Stomach anatomy and motility Flashcards
Describe the path taken by the cervical oesophagus
- upper aspect is on the midline
- Passes to left as approaches thoracic inlet
Describe the thoracic oesophagus
- Begins at thoracic inlet
- Covered by serosa
- Passes through inlet on left
- Moves back to previous position dorsal to trachea
- Runs in mediastinum
Describe the abdominal oesophagus
- following passage through oesophageal hiatus of diaphragm
- very short
- terminates at cardiac of stomach
List the related structures at the cervical portion of the oesophagus
- Common carotid artery
- Internal jugular vein
- Tracheal duct
- Cervical lymph nodes
- Vagosympathetic trunk
- Recurrent laryngeal nerve
What happens to the thickness of the wall of the oesphagus going from cranial to caudal?
- The wall thickens
What are the layers of the oesophageal wall?
- Connective tissue adventitia (outer layer)
- Muscular layer
- Mucuous membrane (stratified squamous epithelium, tight junctions)
Describe the muscular coats of the oesophagus
- Striateed intially in all species
- In cat, pig, horse becomes smooth distally
- get circular and longitudinal muscle neaer to stomach
Describe the tunica mucosa of the oesophagus
- Stratified squamous epi
- cornified in herbivores
- lamina propria without glands
- Lamina muscularis incomplete, most extensive in caudal aspect
- Submucosa well developed
- loosely binds mucosa and muscularis layers
What is different in the tunica mucosa of the oesophagus in dogs, pigs, cats and horses?
- Dogs: mucous glands over entire length
- Pig: glands only cranially
- Cat and horse: glands only at pharyngo-oesophageal junction
Describe the tunica muscularis of the oesophagus
- Inner and outer layers
- Continuous cranilly with pharyngeal muscles
- Mid-region, muscle layers arranged in intercrossing spirals
- Caudally the outer fibres more longitudinal, inner circular and thicker
What are the 3 phases of swallowing?
- Oropharyngeal
- Oesopharyngeal
- Gastro-oesophageal
Outline the process of swallowing
- food to base of tongue
- Pharyngeal muscles contract to push food to laryngopharynx
- after contraction of pharyngeal muscles, cricopharyngeal and thyropharyngeal muscles relax
- Opens oesophagus - bolus can pass
- After bolus passage, muscles contract once more (duration of relaxation determined by sensory input from pharyngeal filling)
- High sphincter pressure prevents aspiration or reflux of oesophageal contents
- Peristaltic wave continues along oesophagus carrying bolus to lower oesophageal sphincter
- if bolus not carried to stomach, secondary wave from local stimulation and carries bolus along
What is the difference in control of pharyngeal and oesophageal phases of swallowing?
- pharyngeal controlled by central pattern generator- no peripheral feedback
- Oesophagus works by peripheral feedback
Explain why conditions can affect pharyngeal or oesophageal phases of swallowing independently?
2 distinct pattern generators for the 2 regions
What is the blood supply to the cervical region of the oesophagus?
- Thyroid and subcalvian arteries
- Branches from left and right common carotids
What is the blood supply to the thoracic region of the oesophagus?
Broncho-oesophageal artery
What is the blood supply to the abdominal region of the oesophagus?
Left gastric and splenic arteries
What is the venous drainage of the cervical portion of the oesophagus?
External jugular veins
What is the venous drainage of the thoracic portion of the oesophagus?
- Oesophageal vein
- In dogs and cats, pair of oesophageal veins drain into azygous via bronchooesophageal veins
- In horse, oesophageal vein drains directly into azygous vein
Describe the lymphatics of the oesophagus
- Cranial aspect into cervical, middle and caudal deep cervical nodes
- Thoracic lymph from oesophagus drains into cranial and caudal mediastinal lymph nodes
where does the sympathetic innervation of the oesophagus come from?
From cervico-thoracic ganglion
where does the parasympathetic innervation of the oesophagus come from?
Branches of the vagus nerve
Where are the ganglia supplying the oesophagus found?
Between the internal and external muscle layers
Describe the physiological characteristics of teh GI tract smooth muscle
- Unitary (syncitial)
- Mass of many fibres, contract as single unit
- Arranged in sheets or bundles
- Cells in contact, joined by gap junctions = free movement of ions between each muscle cell
- Nerves innervating smooth mm branch over many fibres
- Nerves do not make direct contact, diffuse junctions = transmitter secreted into local environment of muscle
- Contraction and relaxation due to transmission of ACh and noradrenaline
- Stretch of lumen = contraction of muscles (peristalsis)
- Local tissue factors and hormones also influence activity
- Some areas have spontaneous activity
What are the main neurotransmitters of the GI smooth muscle?
- Acetylcholine (PS)
- Noradrenaline (S)
What type of synaptic junction is found at neuromuscular junctions of the GI smooth muscle and why?
- Diffuse
- secretes NT into local environment
- Stimulates contraction of lots of muscle cells at once
What are the 4 regions of the stomach?
- Cardia
- Fundus
- Corpus
- Pylorus/antrum
(in that order)
In a dorsoventral radiograph, where would the gas and fluid be in the stomach?
- Gas: fundus
- Fluid: body (corpus)/pylorus
In a ventrodorsal radiograph, where would gas and fluid be found in the stomach?
- Gas: corpus (possibly pylorus)
- Fluid: fundus
In a left lateral radiograph, where would gas and fluid be found in the stomach?
- Gas: pylorus (poss. corpus)
- Fluid: fundus
In a right lateral radiograph, where would gas and fluid be found in the stomach?
- Gas: fundus
- Fluid: pylorus
What are the muscle layers of the stomach?
- Outer longitudinal
- Inner circular
Where is circular muscle found in the stomach?
- Corpus and pylorus
- Does not encircle fundus
- Forms pyloric and cardiac sphincters (with contribution from internal oblique layer)
Where is the internal oblique layer of muscle found in the stomach?
- Fundus and body around greater curvature
- Longest bundles closest to lesser curvature and often reach angular incisura
What is the function of the internal oblique muscle layer of the stomach?
Facilitates greates expansion
Describe the outer muscle layer of the stomach?
- Continuous with oesophagus
- Passes over surface and forms 2 bands alnog each curvature
- Oblique course to long axis - external oblique fibres
- Priminent in horse and pig
- In these species fans out over fundus, not parallel as in cat and dog
- Pyloric canal has heavy longitudinal muscle coat
Describe the parasympathetic supply to the stomach
- Vagal
- Ganglia in wall
- Inhibits fundus
- Stimulates pylorus
Describe the pH gradient of the stomach
- Increased in acidity (decreases in pH) as move from top to bottom
- All regions are acidic
Describe the non-glandular region of the stomach/oesophageal mucosa
- Oesophagus
- Cornified stratified squamous epithelium
Briefly outline the glandular regions of the stomach
- Proper gastric glands (mucous, chief, parietal)
- Pyloric glands (seromucous)
- Cardiac glands (seromucous)
List the mucosal cells of the stomach and their secretions
- Parietal: HCl, intrinsic factor
- Chief cells: pepsinogen
- Endocrine cells includes G cells and ECL cells
- G cells: gastrin
- ECL cells: histamine
Describe monogastric stomach microbiologi
- Barrier to infection
- minimal microbiology
- Acid = barrier to infection and colonisation of lower intestinal tract
- Age may impact barrier function
- Neonatal stomachs may be infected by overgrowth (not as acidic, less flora present)
- Food type affects barrier function
- Pharmacology affects barrier function
- Infectious dose of pathogen depends on tolerance to acid
Describe the formation and secretion of HCl
- DO NOT PRODUCE HCl IN CYTOPLASM
- H+ and Cl+ transported independently and join up outside of parietal cell
- Hydrogen ions generated by carbonic anhydrase on carbon dioxide to carbonic acid, then to hydrogen and bicarbonate
- Bicarbonate actively pumped into interstitial fluid then to blood
- K+ and Cl- transported into gland lumen by facilitated diffusion
- H+ actively pumped into gastric fluid, stimulates uptake of K+
- H+ and Cl- in gland lumen
- Changes osmolarity, outward diffusion of water, HCl produced
What are the factors that regulate HCl secretion?
- ACh from cholinergic neurones
- Gastrin
- Histamine
What happens to the pH of blood and urine of monogastrics after a meal and why?
- Both rise
- Bicarbonate secreted into blood in H+ generation
- Known as alkaline rise
Describe the mechanism of pepsin formation
- Pepsinogen synthesised and stored in chief cells
- Is zymogen for pepsin (inactive)
- HCl creates environment where pepsinogen unfolds, cleaves self (autocatalysis)
- pepsin cleaves off 44 AAs from pepsinogen to produce more pepsin (positive feedback, pepsin = more pepsin)
- Hydrolyse peptide linkages within protein molecules
- Preferentially acts on linkages involving aromatic acids e.g. phenylalanine and tyrosine
- Breakdown to short peptides rather than individual amino acids
Describe the mechanism of pepsin secretion
- pepsinogen release only active in acid environments (pH 2.0)
What are the 3 phases of gastric secretion (in order)?
- Cephalic
- Gastric
- Intestinal
Describe the cephalic phase of gastric secretion
0 CNS derived
- See, smell, taste (think) food - prepares stomach for food
- Neural output via PS fibres running in vagus
- Output synapses in submucosal plexus
- Postganglionic PS fibres innervate mucous cells, chief cells, parietal cells, G-cells
- Short phase, overlaps with gastric phase
Describe the gastric phase of gastric secretion
- Begins when food arrives in stomach
- Distension
- Increase pH of gastric contents
- Presence of undigested materials (esp proteins)
- triggers gastric juice secretion
- lasts several hours, gastrin stimulates contractions (mixing waves)
- Maintains high er pH at mucosal surface and therefore ongoing aacid secretion until thoroughly mixed
Describe the neural component of the gastric secretion phase
- Stretch and chemoreceptors in mucosa trigger local (submucosal and myenteric plexuses) and vagal reflex arcs
- Postganglionic release of ACh at parietal cells
Describe the hormonal response component of the gastric secretion phase
- Neural stimulation and presence of peptides in chyme - stimulates secretion of gastrin
- Gastrin in circulation, stimulates parietal and chief cells via blood steam
- Accelerates secretion rates
Describe the intestinal phase of gastric secretion
- Chyme enters small intestine
- Several hours of mixing
- Waves of contractions sweep down stomach
- Arrival of partly digested proteins to duodenum => secretion of gastrin from duodenal wall G-cells, increased acid and enzyme production
- Higher acid release in response to protein meal
- Quickly dampened by CCK and gastric inhibitory peptide
Describe the neural response in the intestinal phase of gastric secretion
- Chyme leaving stomach releives some of distention - less stimulation
- Food entering duodenum stretches walls => enterogastric reflex
- Less gastric contractions
Describe the hormonal response in the intestinal phase of gastric secretion
- Arrival of lipids and carbohydrates in duodenum => release of CCK and gastric inhibitory peptide
- Reduces gastric secretions and contractions (slow motility)
- Drop in pH below 4.5 stimulates release of secretin by enteroendocrine cells of duodenum
- Inhibits parietal and chief cells
- Stimulates buffer release fromo pancreas
- Pancreas take in bicarbonate
What are some of the negative feedback elements in gastric secretion?
- Duodenal signals
- Stomach pH falling towards 2.0 inhibits gastrin release
Describe how gastrin affects HCl production
- Digestive tract hormone
- Released in response to peptides directly stimulating G-cells
- Also from vagal stimulation
- Acts on ECL cells to produce histamine, and direct to parietal cells
- Both actions increase HCl production
- Inhibited if pH drops below pH 2.0
Describe how histamine affects HCl production
- Stimulates HCl production and secretion
- Stimulated by gastrin action on ECL cells
- Activates adenylate cyclase to use cAMP messenger to stimulate ion pump (K+ into parietal cell, H+ out into lumen of gland)
What are the 2 neural controls of HCl production and secretion
- From CNS and short reflex in wall of stomach
- Both trigger gastrin secretion
Describe the control by the CNS in HCl production
- Long reflex arcs via vagal nerves
- Cephalic phase of digestion
- Sight, smell, taste of food
Describe the control by the short reflexes in teh wall of the stomach in HCl production
- Walls stretch
- Peptide stimulation of sensory nerve endings
Define “zymogen”
- Enzyme precursor
- AKA proenzyme
- Is a protein precursor that is converted into an enzyme when activated by another enzyme e.g. pepsinogen for pepsin
Define “endopeptidase”
An enzyme which breaks peptide bonds other than the terminal ones in a peptide chain (trypsin, chymotrypsin)
Define “exopeptidase”
An enzyme which breaks the terminal bond in a peptide chain (carboxypeptidase)
Give some of the pancreatic proteases and state whether they are endo or exopeptidases
- Trypsin
- Chymotrypsin
- Elastase
- All endopeptidases (different specificities)
- Carboxypeptidase (exo)
What is the action of trypsin?
Breaks peptide bonds involving AAs arginine and lysine
What is the action of chymotrypsin?
Targets peptide bonds involving tyrosine and phenylalanine
What is the action of carboxypeptidase?
- Exopeptidase
- Removes last amino acid of polypeptide chain
- Not specific to identities of AAs in volved
- Generates free AAs
Describe the conversion of trypsinogen to trypsin
- Chyme enters duodenum
- Enteropeptidase from SI triggers conversion of trypsinogen (released from pancrease) to trypsin
What are the basic functions of the stomach?
- Storage of food
- Mechanical degradation of food
- Secretion of gastric juices
Breifly describe the rotation of the stomach in volvulus in the dog
- Stomach twists around longitudinal axis of digestive tract
- Rotation commonly clockwise (viewing animal from behind)
- Can rotate up to 360degrees this way, 90degrees anticlockwise
What are teh effects of distension and volvulus on blood supply to the stomach?
- Prevents blood supply to and from stomach
- Blood flow to heart reduced
- Blood flow to stomach lining cut off
- Blood flow to liver and spleen cut off
List the organs in contact with the stomach
- Diaphragm (left crus bound to fundus of stomach)
- Liver (lesser curvature)
- Spleen (greater curvature)
- Duodenum at pylorus
Briefly describe the histological appearance of the non-glandular regions of stomachs of pigs and herbivores
- Stratified squamous epithelium
- May see division between glandular and non-glandular = margo plicatus
- Mucosa (made up of epithelium, lamina propria, mscularis mucosa)
- Submucosa
- Muscular layers
- Serosa
Briefly describe the histological appearance of the glandular cardiac region of the stomach
- Single layer columar epithelium
- Forms gastric pits (mucus production)
Briefly describe the histological appearance of the glandular fundic region of the stomach
- Parietal cells present (look like fried eggs, pink, dark yolky nucleus)
- Surrounded by darker smaller cells = chief cells
- May also be MALT present
- Mucosa, submucosa, musclaris, serosa
Briefly describe the histological appearance of the glandular pyloric region of the stomach
- Single columnar epithelium
- Very long gastric pits
- Similar to cardia
- Mucosa, submucosa, muscularis, serosa
Where is gatrin synthesised?
G cells in stomach and duodenum
When is gastrin secreted?
- IN response to peptides directly stimulating G-cells
- Also by vala secretion
- Stops when pH<2
What is the activity of gastrin?
- Stimulates parietal cells in stomach to secrete HCl
- Stimulates Chief cells to secrete pepsinogen
Where is somatostatin stored and secreted?
- Stored in pancreas
- Secreted by gastric glands in stomach
Under what conditions is somatostatin secreted?
- Low pH in stomach
What is the action of somatostatin?
- Inhibits release of gastrin and HCl from stomach
- Inhibits secretin and cholecystokinin release from duodenum
- Inhibits release of glucagon and insulin from pancreas
Where is histamine secreted from?
ECL cells in stomach
When is histamine secreted in the stomach?
In response to stimulation of ECL cells by gastrin
What is the function of histamine in the stomach?
Regulates HCl secretion
Where is grehlin secreted from?
Stomach and pancreas
Under what conditions is grehlin secreted?
- When stomach empty, grehlin secreted
- When stomach strethc, secretion stops
What is the action of grehlin?
- Increases food intake by stimulating hunger at the level of the hypothalamus
What is the action of acetylcholine in the stomach?
- Helps regulate HCl secretion
- Causes Chief cells to secrete pepsiinogen
