S2) Physiology of the Gut Flashcards
Which three systems/mechanisms control the gut?
- Autonomic nervous system
- Enteric nervous system
- Hormones and paracrine substances
Identify the two neurons in the autonomic nervous system
- Preganglionic neuron
- Postganglionic neuron
Distinguish between the function of preganglionic and postganglionic fibres
- The preganglionic fibre synapse with an autonomic ganglia outside the CNS
- The postganglionic fibre synapse on peripheral effector organs
Which substances are released by postganglionic and preganglionic nerve fibres respectively?
- All preganglionic fibres release Ach
- Post ganglionic fibres release Ach, neuropeptides or noradrenaline
Describe the anatomical position of the sympathetic nerve fibres
- Found at T5-L3
- Pass through (paravertebral) sympathetic trunk without synapsing
The SNS forms presynaptic splanchnic (abdominopelvic) nerves.
Identify them
- Greater (T5-9)
- Lesser (T10-11)
- Least (T12)
The splanchnic nerves synapse with prevertebral ganglia and mainly innervate blood vessels.
Identify some of these prevertebral ganglia
- Coeliac
- Renal
- Superior mesenteric
- Inferior mesenteric
Identify the nervous structures of the parasympathetic system
- Vagus nerve
- Pelvic splanchnic nerves (S2-4)
Which substances do postganglionic nerve fibres release in the parasympathetic nervous system?
- Acetylcholine
- Peptides
I. Gastrin releasing peptide
II. Vaso inhibitory peptide
What do preganglionic fibres do in the parasympathetic nervous system?
- Preganglionic fibres synapse in walls of the viscera
- Innervate smooth muscle/endocrine and secretory glands
Compare and contrast the structures innervated by the PNS and SNS
- Sympathetic nervous system: coeliac ganglia, superior mesenteric, inferior mesenteric
- Parasympathetic nervous system:
I. Vagus nerve: oesophagus→transverse colon
II. Pelvic nerve: transverse colon→anal canal
Compare and contrast the ganglionic fibres of the PNS and SNS
- Sympathetic nervous system:
I. Short preganglionic fibres
II. Post ganglionic fibres extend to myenteric and submucosal plexuses
III. Noradrenergic
- Parasympathetic nervous system:
I. Long preganglionic fibres
II. Post ganglionic fibres extend to myenteric and submucosal plexuses
III. Cholinergic and peptidergic
The enteric nervous system is a division of the overall system.
Describe its role in the body
- Can function completely independently (from brain)
- Exists from oesophagus to anus
What are the two main plexuses in the enteric nervous system and where are the found?
- Submucosal (Meissner’s) – submucosa
- Myenteric (Auerbach’s) – in between circular and longitudinal muscle
What are the functions of the two plexuses in the enteric nervous system?
- Submucosal (Meissner’s) – secretions and blood flow
- Myenteric (Auerbach’s) – motility
What do the plexuses of the enteric nervous system communicate with?
- Parasympathetic (vagus and pelvic nerves)
- Sympathetic (prevertebral ganglia)
What are the two broad categories for gastro-intestinal hormones?
- Gastrin family:
I. Gastrin
II. Cholecystokinin (CCK)
- Secretin family:
I. Secretin
II. Gastric inhibitory polypeptide (GIP)
Describe the role of G cells in the secretion of gastrin
- G cells in antrum of stomach
- Increases gastric acid secretion
Describe the role of I cells in the secretion of CCK
- I cells in duodenum and jejunum
- Increases pancreatic/gallbladder secretions
I. Stimulated by fat and protein
II. Gallbladder contracts and pancreas stimulated
Describe the role of S cells in the duodenum in the secretion of secretin
- Stimulated by H+ and fatty acids
- Increases HCO3 from pancreas/gallbladder
- Decreases gastric acid secretion
Describe the role of cells in the duodenum and jejunum in the secretion of GIP
- Stimulated by sugars, amino acids and fatty acids
- Increases insulin
- Decreases gastric acid secretion
Describe visceral pain in accordance to the primitive gut tube divisions
- Foregut structures: epigastric pain
- Midgut structures: periumbilical pain
- Hindgut structures: suprapubic/hypogastric pain
What are other sorts of pain that can be felt in the body and their associated conditions?
- Right upper quadrant/right shoulder tip pain: gallstones
- Back pain: pancreatitis/AAA
- Retrosternal pain: oesophagus
Motility is the contraction and relaxation of the GI tract.
What is the purpose of this?
- Moving things along
- Mix contents
- Grind contents
Almost all muscle in the gut is smooth muscle.
Which GI structures are composed of skeletal muscle?
- Pharynx
- Upper 1/3 of oesophagus
- External anal sphincter
Which two forms of smooth muscle are found in the gut?
- Circular muscle
- Longitudinal muscle
Phasic contractions allow for motility in the gut.
Identify and describe its 2 forms
- Periodic – propulsion and mixing
- Tonic – constant level of contraction
In which structures can you observe tonic contractions?
- Upper stomach
- Ileocaecal valve
- Internal anal sphincter
Another form of contraction is peristalsis.
Describe the 3 functions of this mechanism
- Propel contents in one direction
- Contraction proximal to contents
- Relaxation distal to contents
Another form of contraction is segmentation.
Describe the function and purpose of this mechanism
- Contraction splits contents, then relaxes
- To and fro movements that mix contents
Another form of contraction is mass movement.
Where does this mechanism occur and what does it involve?
- Occurs in distal colon
- Rapid movement of contents into rectum
- Gastrocolic reflex to expel residual waste
Describe the role of the nervous system (SNS and PNS) in gut motility
- Sympathetic stimulation (noradrenaline) – decreases motility
- Parasympathetic stimulation (acetylcholine) – increases motility
Identify and describe three forms of motility problems which may arise
- Paralytic ileus – loss of GI contractility
- Achalasia – failure of LOS to relax (dysphagia)
- Hirschsprung’s disease – lack of myenteric and submucosal plexuses
Identify five substances which are secreted in the gut
- Water
- Acid/Alkali
- Enzymes
- Mucus
- Waste products
A fluid balance needs to be maintained between which 3 factors?
- Ingested material
- Secretions from the gut
- Absorption
Where are substances secreted from?
- Saliva
- Gastric
- Intestinal: liver and pancreas
What is the function of stomach acid secretion?
- Innate barrier to infection
- Prepares proteins for digestion
- Activates enzymes
What is the function of HCO3 secretions?
Neutralise the acid (food acidity and stomach acid)
Describe how waste products are secreted
- Mainly from liver
- Done in bile
- E.g. cholesterol and bilirubin*
What three things do emulsifiers do?
- Increase surface area of lipids
- Aids digestion by lipases
- Allows lipid break down products to be transported in the gut (micelles)
Provide an example of an emulsifier
Bile salts
Describe four functions of mucus secretions
- Protects against acidic environment in stomach
- Protects against bacteria in small intestine
- Harbours bacteria in large intestine (provides partial food source)
- Lubricates
Which parts of the GI tract can one observe enzyme secretions?
- Saliva
- Stomach
- Pancreas
- Lining of intestine
What role do enzyme secretions have in the GI tract?
Break down:
- Protein (proteases)
- Fat (lipases)
- Carbohydrates (amylases)
Identify 5 key gands in the GI tract where secretions come from
- Acini of salivary glands
- Acini of exocrine pancreas
- Gastric glands (stomach)
- Brunner’s glands (duodenum)
- Intestinal glands (crypts of Lieberkuhn)
Identify the three requirements for absorption
- Movement across enterocyte e.g. apical membrane, cytosol, basolateral membrane
- Active absorptive cells e.g. simple columnar cells
- Small enough to absorb (mechanical breakdown and chemical digestion)
Which three structures create a large surface area?
- Permanent folds (plica circulares)
- Villi
- Microvilli (brush border)
Identify some conditions which can reduce absorption
- Diarrhoea
- Malnutrition
- Anaemia
I. Crohn’s disease
II. Coeliac disease
Identify three types of substances that get absorbed in the body
- Nutrients:
I. Carbohydrates, proteins, lipids
II. Water & Fat soluble vitamins
III. Bile salts
IV. Fe2+ and Ca2+
- Electrolytes
- Water
Water absorption is passive and follows electrolytes/nutrients.
How does this occur after a meal?
Water uptake driven by nutrients coupled with Na+ (sodium co-transporters)
Water absorption is passive and follows electrolytes/nutrients.
How does this occur in between meals?
Na+ + Cl- are absorbed (sodium/hydrogen & chloride / bicarbonate exchangers)
Water absorption in the colon includes an additional mechanism so that stool can be desiccated.
Identify this
Epithelial Na+ channel (ENaC – identical to distal tubule of kidney)
What is the peritoneal cavity?
The peritoneal cavity is the continous area (potential space) between the parietal peritoneum lining the abdominal wall and the visceral peritoneum surrounding the abdominal organs
What does the peritoneal cavity normally contain?
The peritoneal cavity only contains a thin film of peritoneal fluid
What are the two major divisions of the peritoneal cavity?
- Greater sac: larger, comprises majority of peritoneal cavity
- Lesser sac: smaller, aka omental bursa
What is the connection between the two major divisions of the peritoneal cavity called?
The lesser sac is connected with the greater sac through the epiploic foramen (an opening in the lesser sac)
What are the boundaries of the lesser sac?
- Anterior: quadrate lobe of liver, stomach, lesser omentum
- Posterior: left kidney, adrenal gland, pancreas
The rectovesical pouch is an area in the peritoneal cavity where abnormal fluid can collect.
Where can it be found?
Rectovesical pouch is the pocket that lies between the rectum and the urinary bladder in human males
The hepatorenal recess is an area in the peritoneal cavity where abnormal fluid can collect.
Where can this be found?
Hepatorenal recess is the space that separates the liver from the right kidney
