Anatomy & Physiology

Anatomy & Physiology Study Guide The arrival of an impulse at a cardiac muscle plasma membrane produces an action potential that is comparable to an action potential in a skeletal muscle fiber. As in a skeletal muscle fiber, this action potential triggers the contraction of the cardiac muscle cell. Thanks to the coordination provided by the conducting system, the atria contract first, driving blood into the ventricles through the AV valves, and the ventricles contract next, driving blood out of the heart through the semilunar valves. The SA node generates impulses at regular intervals, and one heartbeat follows another throughout your life. After each heartbeat, there is a brief pause—less than half a second—before the next heartbeat begins. The cardiac cycle is the time between the start of one heartbeat and the start of the next. A heartbeat lasts only about 370 msec. The Conducting System In contrast to skeletal muscle, in the absence of neural or hormonal stimuli, cardiac muscle tissue contracts on its own. This ability is known as automaticity , or autorhythmicity . The cells responsible for initiating and distributing the stimulus to contract are part of the heart’s conducting system, also known as the cardiac conduction system or the nodal system . This system is a network of specialized cardiac muscle cells that initiate and distribute electrical impulses. The actual contraction lags behind the beginning of an electrical impulse (the action potential), with the delay representing the time it takes for calcium ions to enter the sarcoplasm and activate the contraction process. The majority of the cells of the conducting system are smaller than the contractile cells of the myocardium and contain very few myofibrils. Purkinje cells , however, are much larger in diameter than the contractile cells; as a result, they conduct action potentials more quickly than other conducting cells. Conducting cells of the SA and AV nodes share an important characteristic: their excitable membranes cannot maintain a stable resting potential. After each repolarization, the membrane gradually drifts toward threshold. This gradual depolarization is called a prepotential or pacemaker potential . The Sinoatrial (SA) Node The sinoatrial node (SA node) is embedded in the posterior wall of the right atrium, near the entrance of the superior vena cava. The heart rate is established by the pacemaker cells of the SA node. As a result, the SA node is also known as the cardiac pacemaker or the natural pacemaker. The SA node is connected to the larger AV node by the internodal pathways in the atrial walls. It takes roughly 50 msec for an action potential to travel from the SA node to the AV node along these pathways. Along the way, the conducting cells pass the stimulus to contractile cells of both atria. The action potential then spreads across the atrial surfaces by cell-to-cell contact. The Atrioventricular (AV) Node The relatively large atrioventricular (AV) node sits on the floor of the right atrium near the opening of the coronary sinus. The rate of propagation of the impulse decreases as it leaves the internodal pathways and enters the AV node because the nodal cells are smaller in diameter than the conducting cells. Also, the connections between nodal cells are less efficient than those between conducting cells at relaying the impulse from one cell to another. As a result, it takes about 100 msec for the impulse to pass through the AV node. This delay is significant because the atria must contract before the ventricles do. Otherwise, contraction of the powerful ventricles would close the AV valves and Achieve Page 245 of 368 ©2018