Alimentary Canal Flashcards
What is the alimentary canal?
Series of organs from mouth to anus separated by sphincters
Orally?
Moving towards mouth
Aborally?
Moving towards anus
List of organs in alimentary canal?
Mouth & oropharynx, oesophagus, stomach, small intestine, large intestine, rectum & anus
What are the accessory structures of alimentary canal?
Salivary glands, pancreas, liver and gallbladder
What is the function of each part of canal?
Mouth & oropharynx - chops & lubricates food, starts carb digestion, propels food to oesophagus
Oesophagus - muscular tube that propels food to stomach
Stomach - stores/churns food, continues carb and starts protein digestion, regulates delivery of chyme to duodenum
Small intestine - Duodenum, jejunum, ileum, main site of digestion and absorption
Large intestine - Caecum, appendix, colon, colon reabsorbs fluids and electrolytes and stores faecal matter
Rectum & anus - storage and expulsion of faeces’
Superior to inferior parts of the colon?
Descending, ascending, sigmoid.
Activities of alimentary canal?
Motility
Secretion
Digestion
Absorption
Motility?
Mechanical activity mostly involving smooth muscle except pharynx, upper oesophagus and external anal sphincter which is skeletal muscle
Secretion?
Occurs in presence of neural, hormonal or mechanical stimuli.
Required for digestion, protection and lubrication
Digestion?
Chemical breakdown of food by enzymatic hydrolysis to smaller absorbable units
Absorption?
Transfer of absorbable units from GI tract to blood or lymph
Generalised structure of digestive tract wall?
Refer to slide
How does motility arise?
Activity of smooth muscle (skeletal in some cases)
Types of smooth muscle involved in motility?
Effects of contraction of these?
Circular - contraction narrows lumen and lengthens muscle
Longitudinal - contraction shortens intestine and fattens it
Muscularis mucosae - contraction causes change in absorptive and secretory area of mucosa by folding
What is GI smooth muscle coupled by?
Electrically coupled by gap junctions
What is the role of gap junctions in GI smooth muscle?
Allow spread of electrical currents forming functional syncytium - synchronised contraction to act as single unit SM
How is the excitation generated?
Spontaneously by pacemaker cells - Interstitial cells of cajal (ICCs)
Modulated by:
Intrinsic (enteric) and extrinsic (autonomic) nerves
Hormones
How is this excitation described in stomach and small/large intestine?
Slow waves
When does contraction of SM occur?
When slow wave amplitude reaches threshold to trigger AP
Where are ICCs found and how do they link?
Between longitudinal and circular muscle layers and in submucosa
Form gap junctions with each other and SMC
What determines if a slow wave reaches threshold?
What does slow wave determine?
Neural, hormonal or mechanical (food) stimuli
Basic electrical rhythm
BER Frequency for stomach, small intestine and large intestine
Stomach - 3 slow waves per min
Small intestine - 12 in duodenum and 8 in terminal ileum
Drives contents in aboral direction
Large intestine - 8 in proximal colon and 16 in sigmoid
Favours retention of luminal contents
Parasympathetic Stimulation of GI tract?
Preganglionic fibres (releasing ACh) synapse with ganglia (in essence post ganglionic neurones) within enteric nervous system
Parasympathetic excitatory influences?
Increases gastric, pancreatic and small intestinal secretion, blood flow and SM contraction
Parasympathetic inhibitory influences?
Relaxation of some sphincters, receptive relaxation of stomach
Sympathetic stimulation of GI tract?
Preganglionic fibres synapse in the prevertebral ganglia
Postganglionic fibres (releasing NA) innervate mainly enteric neurones
Sympathetic excitatory influences?
Increased sphincter tone
Sympathetic inhibitory influences?
Decreased motility, secretion and blood loss
What make up the prevertebral ganglia?
Celiac, Superior and inferior mesenteric
What is the enteric nervous system?
Approx 100 million neurones in which the bodies are located in ganglia connected by fibre tracts in either;
Myenteric plexus or Submucous plexus
Other name for myenteric plexus and its function?
Auerbach’s
Regulates motility and sphincters
Other name for submucous plexus and function?
Meissner’s
Mainly modulates epithelia and blood vessels
What does the ENS do?
Co-ordinates muscular, secretive and absorptive activities via sensory neurones, interneurons and effector neurones
Sensory neurones?
Mechanoreceptors, chemoreceptors, thermoreceptors
Effector neurones?
Excitatory and inhibitory motor neurones
Types of nerve reflex and examples?
Local reflex - peristalsis
Short reflex : intestine-intestinal inhibitory reflex
Long reflex - gastroileal reflex
What is peristalsis?
What is it triggered by?
Wave of relaxation followed by contraction along gut in aboral direction
Distension of gut wall
What causes circular muscle to contract?
Release of ACh and substance P from excitatory motor neurone
At the same time relaxation of longitudinal muscle by release of VIP and NO from inhibitory motor neurones occurs
What causes circular muscle to relax?
Release of VIP and NO from inhibitory motorneurone
At same time longitudinal muscle contracts due to release of ACh and substance P from excitatory motorneurone
What is segmentation?
Mixing/churning - Rhythmic contractions of circular muscle layer that mix and divide contents
Occurs in small intestine (fed) and large intestine (haustration)
Colonic mass movement?
Powerful sweeping contraction forcing faeces to rectum
Migrating motor complex (MMC)?
Powerful sweeping contraction from stomach to ileum
Tonic contractions?
Sustained contractions
Low pressure - organs with major storage function (stomach)
High pressure - sphincters
How many sphincters in GI tract?
Purpose?
6 excluding sphincter of oddi
Act as one way valves maintaining positive resting pressure relative to 2 adjacent structures (usually smooth circular muscle)
What is the UOS and what does it do?
Upper oesophageal sphincter
Relaxes to allow swallowing
Closes during inspiration
What is the LOS and what does it do?
Lower oesophageal sphincter
Relaxes allowing food to move to stomach
Closes preventing reflux
Pyloric sphincter function?
Regulates gastric emptying
Prevents duodenal gastric reflux
Ileocaecal valve?
Regulates flow from ileum to caecum
Distension of ileum opens and distension of proximal colon closes
Internal and external anal sphincters muscle type and function?
Internal - smooth
External - skeletal
Regulated by defaecation reflex
