Session 2 - Physiology and Development Flashcards
Describe the neuronal structure of the autonomic nervous system?
The cell bodies reside in the CNS. Each pathway = 2 neurones -Preganglionic -Postganglionic (Preganglionic synapse with post ganglionic)
Which neurotransmitters are released by the pre and post ganglionic fibres of the ANS?
Pre ganglionic = Ach
Post ganglionic = Ach, Neuropeptides & Norepinephrine
Describe the presynaptic splanchnic nerves and their roots.
Greater (T5-9)
Lesser (T10-11)
Least (T12)
Which ganglia do the splanchnic nerves synapse with?
With Pre-vertebral ganglia
What is the main effect of sympathetic nerves of the GI?
To control blood flow to the various organs.
E.g. fight/flight to reduce blood flow to the GI
Where do parasympathetic nerves originate from?
Cranial-sacral outflow.
Nerves come from cranium.
And from the sacrum.
Which parasympathetic nerves innervate the gut?
Vagus nerve - main
Some pelvic splanchnic nerves (S2-4)
How do the sympathetic and parasympathetic nervous system affect the guy differently?
Sympathetic
Reduce blood flow, slow gut activity. (Shut down)
Parasympathetic
Stimulates the gut - causing digestion/ movement.
Where do parasympathetic fibres synapse?
Long pre-synaptic, synapse at the organ (viscera)
Which part of the gut do the left and right vagus nerves give parasympathetic innervation to?
Left Vagus = Anterior
Right Vagus = Posterior
What is a ganglion?
A collection of nerve cell bodies, in the sympathetic nervous system, the nerves usually synapse here.
What is the enteric nervous system?
A division of the nervous system.
Exists from oesophagus to anus.
Can operate independently.
Which are the two major plexuses of the enteric nervous system?
Submucosal (Meissner’s)
-submucosa
Myenteric (Auerbach’s)
-In between circular and longitudinal muscle
What does the submucosal plexus control?
Local blood flow/ secretions etc in epithelial area.
What does the myenteric plexus control?
Muscular control / motility
What are enteroendocrine cells?
The many cells of the GI tract that secrete hormones.
Often within crypts.
What are enterochromafin like cells?
Cells that secrete histamine in GI
How do hormones from the GI go on to affect the organs of the GI?
Released into portal circulation
Through liver
Into systemic circulation
Then affect the target organs (in GI)
What is the function of cholecystokinin?
Stimulates gall bladder to contract, pancreas to release enzymes and relaxes sphincter to allow the products to be secreted in the gut.
What is the function of Gastrin?
Secreted by G-cells in antrum of stomach.
Increases gastric acid secretion.
Which two hormones are in the Gastrin family?
Gastrin
Cholecystokinin
Which two hormones are in the Secretin family?
Secretin
Gastric inhibitory polypeptide
Where is secretin secreted? What is it’s function?
S cells in duodenum.
Stimulated by H+ and fatty acids.
Increases HCO3 secretion from pancreas/gall badder.
Decreases gastric acid secretion.
What is the function of gastric inhibitory polypeptide (GIP)?
Increases insulin secretion, decreases gastric acid secretion.
From cells in duodenum/jejunum
Stimulated by sugars, amino acids, fatty acids
Why would a case of appendicitis initially cause central abdominal pain?
Visceral peritoneum becomes distended/inflamed.
Visceral peritoneum hasn’t got its own sensory nerve supply.
Afferent sensory nerve follows splanchnic nerves to the roots.
In appendicitis - least splanchnic nerve (T10-11 - small intestine to transverse colon)
Body makes assumption that pain is from T10 dermatome, and thus gives midline pain.
Why would pain in appendicitis eventually cause localised pain in the Right iliac fossa?
As appendix becomes more distended/ inflamed, it may come into contact with parietal peritoneum.
This has somatic innervation, which can localise pain better.
What are the layers of muscle which allow gut motility?
Circular muscle
Longitudinal muscle
Contractions to move contents
What are periodic and tonic contractions?
Periodic contractions are physical (fast/ short) that move and mix contents.
Tonic contractions are a constant level of contraction (minutes/hours) e.g. upper stomach
What is peristalsis?
Propulsion of contents in one direction, with contractions proximal to the contents.
What is segmentation (motility)?
Contraction splits contents, then relaxes.
These to and fro movements mix contents.
What is a paralytic ileum?
Temporary impairment of movement of gut contents (following GI surgery)
Functional obstruction - not real obstruction
What is Hirschprung’s disease?
Lack of myenteric and submucosal plexuses.
Causes functional obstruction, as muscles cannot contract properly.
(In newborns, first faeces may not be passed)
Why is mucus vital for a functional GI?
- Protects against acidic environment in stomach.
- Protect against bacteria in small intestine (contains antibacterial compounds)
- Harbours bacteria in large intestine.
- Lubricates contents
How is the large surface area in the GI formed?
Permanent folds (plicae circulates) [SA x3]
Villi [SA x 10]
Microvilli [SA x20]
How is the last small amount of water removed from the contents in the colon?
ENaC channels
Desiccate the stool (dry it out) so as little water as possible is lost.
What is the gut tube? Describe its structure.
The endoderm lines tube, running the length of the body, containing blind pouches at the head and tale, with an opening at the umbilicus.
It has a splanchnic mesoderm covering.
What are the embryonic divisions of the gut?
Foregut, midgut and hindgut.
Foregut and hindgut begin as blind diverticula.
Midgut has opening at first, is continuos with the yolk sac.
Which structures are derivatives of the foregut?
- Oesophagus
- Stomach
- Pancreas, liver & Gall bladder
- Duodenum (proximal to bile duct entrance)
Which structures are derived from the midgut?
- Duodenum (distal to bile duct entrance)
- Jejunum
- Ileum
- Caecum
- Ascending Colon
- Proximal 2/3 of transverse colon
Which structures are derivatives of the hindgu?
- Distal 1/3 of transverse colon
- Descending & sigmoid colon
- Rectum
- Upper anal canal
- Internal lining of bladder and urethra
How does the embryonic development affect blood supply?
Arterial supply to each embryonic segment from a distinct branch of the abdominal aorta.
Which blood vessels supply which parts of the gut?
- Celiac trunk: foregut
- Superior Mesenteric artery: midgut
- Inferior Mesenteric artery: Hindgut
Which exceptions in the gut have mixed blood supply?
- Duodenum:
Proximal to entry of bile duct = branches of celiac trunk
Distal = branches of Superior mesenteric artery - Pancreas:
Head = branches of Celiac trunk AND Superior Mesenteric artery.
What is the intra-embryonic coelum?
The cavity formed when the embryo folds, surrounded by mesoderm, containing the gut tube.
It is eventually gives rise to the thoracic and abdominal cavities.
What does the membrane lining the whole intra-embryonic cavity specialise into through development?
- Pericadrium
- Pleural membrane
also
- Peritoneum & peritoneal cavity
(peritoneal membrane lines abdominal cavity, and invests the viscera)
How are mesenteries formed in the embryo?
Primitive gut is suspended in intraembryonic coelum.
- Splanchnic mesoderm surrounds the new gut.
- Mesentery formed by a condensation of this mesoderm.
Where do mesenteries suspend the parts of the gut tube from?
- Doral mesentery = suspends entire gut tube from dorsal body wall.
- Ventral mesentery = ONLY in the region of FOREGUT.
How are the peritoneal sacs organised in the body?
Dorsal and ventral mesenteries in foregut region divide cavity into left and right.
Left sac - contributes to the Greater sac.
Right sac - contributes to the Lesser sac.
(comes to lie behind stomach)
How are the greater and lesser omentum formed in the embryo?
Greater omentum.
- Formed from the dorsal mesentry
- First structure seen when abdo cavity opened anteriorly.
Lesser Omentum.
- Formed from the ventral mesentery
- Free edge conducts the portal triad
What developmental process forms the greater omentum?
The folding of the stomach.
The ventral and dorsal mesenteries are flexible, and move witht he development of the stomach, forming this greater omentum.
What is the infracolic compartment? What does it contain?
Infracolic = lies below the transverse mesocolon, contains Small intestine, ascending and descending colon.
What is in the supracolic compartment?
Lies above the transverse mesocolon, contains stomach, liver and spleen.
What is a peritoneal reflection?
A change in direction.
E.g. - from parietal peritoneum to mesentery.
- from mesentery to visceral peritoneum.
What are retroperitoneal and secondary retroperitoneal structures?
Structures NOT suspended within the abdominal cavity.
Retroperitoneal = were never in peritoneal cavity, no mesentery.
Secondary = began development in peritoneum, had mesentery, BUT, with growth/development, mesentery is lost (fusion at posterior wall).
What happens developmentally in the foregut in the 4th week?
A respiratory diverticulum forms in the VENTRAL wall of the foregut (at junction of pharyngeal gut)
- respiratory primordium (ventrally)
- Oesophagus (dorsally)
How are respiratory and GI tracts separated? What can go wrong?
diverticulum separated by tracheoesophageal septum.
Abnormal positioning (1/3000 live births) can happen. Could give blind-end oesophagus, or fistulae.
How does the liver develop in the embryo?
Appears as a liver bud.
contribution from endoderm of foregut, and also from developing diaphragm
How is the liver connected to the abdominal wall?
By ventral mesentery.
The falciform ligament!
Where does the pancreas develop?
Partly in the ventral, and partly in the dorsal mesentery.
Which two folds of peritoneum hold the liver?
Lesser omentum (between liver and stomach) Falciform ligament (between liver and anterior wall of abdomen)
What gives the duodenum its C shape?
Rotation of the stomach.
What type of organ is the duodenum? Why?
Secondarily retropetitoneal.
Rotation of stomach pushes duodenum to right, then against the posterior abdominal wall, where most of it loses its mesentery.
What are the two secondary retroperitoneal structures of the foregut?
Duodenum (except duodenal cap)
Pancreas
What are fusion fascia?
Why is this important.
Where two layers of fascia fuse, e.g. when a peritoneal structure becomes retroperitoneal.
Structures can become mobilised, by freeing up the fusion fascia. (don’t have to enter retroperitoneum)
