Alimentary Canal Flashcards
What are the four activities of the alimentary canal?
Motility, secretion (required for digestion, protection and lubrication), digestion, absorption.
What are the four main layers of the digestive tract wall?
Mucosa -> submucosa -> muscularis externa -> serosa
What makes up the mucosa (from lumen outwards)?
Epithelial cells -> exocrine cells -> endocrine gland cells -> lamina propria (capillaries, enteric neurones, immune cells) -> muscularis mucosae.
What makes up the submucosa?
Connective tissue, larger blood and lymph vessels, glands, nerve network (submucous plexus).
What makes up the muscularis externa?
Circular muscle layer, nerve network (myenteric plexus), longitudinal muscle layer.
What is the serosa composed of?
Connective tissue.
Where is there skeletal muscle in the GI tract?
Mouth, pharynx, upper oesophagus and external anal sphincter.
What are the 3 smooth muscle layers in the GI tract?
Circular layer, longitudinal layer and the muscular mucosae..
What does contraction of the muscularis mucosae cause?
Change in absorptive and secretory area of mucosa (folding), mixing activity.
What allows the smooth muscle cells of the GI tract to become depolarised and contract at the same time as a synchronous wave (functional syncytium)?
Gap junctions allowing spread of electrical currents from cell to cell.
What is single unit smooth muscle?
Smooth muscle that all contracts together.
What is the activity of the pacemaker cells modified by?
Intrinsic (enteric) nerves, extrinsic (autonomic) nerves and numerous hormones.
What are the waves of depolarisation that occur in the digestive tract called?
Slow waves.
What are the pacemaker cells in the GI tract called?
Interstitial cells of Cajal (ICCs).
What does the slow wave amplitude have to be to cause contraction?
Sufficient to trigger SMC action potentials.
What ion currents cause the upstroke and downstroke of smooth muscle cell action potentials?
Upstroke is calcium current, downstroke is potassium current.
Where in the GI tract wall are ICCs located?
Between the longitudinal and circular muscle layers and in the submucosa.
What is the basic electrical rhythm?
The rate at which slow waves occur in the different organs.
What determines whether slow wave amplitude reaches threshold?
Neuronal stimuli, hormonal stimuli, mechanical stimuli.
What effect do these stimuli have on the membrane potential of smooth muscle cells?
They depolarise smooth muscle cells rather than influence slow waves directly.
What is the BER frequency in the stomach?
3 slow waves per minute.
What is the BER in the duodenum and terminal ileum respectively and in what direction does this drive luminal contents?
Duodenum - approximately 12 waves per minute. Terminal ileum - approximately 8 waves per minute. Drives in aboral direction.
What is the BER for the proximal and distal (sigmoid) colon respectively and what direction does this drive luminal contents?
Proximal - 8 per minute.
Distal - 16 per minute. Drives in oral direction.
Why does the large intestine drive luminal contents in oral direction?
It favours retention of luminal contents facilitating reabsorption of water and electrolytes.
Where do pregangionlic parasympathetic nerves synapse in the GI tract?
Within the enteric nervous system.
Where does the parasympathetic innervation of the GI tract come from?
Vagal nerves and sacral spinal nerves (form the pelvic nerves).
What are the influences of parasympathetic excitation on the GI tract?
Excitatory: increased gastric, pancreatic and small intestinal secretion, blood flow and smooth muscle contraction.
Inhibitory: relaxation of some sphincters, receptive relaxation of stomach.
What are the ganglia that sympathetic fibres innervating the GI tract synapse in?
The prevertebral ganglia (particularly celiac, superior mesenteric and inferior mesenteric.
What do postganglionic sympathetic fibres innervate?
Mainly enteric neurones.
What effects does sympathetic excitation have on the GI tract?
Excitatory influences: increased sphincter tone.
Inhibitory influences: decreased motility, secretion and blood flow.
Where are the cell bodies of the enteric nervous system (ENS) located?
In ganglia connected by fibre tracts within the myenteric (Auerbach’s) plexus and the submucous (Meissner’s) plexus.
What are the roles of the 2 enteric plexuses?
Myenteric: mainly regulates motility and sphincters.
Submucous: mainly modulates epithelia and blood vessels.
What is meant when it is said that the ENS is intrinsic to GI tissue?
Reflex circuits can operate independently.
What are the 3 types of neurones in the enteric nervous system?
Sensory neurones, interneurones and effector neurones
What type of sensory neurones are there in the ENS?
Mechanoreceptors, chemoreceptors and thermoreceptors.
What is the function of interneurones?
Co-ordinate reflexes and motor programs.
What do the effector neurones of the ENS supply?
Both smooth muscle layers, secretory epithelium, endocrine cells and blood vessels.
Describe where a local reflex, a short reflex and a long reflex go to?
Local reflex - just in ENS.
Short reflex - prevertebral ganglion.
Long reflex - brain.
Give an example of a local, short and long reflex.
Local - peristalsis.
Short - interstino-intestinal inhibitory reflex (local distension causes inhibition of muscle activity in adjacent areas).
Long - gastroileal reflex (increase in gastric activity causes increase propulsive activity in the terminal ileum).
Define peristalsis.
A wave of relaxation, followed by contraction, that normally proceeds along the gut in an aboral direction, triggered by distension of the gut wall.
What happens in the oral side of the food bolus when distension activates sensory neurones?
Alters activity of interneurones -> alters activity of motor neurones -> longitudinal muscle relaxes (release of VIP and NO from inhibitory motorneurone) and circular muscle contracts (release of ACh and substance P from excitatory motoneurone).
What happens to the aboral side of the food bolus when distension activates sensory neurones?
Alters activity of interneurones -> alters activity of motor neurones -> longitudinal muscle contracts (release of ACh and substance P from excitatory motorneurone and circular muscle relaxes (release of VIP and NO from inhibitory motorneurone).
Other than peristalsis, name all the major motility patterns in the GI tract.
Segmentation, colonic mass movement, migrating motor complex (MMC), tonic contractions.
What is segmentation and where does it occur?
Rhythmic contractions of the circular muscle layer that mix and divide luminal contents. Small intestine (in fed state) and large intestine (called haustration).
What is colonic mass movement?
Powerful sweeping contraction that forces faeces into the rectum (occurs few times a day).
What is the migrating motor complex?
Powerful sweeping contraction from stomach to terminal ileum.
Give examples of low pressure and high pressure tonic contractions (sustained contractions).
Low pressure - organs with a major storage function e.g. stomach.
High pressure - sphincters.
What causes opening and closing of a sphincter in general?
Stimuli proximal: opening. Stimuli distal: closing.
What are all the sphincters called?
Upper oesophageal sphincter (UOS), lower oesophageal sphincter (LOS), pyloric sphincter, ileocecal valve, internal and external anal sphincters.
When does the upper oesophageal sphincter open and close?
Opens to allow swallowing, closes during inspiration.
What is the function of the pyloric sphincter?
Regulates gastric empyting, usually prevents duodenal gastric reflux.
