Final answer:
The left and right bundle branches are part of the heart's electrical conduction system, receiving stimulus from the AV node and transmitting it to the ventricles. The left branch serves the left ventricle and the right branch, aided by the moderator band, serves the right ventricle and papillary muscles. They connect with the Purkinje fibers to complete the ventricular contraction cycle.
Step-by-step explanation:
The left and right bundle branches are essential components of the heart's electrical conduction system. They arise from the bifurcation of the atrioventricular bundle (also known as the bundle of His), which receives its stimulus from the atrioventricular node (AV node). The atrioventricular bundle branches pass through the interventricular septum, carrying electric impulses to the left and right ventricles respectively. The left bundle branch, which is larger due to the larger size of the left ventricle, has two fascicles and provides stimulus to the left ventricle. Meanwhile, the right bundle branch, which is aided by the moderator band, stimulates the right papillary muscles and ventricle, ensuring that the papillary muscles begin contracting simultaneously just before the ventricles. Both branches reach the apex of the heart and then connect with the Purkinje fibers.
This conduction system ensures that the ventricles contract in a coordinated manner, following the atrial contraction, which allows for efficient blood pumping. The entire sequence from the stimulus at the AV node to the ventricles takes approximately 25 milliseconds (ms). The Purkinje fibers are the final pathway for the electrical signal before causing the myocardial contractile cells of the ventricles to contract, completing the process of ventricular contraction initiated by the impulse traveled through the bundle branches.