L25 Flashcards
what are examples of Phasic contractions
Phasic contractions
• Esophagus, gastric antrum (stomach), small intestine
• Contract and relax in seconds
- Tonic contractions
- Lower esophageal sphincter, orad (near mouth stomach, ileocecal and internal anal sphincters
- Sustained contractions that last minutes to hours
what are examples of where tonic contractions happen
• Tonic contractions • Lower esophageal sphincter, orad (near mouth) stomach, ileocecal and internal anal sphincters • Sustained contractions that
eg you want the shinters to be constantly contracted and then relax when you need them to (which is opposite to normal)
what is the main difference between phasic and tonic contractions
phasic = seconds tonic = minutes to hours
there are 2 main stages of motility patterns. what are they
fasting and fed
describe the fasting motility pattern
- Clearing undigested material & secretions
- Regulating intestinal microflora
Fasting is a period of time where you are not eating eg when you are sleeping at night
Fasting has many different functions, it is not a time of absolute rest of GI tract as there are motility patteren to help clear the GI tract and it also helps with the microflora in the gut
what is the fed motility pattern used for
- Storage
- Movement at a controlled rate
- Mixing
- Exposure to absorptive surfaces
what are some other names for the fasting motility pattern
Fasting Motility = Migrating Motor OR Myoelectrical Complex (MMC)
when does the Fasting Motility pattern start and how long does it last
Starts 4-5 hours post meal absorption
Duration – 2 hours from the stomach to the end of the large intestine
the MMC has 3 phases Coordinated in stomach then small intestine. what are they
(1) intense, (2) inactive, (3) intermittent
what is the function of the MMC
Clears undigested material & secretions
Regulates intestinal microflora (makes sure you don’t get too much or that one bacteria doesn’t overpower others)
Epithelial cell turnover
last 2 part of immune response
what is the MMC regulated by
Hormonal = Motilin released by intestinal m-cells
Neuronal = Motilin stimulates both the enteric and autonomic NS
hormonal and neural elements act on eachother
fed state has 3 main functions. what are these
Reservoir/Storage Function
- Mainly stomach but also the Colon
- Relaxation of smooth muscle allows the volume of luminal
contents to increase without change in pressure
Peristalsis - Lower esophagus, stomach - Small & large intestine - Propulsive (but also mixing as part of retropulsion in the stomach)
Segmentation
• Small and large intestine
• Mixing & exposure to absorptive surfaces
there are 2 types of reservoir functions in the stomach. what are these
receptive relaxation and accommodation
what is receptive relaxation
- Receptive relaxation
An event, (e.g. swallowing in the case of the stomach) triggers reduced muscle tone as content is moved along the tract (less resistance as content
arrives).
Allows food to enter and not be pushed back.
what is accommodation
- Accommodation
Progressive relaxation in response to a volume
change.
This is due to receptors in the stomach which detect distention and stretch which causes a locals and something long loop reflex which causes the stomach to stretch
describe the muscle contraction responsible for peristalsis
Ascending circular muscle contraction behind bolus.
Descending circular muscle relaxation ahead of bolus.
Longitudinal muscle
shortening (contraction)
Causes intestine to bunch up meaning that the cyme only needs to travel a shorter distance
Combines with other motility to produce complex patterns
(e.g. retropulsion)
causes movement from the mouth towards the anus
describe segmentation
Circular muscle contraction in alternating
segments.
Mixing function.
The major motility pattern in the small
intestine.
which is the main motility pattern in the small intestine
segmentation
Barium acts as a contrast die and then they have taken xray videos of the the motility patterns which is how you diagnose defects with motility patteren
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what is the difference between smooth muscle in the body and in the GI tract
Intestinal smooth muscle is different from other types of smooth muscles
The big difference is how the cells are interconnected and how they get their messaging
In the GI smooth muscle we call it signal unit or interconnected smooth muscle as they are are all connected via gap and adherence junctions therefore they contract all together fin a sinciduim
This differs from other smooth muscle where each individual cells is directly innervated
describe the arrangement of intestinal smooth muscle
- Single unit or unitary smooth muscle
- Acts as a syncytium
- Extensive intercellular communication through Gap junctions (electrical)
- Cells physically connected through Adherens junctions
• Muscle cells are arranged in layers with a neural plexus between
• circular and longitudinal intestinal smooth muscle
sandwiching neural components of the myenteric plexus
There are also intestinal cells of cajal which are the pacemaker cells
They are long cells and each individual cell is going to interact with 10 or more cells
They have a very irregular arrangement they are not striated like skeletal mucles s
what are the measurements of intestinal smooth muscle cells
Cells are 5 – 20 µM diameter and ~ 500 µM long
how many other cells with a smooth muscle cell interact with
10 through gap and adherences junctions
how does smooth muscle contract
You stil have actin and myosin filaments but they are not regulated my troponin or tropomyosin instead they are modulated by modulin and myosin like kinase
They still get modulated by action potentials and membrane depolerisation which causes voltage gated Ca channels causeing the entry of Ca. Ca binds to a molecule called calmodulin.
This complex activated a kinase (myosin like calcium kinase). This phosphorylates the myosin like chain. When this is phosphorylated it interacts with actin which leads to the contraction
Because all of these cels are linked electrical then all of these cells are going to contract as a syncytium
You can also remove the phosphate with a myosin like chain phosphatase (removes phosphate whereas a kinase adds it)
NO (neurotransmitter) will activated myosin like chane phosphatase which removes the phosphate causing relaxation
The membrane potential of intestinal smooth muscle is not stable. what is its range
- Varies between -40 and -80 mV
- Fluctuates in a pattern of Slow Waves or Basic Electrical Rhythm
- Generated by Interstitial Cells of Cajal
what is the MP of smooth muscle cells in the intestine generated by
• Generated by Interstitial Cells of Cajal
Interstitial cells of cajal which spontainiously depolerisae and reploerise. These are connected to the smooth muscle cells therefore it causes them to contract and relax
this facilitates a pattern of slow waves or basic electrical rhythm
how do Interstitial cells of Cajal (ICC) Generate slow waves
Function as pacemakers by causing rhythmic changes in:
• Activity of Na+/K+ ATPase and Membrane K+ conductance
where are interstitial cells of cajal located
Stomach and small intestine
- In smooth muscle layer close to myenteric
plexus
• Colon
- Boundary of muscle layer and submucosal
layer
describe the arrangement of ICC
• Send multiple branching processes to surrounding smooth muscle cells
• Electrically linked to other ICC & muscle
cells by gap junctions
what would happen if you knocked out the ICC
If you take away the interstitial cells of cajal then you don’t get the slow waves therefore you don’t get the regular fluctuations of membrane potential
what determins the frequency of contraction of smooth muscle
slow waves which are generated by ICC
Therefore the frequency of contraction does not chane in different areas of the GI tract (except in pathological conditiins)
Different areas of the GI tract will have different ICC with have different firing rates. This has a functional purpose. what is this
The higher up you go the more you want to digest therefore it is faster and the lower region is slower beacyse you want to give more time for absorption
the firing rate of the ICC differs in different parts of the GI tract. what are these different rates
Faster in the proximal regions of the GI tract (digestion) = Duodenum 12 – 20 per minute
Slower in the distal regions of the GI tract (absorption) = Colon 6-8 per minute
describe how Action potentials determine the force (size/amplitude) of contraction in smooth muscle
- Small / no contractions occur in absence of action potentials
- If the membrane potential does not reach threshold no contraction.
• Greater the number of action potentials the greater the force of contraction
• The longer the potential is above threshold, the more APs, the bigger the
contraction.
• Calcium also contributes – elevated Ca2+ = more calmodulin / myosin light chain kinase activation.
Elevation of intracellular Ca2+ initiates
contraction – two mechanisms
describe the one to do with depolerisation
Depolarisation of cell membrane potential Opens voltage gated Na+ and Ca2+
channels in sarcolemma (cell
membrane)
Results in an Influx of Ca2+ which activates contraction (calmodulin & MLCK) as well as activates the ryanodine receptor resulting in release of Ca2+ from the sarcoplasmic reticulum (Ca2+
induced Ca2+ release)
This increases the contraction amplitude.
Elevation of intracellular Ca2+ initiates
contraction – two mechanisms
describe the one via the Gq pathway
this is pharmomechanical contraction.
is is caused when Hormones & neurotransmitters bind
to a receptor to induce Ca2+ release.
• Gαq - Signaling pathway activated
• Receptor activation of
phospholipase C
• Production of IP3
• IP3 induces release of Ca2+ from sarcoplasmic reticulum
This increases the contraction amplitude
both mechanisms of Ca elevation lead to increased
contraction amplitude and can
occur at the same time.
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what 3 things down regulate smooth muscle contraction
hyperpolarisation
increased activity of myosin light chain phosphatase
inhibition of excitatory ENS neurotransmittion
describe how hyperpolarisation of smooth muscle cell membrane (or decrease Ca2+) happens
- Slow waves do not pass threshold - No contraction
* Or Pass threshold but reduce the number of action potentials - Reduced force
- Increased activity of Myosin Light Chain Phosphatase
• Dephosphorylates myosin so reduced force of contraction
describe hormone and neurotransmitters effect on regulating contractions
Hormones and neurotransmitters regulate contractions
• Modulate magnitude/strength of contraction only
• Little effect on frequency (this is determined by ICC and varies by region)
describe an example here neural control can effect smooth muscle activity
increase in parasympathetic activity = increase in ACh binding to muscarinic receptors
this activates the ENS which increases smooth muscle activity
this causes More time above threshold means greater
force of contraction and more GI tract
motility
if this was sympathetic activation noradrenalin would be released which would bind to the a1 receptor decreasing ENS activity therefore Less time above threshold means smaller
force of contraction and less GI tract
motility
what receptor do opiods bind to
Opioids can be very specific for receptors in the GI tract called reperiomid or amodium which is used to treat diarrhea
G alpha inhibbitory
when opioids bind to their receptors what happens
These inhibit an expiratory signal which would normally tell the ENS to contract
The smooth muscle doesn’t contact and then it doesn’t aid in the digestion and movemt of chime through the GI tract which halts a lot of the digestive processes. If you had diarrhea this would be a good thing as you would have more time to reabsorbe water and salts loosed
In a healthy perosn however this would lead to constipation, constipation causes stretching damage to the GI tract which over time will cause pain and mellabsorbtion
what is Impacted bowel
is when the facies is so hard that the intestine cant move it
what did elvis presley’s autopsy show
He had a heart attack but when they looked at his GI tract he as constipated and a megacolon (because it had been stretched)
The enlarged colon mean that you needed more contraction to induce defecation. What he was doing in called Valsalva maneuver (what you do when you take a massive shit).
what is the Valsalva menover and what does it cause (how does it cause a heart attack)
Valsalva menover (what you do when you take a massive shit). This increased the thoracic pressure, increasing the abdominal pressure which puts pressure on the bowl to force poop out
the Valsalva menover it increases the intrathoracic pressure which created a compression on the aorta. This compression pushes everything on the aorta backwards. When the aortic pressure is higher then you ventricular pressure the openings to the coronary arteries are open. Therefore if you have anything that could cause blockage of the coronary arteries (clots) and you perform this Valsalva menover and they get pushed backwards then you are going to have a very large heart attack.
Which of the following statements about the spontaneous contractions of intestinal
smooth muscle is not correct?
A. These are phasic contractions generated by interstitial cells of Cajal.
B. Spread of spontaneous contractions depends on gap junctions between intestinal
smooth muscle cells.
C. The frequency of contractions is dependent on the region of the intestine.
D. Spontaneous contractions are generated by the parasympathetic nervous system.
E. Intestinal smooth muscle can contract independently of the central nervous system.
D
