Large Intestines, Constipation, Diarrhoea and Lactose intolerance Flashcards
What type of muscle aids GI motility?
Smooth Muscle
Where in the body is skeletal muscle the most important?
mouth, pharynx, upper oesophagus, external anal sphincter
What happens to the lumen of the large intestines when circular muscles contract?
it becomes narrower and longer
What happens to the intestine when longitudinal muscles contract?
It becomes shorter and fatter
What happens to the mucosa when the muscularis mucosae contracts?
There is a change in the absorptive and secretory area
Where are the neurones of the enteric nervous system located?
In the ganglia within the myenteric and submucous plexus solely in the GI tissue (making it intrinsic)
What is used to connect the myenteric and submucous plexus?
Integanglionic fibre tracts
What is a pro of the GI tissue having an intrinsic nervous system?
it forms a complete reflex circuit that can operate independently to the rest of the nervous system, but is strongly modulated by hormones and extrinsic nerve input.
What is the enteric nervous system comprised of?
Sensory neurones, interneurones, effector neurones
Examples of sensory neurones?
mechanoreceptors, chemoreceptors, thermoreceptors
What are effector neurones?
excitatory and inhibitory motor neurones which supply the longitudinal and circular smooth muscles, secretory epithelium, endocrine cells and blood vessels.
How does the parasympathetic system work?
the preganglionic fibres send ACh across the synapse to the post ganglionic neurones within the enteric nervous system (ENS) and have a excitatory and inhibitory influence.
Excitatory influence of the parasympathetic system in the ENS in the GI?
1) increased gastric, pancreatic and small intestinal secretion, blood flow and smooth muscle contraction
Inhibitory influence of the parasympathetic system in the ENS in the GI?
1) relaxation of some sphincters, receptive relaxation of stomach
How does the sympathetic system work?
The preganglionic fibres release ACh across the synapse to the postganglionic fibres. These then release NA (noradrenaline) supplying mainly the enteric neurones but also other structures. This all occursr in the prevertebral ganglia.
Three prevertebral ganglia supplied to the GI?
1) celiac
2) superior mesenteric
3) inferior mesenteric
Which of the two is more fuctionally important, the sympathetic or the parasympathetic system?
the parasympathetic system
What is the inhibitory influence of the sympathetic system in the ENS on the GI?
it decreases the motility, secretion and blood flow.
Smooth muscle cells of the GI tract are coupled by what and what is the effect of this coupling?
coupled by gap junctions and allows the spread of electrical currents from cell to cell forming a functional syncytium.
What is a functional syncytium?
When hundreds of cells are depolarized and contract at the same time
What is the spontaneous activity of the smooth muscle in the GI tract modulated by?
1) intrinsic (enteric) and extrinsic (autonomic) nerves
2) hormones
What are slow waves?
They’re rhythmic patterns of membrane depolarization and repolarization that spread from cell to cell via gap junctions. They occur in the stomach, SI and LI
What drives slow waves?
Interstitial Cells of Cajal (ICCs)
What do ICCs do?
they’re the pacemaker cells in the stomach, SI and LI stimulating spontaneous electrical activity
What causes contraction of smooth muscle?
if the slow wave amplitude is big enough to trigger action potentials. The contraction force is related to the number of action potentials discharged
What mediates the slow wave?
voltage-activated Ca2+ channel
Where are ICCs located?
between longitudinal and circular muscle layers and in the submucosa. They form gap junctions with each other and smooth muscle.
What do slow waves determine?
The Basic Electrical Rhythm (BER) - not all slow waves trigger contraction
three factors that affect if the slow wave amplitude reaches threshold?
1) neuronal stimuli
2) hormonal stimuli
3) mechanical stimuli
What happens to the BER along the GI tract?
it varies along the length of the G.I. tract
What is the BER in the stomach?
3 slow waves per minute
What is the BER in the duodenum?
1-12 waves per minute
What is the BER in the terminal ileum?
8 waves per minute. It’s used to drive luminal contents in the aboral directon
What is aboral?
side or end that is furthest from the mouth
What is the BER in the proximal colon?
8 waves per minute
What is the BER in the distal colon?
16 waves per minute for the retention of luminal contents and to facilitate water and electrolyte absorption
What is peristalsis?
a wave contraction that normally proceeds along the gut in the aboral direction and is triggered by the distension of the gut wall.
What is segmentation?
the mixing or churning movements caused by rhythmic contractions of the circular muscle layer that mix and divide the luminal contents.
What is tonic contractions?
sustained contractions found in sphincters of the GI tract