Therefore, drugs that block beta-adrenoceptors ( beta-blockers) decrease conduction velocity and can produce AV block. Sympathetic nerves exert their actions on the AV node by releasing the neurotransmitter norepinephrine that binds to beta-adrenoceptors, leading to an increase in intracellular cAMP. All the ventricular mass is activated by about 0.23 sec. After a delay at the AV node, the septum becomes activated (0.16 sec). Atrial activation is complete within about 0.09 sec (90 msec) following SA nodal firing. The time (in seconds) to activate the different regions of the heart are shown in the figure to the right. The conduction system within the heart is very important because it permits a rapid and organized depolarization of ventricular myocytes that is necessary for the efficient generation of pressure during systole. This results in rapid depolarization of ventricular myocytes throughout both ventricles. The bundle branches then divide into an extensive system of Purkinje fibers that conduct the impulses at high velocity (about 4 m/sec) throughout the ventricles. These specialized fibers conduct the impulses at a very rapid velocity (about 2 m/sec).
The impulses then enter the base of the ventricle at the Bundle of His and then follow the left and right bundle branches along the interventricular septum.
