The Cardiovascular System, Rhythmic Excitation of the Heart
The atria contract 1/6th of a second before the ventricles. Specialised excitatory and conductive system of the Heart. SA node - Sinus node or sinoatrial node, initiates the cardiac impulse Internodal pathway - SA to AV node AV node - delays impulse from the atria to the ventricles AV bundle - conducts from AV node to the ventricles Purkinje Fibers - conducts impulses to all parts of the ventricles. The Sinus Node controls the heart rate: The membrane potential of sinus node is -55 to -60 mV The heart rate is caused by: The fast sodium channels are inactivated at the normal resting potential, but there is slow leakage of sodium in the fiber. Between action potentials the resting potential gradually increases because of slow leakage sodium until the potential reaches -40mV At this time, the calcium-sodium channels become activated, allowing rapid entry of sodium and calcium and thus causing the action potential. Greatly increased numbers of potassium channe...