Control of Smooth Muscle Flashcards
What are the types of muscle in the GI tract and where are they found?
- smooth muscle in most regions
- skeletal muscle in pharynx and top third of oesophagus as well as anal sphincter
What is motility in the GI tract supported by?
intestinal smooth muscle
What are the two types of smooth muscle in the GI tract?
- phasic (rapid contraction and relaxation) in oesophagus, stomach antrum and intestines
- tonic (sustained contractions) sphincters and orad (upper stomach)
What cells are important in pace making the contractions and how?
interstitial cells of CAJAL, these cells are electrically coupled to the smooth muscle through gap junctions allowing the spread of the slow wave and a have spontaneous oscillating membrane potential
What is depolarisation (contraction) of the smooth muscle fibres stimulated by?
- stretch
- acetylcholine
- parasympathetics
What is hyper polarisation (relaxation) of the smooth muscle fibres stimulated by?
- noradrenaline
- sympathetics
What is the connection between electrical activity and contractile activity?
The longer the electrical activity is over the threshold, the greater the contractile event is
What is the difference between phasic and tonic electrical activity?
- phasic contraction the electrical activity is on and off
- tonic contraction electric activity constantly on and above threshold
How does the smooth muscle contract?
- calcium binds calmodulin which activates a myosin light chain kinase which phosphorylates myosin
- myosin can interact with actin which causes a contractile event
In terms of calcium, how is relaxation enabled to occur?
Ca2+ reuptake into the sarcoplasmic reticulum is mediated by a pump and the calcium itself binds to molecules to prevent it being free and binding to calmodulin
Why is reducing Ca2+ conc alone not enough to cause relaxation?
because MLC required dephosphorylation to prevent interaction with actin, this occurs using myosin light chain phosphatase
What are the two types of innervation in the GI tract?
- intrinsic innervation of the enteric nervous system
- extrinsic innervation of the autonomic nervous system
What is the enteric nervous system made up of?
- submucosal plexus (adjacent to mucosa)
- myenteric plexus (between circular muscle and longitudinal muscle)
What does activation of the myenteric plexus cause?
- increases tonic contraction
- increases intensity of rhythmic contractions
- increases the rate of rhythmic contractions
- increases velocity of contractions
What does the activation of the submucosal plexus cause?
- increases secretory activity
- modulates intestinal absorption
What are the types of movement in the GIT?
- propulsive movements (peristalsis)
- non-propulsive movements (segmentation)
- interdigestive (migrating motor complex)
What is the intrinsic pathway?
- motor neurone
- interneurone
- sensory neurone
What nerve is important in the extrinsic pathway?
-vagus nerve innervates most structures of the GI tract down to the top of the large intestine where it is then innervated by the pelvic nerve
What are three diseases that occur as a result of lack of GIT motility?
- Hirschsprung disease (congenital lack of neuronal ganglionic cells in the ENS plexus) (megacolon usually results)
- Chagas disease (infectious disease of a parasitic nature resulting in reduction in ganglionic cells in ENS)
- Achalasia (reduction in no of neuronal cells in lower oesophageal segment)
Symptoms of HD?
vomiting, constipation, distention of abdomen, intestinal obstruction
How do you treat HD?
remove the ganglionic segment
What causes achalasia?
failure of the LOS to relax due to lack of inhibitory neurones
How are changes in wall tension and intraluminal pressure in the stomach minimised?
stomach is highly folded and upon filling the folds flatten out
What causes relaxation in the fungus?
the vago-vagl reflex, receptive relaxation
What are the phases of gastric motility?
- propulsion
- grinding
- retropulsion (of large particles and clearing of terminal antrum)
Functions of pylorus?
- carefully regulates emptying of gastric contents
- prevents regurgitation of duodenal contents into stomach
What are the three phases of gastric emptying?
- cephalic (inhibitory phase which relaxes stomach which allows it to store large volumes)
- gastric (excitatory phase where stomach empties at rate proportional to volume)
- intestinal
What causes the gastric phase?
- myogenic reflex (stomach stretching)
- activation of pressure receptors (send signals to vagus nerve)
- gastrin released in response to peptides
How does the intestinal phase occur?
- low PH activates secretion of secretin
- high levels of lipids increase secretion of CCK
- high levels of amino acids increase secretion of gastrin
- high levels of carbs increase secretion of GIP
What is the benefit of the intestinal phase?
allows more time for digestion
What happens in the intestinal phase?
an inhibitory event that prevents the stomach from emptying contents when there is chyme in the duodenum
