Electrical properties of the heart Flashcards
functional syncyitium
mass of cells that have merged together. The syncytium of cardiac muscle is important because it allows rapid coordinated contraction of muscles along their entire length.
how can cardiac muscle regulate the strength of contraction
longer and sites are not saturated so can regulate how many sites are activated by regulating how much calcium comes in from the outside during action potential. don’t want it to stay contracted, needs to relax so long action potential allows it to do that
intercalated discs
series of gap junction (so electrically connected) and desmosomes (so physically connected) all linked together which means if neighbouring cell depolarises it is likely that this cell will too
how can cardiac muscle regulate the strength of contraction
longer action potential and sites are not saturated so can regulate how many sites are activated by regulating how much calcium comes in from the outside during action potential. also long refractory period so cannot exhibit tetanic contraction. don’t want it to stay contracted, needs to relax so long action potential allows it to d0 that
pacemakers action potentials
to get to threshold there is increase in opening of voltage gated calcium channels which is slower than voltage gated sodium channels but still long lasting. then pacemaker potential
non pacemaker cells action potential
resting membrane potential is high, initial depolarisation due to rise in sodium and opening of more voltage gated sodium channels, then during plateau- voltage gated calcium channels L open too and stay open for a very long time which maintains the depolarisation also some leaky potassium channels shut, then voltage gated calcium channels shut and leaky potassium channels open again and return the membrane potential back to normal
pacemaker potential
gradual decrease in permeability to potassium, early increase in permeability to sodium PF (opened by repolarisation from the previous action potential), late increase in permeability to calcium T (only small amount of calcium allowed)
pacemakers function
don’t need to be told to beat, they just do it themselves because they spontaneously depolarise to threshold. spread the action potential through the gap junctions cell to cell and the action potential will cause the excitation contraction coupling and contraction will spread from cell to cell
effect of Administration of Ca2+-channel blockers
Voltage-gated Ca2+ channels are involved in the action potentials of pacemaker and non-pacemaker cells. Ca2+ channel blockers, will reduce the speed with which Ca2+ enters the cell and the total amount of Ca2+ that enters the cell. Voltage-gated Ca2+ channels blockers will therefore reduce heart rate, and reduced the strength of contraction.
effect of Hypercalcemia (high plasma Ca2+)
Hypercalcemia will increase the concentration gradient for Ca2+ entering the cell so it enters faster and more of it enters. Hypercalcaemia will therefore increase heart rate and strength of contraction.
effect of Hypocalcemia (low plasma Ca2+)
Hypocalcemia will decrease the concentration gradient for Ca2+ entering the cell so it enters slower and less of it enters. Hypocalcaemia will therefore decrease heart rate and strength of contraction.
effect of Hyperkalemia (high plasma K+)
Hyperkalemia will depolarise myocardial cells and therefore trigger spontaneous uncoordinated contraction (ie fibrillation). The depolarisation also reduces the electrical gradient for positive ions entering the cell and therefore slows down conduction of the action potential. This may mean that conduction through the atrioventricular node fails and depolarisation in the atria never reaches the ventricles. This is known as heart block.
effect of Hypokalemia (low plasma K+)
You would expect this to do the opposite of hyperkalemia and hyperpolarise cells, but for some reason it soon causes depolarisation and therefore produces the same effects as hyperkalemia.
effect of increased body temp(fever)
Most chemical events occur faster at higher temperatures and this includes those that govern heart rate. A 1°C rise in body temperature will cause an increase in heart rate of about 10 beats per minute.
order of special conducting system
sinoatrial node, atrioventricular node, bundle of his, bundle branches, Purkinje fibres
