Anatomy & Physiology

Anatomy & Physiology Study Guide positive charges discontinues because as the sodium channels are closing, voltage-gated calcium channels are opening. These channels open slowly and remain open for a relatively long period (roughly 175 msec) and are called slow calcium channels. While the slow calcium channels are open, calcium ions enter the sarcoplasm. The entrance of positive charges through the calcium channels, in the form of Ca 2+ , roughly balances the loss of positive ions through the active transport of Na + , and the transmembrane potential remains near 0 mV for an extended period. This portion of the action potential curve is called the plateau. The major difference between action potentials in skeletal muscle cells and cardiac muscle fibers is the presence of this plateau. In a skeletal muscle fiber, rapid depolarization is immediately followed by rapid repolarization. • Step 3 Repolarization : As the plateau continues, slow calcium channels begin closing, and slow potassium channels begin opening. As the channels open, potassium ions (K + ) rush out of the cell, and the net result is a period of rapid repolarization that restores the resting potential. The Refractory Period As with skeletal muscle contractions, for some time after an action potential begins, the membrane will not respond normally to a second stimulus. This time is called the refractory period. Initially, in the absolute refractory period, the membrane cannot respond at all because the sodium channels are already open or are closed and inactivated. In a ventricular muscle cell, the absolute refractory period lasts approximately 200 msec, spanning the duration of the plateau and the initial period of rapid repolarization. The absolute refractory period is followed by a shorter (50 msec) relative refractory period. During this period, the voltage-gated sodium channels are closed but can open. The membrane will respond to a stronger-than-normal stimulus by initiating another action potential. In total, an action potential in a ventricular contractile cell lasts 250–300 msec, roughly 30 times as long as a typical action potential in a skeletal muscle fiber. The Energy for Cardiac Contractions When a normal heart is beating, the energy required is obtained by the mitochondrial breakdown of fatty acids (stored as lipid droplets) and glucose (stored as glycogen). These aerobic reactions can occur only when oxygen is readily available. In addition to obtaining oxygen from the coronary circulation, cardiac muscle cells maintain their own sizable reserves of oxygen. In these cells, oxygen molecules are bound to the heme units of myoglobin molecules. Normally, the combination of circulatory supplies plus myoglobin reserves is enough to meet the oxygen demands of the heart, even when it is working at maximum capacity. 19.9 Cardiac Cycle Each heartbeat is followed by a temporary resting phase, allowing time for the chambers to relax and prepare for the next heartbeat. The period between the start of one heartbeat and the beginning of the next is a single cardiac cycle . The cardiac cycle, therefore, includes alternating periods of contraction and relaxation. For any single chamber in the heart, the cardiac cycle can be divided into two phases: (1) systole (contraction) and (2) diastole (relaxation). The chamber contracts during Achieve Page 247 of 368 ©2018