Describe and compare action potentials in cardiac pacemaker and contractile cells. Cardiac pacemaker cells are present in the sinoatrial and atrioventricular nodes. They are the 1% of cardiac fibers that are autorhythmic. This means they can depolarize and pace the heart. These cells play a role in the intrinsic conduction system. These cells have an unstable resting potential that depolarizes continuously, getting closer to threshold. These changes in membrane potential are referred to as pacemaker potentials. They are responsible for initiation of the action potentials that cause contraction of the heart. The first step is known as pacemaker potential. This potential is caused by the unique properties of the channels in the sarcolemma. Hyperpolarization at the end of an action potential causes opening of the sodium channels and closing of the potassium channels. This causes the inside of the membrane to become more positive. The next step is known as depolarization. The calcium channels open at threshold allowing the calcium ions to flow in from the extracellular matrix. This action generates the rising phase of the action potential and reverses membrane potential. Lastly, repolarization occurs this step is due to inactivation of the calcium channels causing the opening of the potassium channels. This causes potassium efflux which brings membrane potential back to its most negative voltage. When repolarization is finalized, potassium channels close and efflux decreases. The slow depolarization to threshold starts again. Neurons and skeletal muscle fibers are examples of unstimulated contractile cells of the heart that are responsible for keeping the membrane potential stable. Contractile cells are the major component of heart muscle. The action potential of contractile cardiac muscle cells begins with depolarization. This is caused by an influx of sodium through the voltage gated sodium channels in the sarcolemma. This influx begins a positive feedback cycle that initiates rising of action potential and the reversal of membrane potential from -90mV to +30mV. Sodium channels inactivate fast therefore the sodium influx does not last very long. The next step is the unique plateau phase. When sodium dependent membrane depolarization occurs it leads to a change in voltage that allows calcium to flow in from the extracellular fluid. The opening of the channels is delayed therefore they are referred to as slow calcium channels. The calcium efflux prolongs the depolarization causing a hump or plateau. The plateau phase is extended because there are very few voltage-gated potassium channels open. The cells continue to contract as calcium enters in. The final step is repolarization. This phase is due to inactivation of calcium channels and opening of voltage-gated potassium channels. The resting membrane potential is restored by the loss of potassium form the cells through the channels. During this phase, calcium is pumped back into the sarcoplasmic reticulum and the extracellular space. The action potential and contractile phase lasts noticeably longer in cardiac muscle than in skeletal muscle. The action potential in a skeletal muscle lasts 1-2 ms and contraction for a single stimulus 15-100 ms. In cardiac muscle, the action potential lasts 200 ms or more due to the unique plateau phase. This ensures that contraction is sustained so blood is removed effectively from the heart. It also causes a long refractory period. This allows the heart to fill again for the next beat.Describe the timing and events of the cardiac cycle (Wigger’s Diagram) The cardiac cycle determined by a sequence of changes of pressure and blood volume in the heart. It includes events related with blood flow through the heart during a single heartbeat. Systole is defined as depolarization and contraction. Diastole is the opposite. It is associated with repolarization and relaxation. The starting point of the cardiac cycle is when the heart is in complete relaxation. The atria and ventricle are nearly silent and it’s mid-to-late diastole. The first step is known as ventricular filling. There is low pressure in the heart, blood from the circulation is flowing passively through the atria and open AV valves into the ventricles. The aortic and pulmonary valves are closed. Over 80% of ventricular filling occurs during this step. The other 20% is delivered to the ventricle when the atria contract at the end of this phase. Now everything is in place for atrial systole. After depolarization, the atria contract, this causes the compression of blood in its chambers. This leads to an increase in atrial pressure which sends residual blood out of the atria into the ventricles. The ventricles now have their maximum volume of blood. This is referred to as end diastolic volume (EDV). Next, the QRS complex is generated by the relaxation of the atria and the depolarization of the ventricles. Atrial relaxation continues through the remainder of the cycle. The second step is known as isovolumetric contraction. The ventricles begin to contract when the atria relax. Ventricular pressure increases rapidly as the walls close in on the blood in their chambers. The short period of time when the ventricles are closed and the blood volume in the chambers are constant as the ventricles contract is the isovolumetric contraction phase. As ventricular pressure increases, it surpasses the pressure in the large arteries from the ventricles. As the semilunar valves are opened, the isovolumetric stage concludes. The third phase is ventricular ejection. This is when blood flows from the ventricles into aorta and pulmonary trunk. The aorta pressure gets to nearly 120 mmHg. The final step is isovolumetric relaxation. After the peak of the T wave the ventricles relax. Ventricular pressure decreases and blood in the aorta and pulmonary trunk flows back to the heart closing the semilunar valves. Once the ventricles are closed completely this marks the isovolumetric relaxation phase. Closure of the aortic valve increases aortic pressure for a short period of time as backflowing blood rebounds off the valve cusps. This starts at the dicrotic notch. The AV valves are open and ventricular filling starts again when blood pressure on the atrial side of the AV valves surpasses that in the ventricles. Pressure of the atria decreases to its lowest point and ventricular pressure rises