Cardiac Contractility and events of the cardiac cycle Flashcards
what percentage of extracellular calcium contributes to contraction
10%
how does extracellular calcium enter the cell
L-type DHP channels
how much bigger are t tubules in cardiac cells than skeletal cells?
5x in diameter
25x in volume
what part of the t tubule sequesters Ca2+
muscopolysaccharides
what is contractility
the ability of the heart to contract
how is contractility altered without changing volume
increasing Ca2+ release from SR
modifying pumps at the membrane to increase uptake of Ca2+ into the SR
How is contractility modified by extrinsic mechanisms
sympathetic nervous system
describe the extrinsic mechanisms affecting contractility
noradrenaline on B1 receptors increases cAMP
enhances Ca2+ influx and promotes storage and release from SR
Parasympathetic has a indirect inotropic effect by prolonging relaxation of the heart
how long does the absolute refractory period of cardiac muscle last
245 ms
what is meant by the relative refractory period
some Na channels have been reset and can be stimulated
need a greater than normal stimulation to produce AP
what is meant by the period of supranormal excitability
enough Na channels have closed to cause a AP
As membrane potential has not reached -80mV, a less than normal stimuli can cause a AP
how long does the period of contraction of cardiac muscle last?
250 ms
why can cardiac AP not be significantly summated
each twitch uses all fibres of the myocardium
Describe how aortic pressure changes
Increase - During systole when the aortic valve opens and AV valve closes
Decrease - as ventricles empty and aortic valve closes
when does the AV valve close?
when the ventricular pressure is greater the atrial pressure
