structure and function of the alimentary canal Flashcards
What does the GI tract consist of?
series of hollow organs between mouth and anus (alimentary canal, separated by sphincters.
also several accessory structures
what is the function of the mouth and oropharynx?
1) chops up and lubricates food
2) starts carbohydrate digestion (amylase)
3) propels food to oesophagus
what is the function of the oesophagus?
actively propels food from the oropharynx to the stomach via peristalsis
what is the function of the stomach?
1) storage organ
2) churns food
3) continues carbohydrate digestion and begins protein digestion
4) regulates delivery of chyme to duodenum (via enzyme/churning activity and secretions)
what 3 structures does the small intestine consist of?
duodenum (short)
jejunum (longer)
ileum
what is the function of the small intestine?
principal site of digestion and absorption via secretions from small intestine and pancreas
begins fat digestion
what parts of the small intestine are mainly involved in absorption?
duodenum and jejunum
what are the 3 component structures of the large intestine?
caecum, appendix and colon
what are the functions of the large intestine?
1) contains left over substances
2) colon reabsorbs water and electrolytes
3) delivers fairly solid stool to the rectum
what is the function of the rectum?
storage and expulsion of faeces
what are the 3 accessory structures?
pancreas
salivary glands
liver and gall bladder (hepatobiliary system)
describe the motility activity of the alimentary canal
mechanical activity mainly involving smooth muscle
describe the secretion activity action of the alimentary canal
secretion into the digestive tract from itself and accessory structures in response to food.
required for digestion, protection and lubrication
describe the digestion activity of the alimentary canal
chemical breakdown by enzymatic hydrolysis of complex foodstuffs into smaller absorbable units
describe the absorptive activity of the alimentary canal
transfer of absorbable products of digestion from the digestive tract to the blood or lymph
(involves specialised transport mechanisms)
name the 4 layers of the GI wall from the lumen inwards (7-10 cm)
mucosa
submucosa
muscularis externa
serosa
what is the mucosa?
epithelial/endocrine/exocrine cells, lamina propria, muscularis mucosae (thin, circular smooth muscle that changes shape of mucosa so involved in mixing of contents of lumen)
what is the submucosa?
elastic connective tissue, larger blood and lymph vessels, glands, submucous plexus
what is the muscularis externa?
circular muscle layer and longitudinal muscle layer with nerve network in between (myenteric plexus)
what is the serosa?
connective tissue, squamous cell layer covering it allowing smooth movement of lumen contents over it
what special adaptations exist in the GI wall in the stomach?
another oblique smooth muscle layer in muscularis externa
what drives GI motility?
smooth muscle activity (circular and longitudinal)
what parts of the GI tract are skeletal muscle?
under autonomic control
mouth pharynx upper oesophagus external anal sphincter (rest is smooth)
GI smooth muscle acts as a single unit, how does this happen?
cells linked via gap junctions allowing electrical continuity between cells and depolarization spreads through all of them as a synchronous wave and they contract as one
what drives spontaneous activity across the syncytium (group of SM cells) and how is this modulated?
driven by specialised pacemaker cells
modulated by intrinsic (enteric) and extrinsic (autonomic) nerves and numerous hormones
what are “slow waves”?
rhythmic patterns of depolarization spreading via gap junctions (occurs in stomach and both intestines)
how so slow waves determine smooth muscle activity?
contraction only occurs if small wave peak reaches threshold and action potential fires
Number/direction/velocity proportional to number of AP’s
what drives slow waves?
interstitial cells of cajal (ICCs) - pacemaker cells interspersed between muscle layers
why does basic electrical rhythm vary along the length of the GI tract?
not all slow waves reach threshold
what determines whether slow wave amplitude reaches threshold?
neuronal stimuli
hormonal stimuli
mechanical stimuli - eg. food
(don’t affect amplitude of wave, just shift starting point of wave)
BER of stomach?
3 slow waves per min
3 contraction per min when food present
BER of small intestine?
12 per min in duodenum
8 per min in terminal ileum
propels matter downwards
BER of large intestine?
8 per min proximal colon
16 per min distal colon
pushes matter back up so more time to reabsorb
describe parasympathetic innervation of smooth muscle cells
preganglionic fibres synapse with post ganglionic within the enteric NS
excitory = increased gastric/pancreatic/small intestine secretion, blood flow and muscle contraction
Inhibitory = relaxes smooth muscle and sphincters
describe sympathetic innervation of smooth muscle cells
preganglionic fibres synapse in the prevertebral ganglia (celiac, superior and inferior mesenteric)
Excitory = increased sphincter tone
Inhibitory = decreased motility, secretion and blood flow
what 2 plexuses is the enteric nervous system contained within?
myenteric (auerbach’s) plexus (motility and sphincters)
Submucous (Meissner’s) plexus (epithelia and blood vessels)
All within the GI tissue
what 3 types of neurones of the enteric nervous system co-ordinate muscular, secretive and absorptive activities?
sensory neurons interneurons (majority) effector neurons (excitory and inhibitory motor neurones supplying the 2 smooth muscle layers, secretory epithelium, endocrine cells and blood vessels)
describe local nerve reflex in the GI tract
occurs completely in the walls of the GI tract
describe the short nerve reflex of the GI tract
intestine-intestinal inhibitory reflex (neurone leaves GI wall, activating sensory neurones exciting sympathetic fibres that inhibit muscle activity in adjacent areas)
describe the long nerve reflex in the GI tract
neurone goes all the way to CNS (nucleus of vagus in medulla)
gastroileal reflex - brain tells muscles to deficate
how does peristalsis happen?
before bolus: circular muscle contracts, longitudinal relaxes
after bolus: circulator relaxes, longitudinal contracts
What is segmentation?
rhythmic contractions of adjacent portions of circular smooth muscle that mix and divide lumen contents
(called haustration in large intestine)
what is colonic mass movement?
powerful sweeping contraction wave that forces faeces into rectum
what is migrating motor complex?
powerful sweeping contraction wave from the stomach t the terminal ileum (cleans out small intestine)
describe tonic contraction in the sphincters and storage organs
high pressure in sphincters
low pressure in storage organs
how many sphincters are there in the GI tract?
6 (excluding sphincter of Oddi)
what are sphincters made of?
circular smooth muscle
external anal sphincter and …. are skeletal
how do sphincters work?
act as one way valves by maintaining a positive resting pressure (tone) relative to two adjacent structures (eg. oesophagus and stomach)
describe the function of each sphincter
upper oesophageal = entry to oesophagus, open in swallowing, closed on inspiration
lower oesophageal = permits entry of food into stomach then closes to prevent reflux
Pyloric = regulates gastric emptying, prevents reflux of duodenal contents)
ileocaeccal = one way movement of ileum contents to caecum
internal/external anal = control exit of faeces
what is gastric outlet obstruction?
overactive pyloric sphincter so stomach doesn’t empty into duodenum so backs up causing vomiting
