heart Flashcards
myocardial control mechanisms: explain the mechanisms regulating heart rate, contractility and myocardial oxygen supply
primary pacemaker cells in heart
SAN (sino-atrial node)
key features of SAN cells
no true resting potential, so instead generate regular, spontaneous action potentials
speed at which depolarising current is carried into SAN cell primarily, and method
relatively slow by Ca2+ currents as opposed to fast Na+ currents
3 channels regulating heart rate
I f (“funny” hyperpolarisation-activated cyclic nucleotide-gated (HCN) channels), I Ca (repolarising transient T-type Ca2+ channel or repolarising long lasting L-type Ca2+ channels), I K (K+ channels)
processes of heart rate action potential generation, and SNS and PSNS influences on cAMP and channels
HCN channels switch on during hyperpolarisation, utilising cAMP and drive Na+ influx to initiate depolarisation -> as current rises, T-type then L-type Ca2+ channels open (predominant driver) -> K+ channels open, initating repolarisation -> spontaneous depolarisation that triggers action potential; SNS increases cAMP, I f and I Ca; PSNS decreases cAMP (by I f) and increases I K
electrical excitation causing cardiac contraction in cardiac myocytes pathway
action potentials from SAN -> membrane depolarisation of myocytes -> VGCC open -> small release of Ca2+ into cytoplasm -> induces release of Ca2+ from SR by depolarisation or ryanodine receptors (RyR2; Ca-induced Ca-release) -> promotes troponin to initiate contraction
% of free Ca2+ in a cardiac twitch necessary for cardiac contraction from induced influx of Ca2+ current (I Ca) through L-type channels vs Ca-induced Ca-release
20-25% vs 75-80%
how is repolarisation of cardiac tissue achieved
Na+/Ca2+ provides Ca2+ efflux and Na+ influx, meaning Na+ can efflux in exchange for a K+ influx via Na+/K+-ATPase
2 components of increased myocardial oxygen supply
increased coronary blood flow, increased arterial O2 content
4 components of increased myocardial oxygen demand (for myocyte contraction)
increased HR, increased preload, increased afterload, increased contractility
which component increases number of contractions
increased HR
which components increase force of contraction most
afterload and contractility (preload only causes a small increase in force of contraction)
how does afterload increase work of heart
increased resistance in system means heart has to work harder
how does preload increase work of heart
Starling’s law; high blood return so dealing with more volume (stroke volume), so heart works harder
