Physiology of Alimentary Canal Flashcards
what is the alimentary canal
the whole passage along which food passes through the body from mouth to anus during digestion
what is the role of sphincters
separate functional compartments of the alimentary canal and help control movement of matter inside it
what is the role of the accessory structures
produce fluid that drains into the alimentary canal
what are the accessory structures
salivary glands, pancreas, liver and gall bladder
what organs and structures make up the alimentary canal
mouth and oropharynx, oesophagus, stomach, small intestine, large intestine, rectum and anus, accessory structures
what happens in the mouth and oropharynx
chops and lubricates food, carbohydrate digestion started, food propelled to oesophagus
what is the role of the oesophagus
muscular tube that propels food to the stomach
what is the role of the stomach
stores/churns food, continues carbohydrate and initiates protein digestion, regulates delivery of chyme to duodenum- stops small intestine from being overwhelmed
what is chyme
the pulpy acidic fluid which passes from the stomach to the small intestine, consisting of gastric juices and partly digested food
what is the role of the small intestine
principle site of digestion and absorption of nutrients
what are the parts of the small intestine
duodenum, jejunum and ileum
what are the parts of the large intestine
caecum, appendix and colon
what is the role of the colon
reabsorbs fluids and electrolytes, stores faecal matter before delivery to rectum
what is the role of the rectum and anus
storage and regulated expulsion of faeces
how small do food particles have to be to pass from the stomach into the duodenum
2mm
where is fat digestion initiated
in the small intestine
what is pepsin
enzyme produced in stomach that breaks down proteins
what is amalyse
enzyme that breaks down starch into sugars in the small intestine
what organs make the hepatobiliary system
liver and gall bladder
what controls the motility of the alimentary canal
mostly smooth muscle
skeletal at mouth, pharynx, upper oesophagus and external anal sphincter
what causes secretion in the digestive tract
in response to presence of food, hormonal and neural signals
what is the role of secretion in the digestive tract
required for digestion, protection and lubrication
what is digestion
chemical breakdown by enzymatic hydrolysis of complex foodstuffs to smaller absorbable units
where does physical, mechanical digestion take place
mouth, stomach, small intestine
describe absorption in the alimentary canal
transfer of the absorbable products of digestion (along with water, electrolytes and vitamins) from the digestive tract to the blood or lymph
what is absorption largely mediated by
numerous transport mechanisms
what is the innermost part of the digestive tract wall and what is its role
mucosa- absorptive function (eneterocytes)
what makes up the mucosa
epithelial cells, exocrine cells, endocrine gland cells, lamina propria
what is the lamina propria composed of
capillaries, enteric neurons, immune cells, muscularis mucosae
what is the layer surrounding the mucosa
submucosa
what is the submucosa made of
elastic connective tissue, larger blood and lymph vessels, glands, nerve network (submucosa plexus)
what is the layer surrounding the submucosa
muscularis externa
what is the muscularis externa made of
2 layers of muscle and nerve network;
circular muscle layer (controls diameter of lumen), nerve network (myenteric plexus), longitudinal muscle layer (controls length of digestive tract)
what is the outermost layer of the digestive tract wall
serosa
what is the serosa made of
connective tissue
what immune cells are in the lamina propria
lymphocytes and mast cells
how long is the digestive tract
7-10 m
what is a role of the serosa
secrete lubricant to move abdominal content smoothly
what causes the lumen to become narrower and longer
circular muscle contraction
what causes the intestine to become shorter and fatter
longitudinal muscle contraction
what happens when muscularis mucosae contract
there is changes in the absorptive and secretory area of the mucosa (folding), mixing activity
what controls the skeletal muscle involved with gastrointestinal motility
autonomic control
describe how smooth muscle in the GI tract is electrically coupled
adjacent smooth muscle cells are coupled by gap junctions which allow the spread of electrical currents forming a functional syncytium which allows a slow wave of depolarisation to spread across the muscle sheet
what is the term used to describe hundreds of muscle cells contracting at the same time
single unit smooth muscle
what modulates the spontaneous activity across the syncytium which is driven by the pacemaker cells
intrinsic (enteric) and extrinsic (autonomic) nerves
hormones
how does spontaneous electrical activity occur as in the stomach and small intestine
in slow waves - rhythmic patterns of membrane depolarization and repolarization that spread from cell to cell via gap junctions
what does the slow wave electrical activity determin
max frequency, direction and velocity of rhythmic contractions
what pacemaker cells that are interspersed between the far more numerous smooth muscle cells
interstitial cells of Cajal (ICCs)
when only can contraction occur
only when the slow wave amplitude is sufficient to trigger smooth muscle cell action potentials
what is the force of contraction related to
number of action potentials discharged
where are the interstitial cells of cajal located
in the submucosa
do all slow waves trigger contraction
no (slow waves in background- BER- only reach threshold when stimulated)
what determines whether slow wave amplitude reaches contraction
neuronal stimuli, hormonal stimuli, mechanical stimuli
what does BER (basic electrical rhythm do to luminal contents in the small intestine
drive it in the aboral (rectum) direction
what does BER (basic electrical rhythm do to luminal contents in the large intestine
favours retention of luminal contents, facilitating absorption of water and electrolytes
how is the GI tract innervated with parasympathetics
pre ganglionic fibres (that release ACh) synapse with ganglion cells within the ENS
what is the ENS
division of autonomic nervous system in the GI tract, pancreas and gallbladder
what are the parasympathetic excitatory influences of the GI tract
causes increased;
- gastric, pancreatic and small intestine secretion
- blood flow
- smooth muscle contraction
what are the parasympathetic inhibitory influences of the GI tract
causes relaxation of some sphincters, receptive relaxation of stomach
where does the parasympathetic stimulation of the GI tract arise from
medulla oblongata for majority, sacral spinal chord for the distal ends on the large intestine
what is more important in the GI tract para or symp
para
how does the sympathetic system innervate the GI tract
pre ganglionic fibres releasing ACh synapse in the prevertebral ganglia. postganglionic fibres releasing NA innervate mainly enteric neurones
what excitatory influences does the sympathetic system have on the GI tract
increased sphincter tone
what inhibitory influences does the sympathetic system have on the GI tract
decreased motility, secretion and blood flow
what are the 3 prevertebral ganglia involved with the sympathetic system
celiac, superior mesenteric, inferior mesenteric
what do nerves from the thoraco-lumbar region innervate
distal end of the stomach, small intestine, oesophagus, proximal large intestine, distal large intestine
what is the main role of the myenteric (auerbach’s plexus)
mainly regulates motility and sphincters
what is the main role of the submucous (meissner’s) plexus
mainly modulates epithelia and blood vessels
is the enteric nervous system intrinsic or extrinsic to the GI tissue, explain
intrinsic- many of the nerve cell bodies in the submucosa are post ganglionic parasympathetic fibres and are classed as part of the ENS
how do reflex circuit in the ENS operate
can run independantly but hormones and extrinsic nerves exert a strong regulatory influence
what does the ENS coordinate
musclur, secretive and absorptive activities
what neurons produce the activities that the ENS co ordinates
sensory neurons (mechano, chemo and thermo receptors) interneurons (majority, co ordinate reflexes and motor programs) effector neurons (excitatory and inhibitory motor neurones supplying both smooth muscle layers, secretory epithelium, endocrine cells and blood vessels)
what do nerve reflexes in the GI tract control
motor and secretory activity
what is the relfex pathway
sensory neuron, interneurone, effector neurone
give an example of a local relflex
peristalsis
give and example of a short reflex
intestino-intestinal inhibitory reflex (local distension activates sensory neurones exciting sympathetic pre-ganglionic fibres that cause inhibition of muscle activity in adjacent areas)
give an example of a long relfex
gastroileal reflex (increase in gastric activity causes increased propulsive activity in the terminal ileum)
what is peristalsis
a wave on relaxation, followed by contraction that normally proceeds along the gut in an aboral direction
what triggers peristasis
distension of the gut wall
what is released when muscles relax and from what
VIP and NO released from inhibitory motoneurone
what is released when circular muscles contract and from what
ACh and substance P from excitatory motor motorneurone
what does distention of the GI tract activate
sensory neurones- mechanoreceptors- which alter the activity of the smooth muscle layers which propel the matter through the tract
describe segmentation
motility pattern- contraction of alternative places in that causes mixing/ churning movements, rhythmic contractions of the circular muscle layer that mix and divide luminal contents. happens in small and large intestine. In large intestine is called haustration.
describe colonic mass movement
motility pattern- powerful sweeping contraction across the large intestine that forces faeces into the rectum from the colon
describe migrating motor complex
motility pattern-
powerful sweeping contraction from stomach to terminal ileum, cleans out the small intestine from the duodenum to the colon to make room for next meal
describe tonic contractions
motility contractions- sustained contractions: low pressure (organs with a major storage function e.g. stomach) maintain a degree of tone high pressure (sphincters)
what is the role of the sphincters
usually one way valves, maintain positive resting pressure relative to two adjacent structures, exert a constant tone
what in general causes opening and closing of the sphincters
stimuli (pressure) proximal and distal
what is the role of the upper oesophageal sphincter and what type of muscle is it
relaxes to allow swallowing and closes during inspiration
what is the role of the lower oesophageal sphincter
relaxes to permit entry of food into the stomach, closes to prevent reflux of gastric contents to the oesophagus
what is the role of the pyloric sphincter
regulates gastric emptying and usually prevents duodenal gastric reflux
what is the role of the ileocaecal valve
regulates flow from ileum to caecum, distention of ileum opensm distention of proximal colon closes
what are the role of internal and external anal sphincters regulated by
defaecation reflex
most sphincters are smooth muscle, which ones are skeletal
upper oesophageal, external anal
