Control of the beating of the heart

The beating of the heart

Cardiac muscle is myogenic, that is, generate rhythmical contraction from within its muscle. The heart rhythm is initiated by specialized plexus (network)of fine cardiac muscle fibers embedded in the walls of the right atrium close to where the vena cava enters the heart, called the sinoatrial node (SAN) or the pacemaker. The contraction of the heart is preceded by a wave of electrical excitation from SAN and then spread over the heart atria. When the wave reaches the junction between the atria and ventricle, it excites another specialized group of modified cardiac muscle fibers called the atrioventricular node (AVN), Continuous with the AVN is a strand of modified cardiac muscles fibers called Bundle of Hiss. The Bundle of Hiss runs down the inter-ventricular septum and fans out over the walls of the ventricle where it breaks up into a network of fibers called Purkinje tissue just beneath the endothelial lining. When the AVN receives excitation from the atria, it sends impulses down the bundle of Hiss to Purkinje tissue. The impulse then spread out to the cardiac muscle tissue in the walls of the ventricle making it contract. The role of AVN is to delay electrical impulses from the atria to the ventricle to allow atria contract before the ventricle. The Purkinje tissue ensures a slow uniform spreading of excitation to the ventricle after the contraction of the atria. The diagram below is a ventral view of the heart showing the spreading of the electrical excitation that causes contraction.

The nerves simply serve to speed up or slow down the heart

Control of contraction The cardiovascular center in the medulla of the brain controls the rate of heartbeat. Depending on circumstances, the cardiovascular center sends impulse down the sympathetic nerve to the heart, increasing its rate of heartbeat or down the vagus nerves decreasing the rate of heartbeat. Please find free downloadable notes, exams and marking guides of agriculture, biology, Physics, chemistry etc.  from digitalteachers.co.ug website. Dr. Bbosa Science