
How is the greater omentum formed?
- gets pulled to the right creating a space behind the stomach called the lesser sac
- bulges down and continues to grow forming a double layered sac in from of the transverse colon
- the 4 layers fuse together to form the greater omentum
what happens to ventral mesentery during rotation of the stomach?
- gets pulled to the right
- liver develops in it, dividing it into the lesser omentum and falciform ligament
