Cardiac Contraction Flashcards
Structure of cardiomyocytes
60-140 micrometers in length
17-25 micrometer in diameter
Multiple rod like cross banded strands , composed of repeating sarcomeres
Function of sacromeres
Cause muscle contraction when actin and myosin move relative to each other
How is calcium involved in contraction
T tubules have calcium channels where calcium enters and goes into the cell close to the sacromere
Calcium enters via calcium channel that open in response to depolarisation that travels along sacrolemma , calcium is released from sacrolaplasmic reticulum which initiates contraction
When calcium ions are maximal the varying actin myosin overlap is shown for ..
Systole
How does electrical excitability contract cardiac myocytes
-Contraction is determined by an increase in calcium ions
-intracellular increase of 0.1um to 10um
-Action potential causes cell depolarisation which opens calcium channels ,calcium influx,
-calcium binds to ryanodine receptor which causes release of calcium stores
Steps for intracellular rise in calcium
-action potential depolarises t tubules and activate VGC causing calcium influx
-calcium binds to ryanodine receptor in the sacroplasmic reticulum
-calcium is released from the sacroplasmic reticulum
-calcium binds to troponin, displacement of tropomyosin , exposing active sites
Myosin head bind to actin
What subunits are composed in troponin and their roles
troponin T- binds to tropomyosin
troponin I - binds to actin filaments
Troponin C - bind to calcium ions
Steps of contraction
- binding of myosin to actin , releasing an inorganic phosphate
-power stroke as ADP is released , actin gets pulled to middle of sacromere
-myosin in low energy form
-new ATP binds to myosin head causing unbinding of myosin
-cocking of myosin head , myosin now in high energy form
Steps when there is a decrease in calcium and relaxation occurs ( after fibres have done contraction)
-AP repolarises t tubules , potassium ions leaves
-close of VGCC, less calcium ion influx
-extrusion of calcium ions from cell by the sodium / calcium exchanger
-calcium ion uptake in SR via SR membrane , calcium is in SR for next contraction
-uptake of calcium ions in mitochondria
3 ways to control cardiac contractility
-noradrenaline acts via beta 1 receptors , this phosphorylates calcium channels allow greater calcium influx
-increasing VGCC activity
- reducing calcium ion extrusion
List the actions of the heart when it experiences sympathetic stimulation
-increased calcium influx , so higher force of contraction
-increased SR calcium atpase , faster relaxation
-potassium channels opening so faster repolarisation
- heart rate and conduction increase
What cardiac glycosides increases contractility by reducing calcium extrusion
Digoxin
Mechanism of action of digoxin
- digoxin inhibits sodium potassium ATPase
- build up of intracellular sodim lowers concentration gradient
-less calcium ion explosion by sodium calcium exchanger
-more. Calcium uptake into stores and greater CICR
Where is digoxin used
Atrial fibrillation,
Atrial flutters
Side effects of digoxin
Fatigue , tiredness
Hallucinations
Anxiety
Vomiting
Nausea
Electrolytes imbalance
Can be worsened by other medicines
