 This video will cover the following objective. Describe the intrinsic conduction system of the heart and explain the pathway and action potential travels through the heart. There are two major types of cardiac muscle cells. There are contractile cells that contain the sarcomeres, consisting of thick and thin filaments that perform the sliding filament theory, a mechanism of contraction in order to produce the force of contraction that pumps blood. And there are auto-rhythmic cells, the cells that stimulate contraction. So the heart can contract without a stimulus from the nervous system. If there were no nerves attached to the heart, it would be able to keep beating because of the auto-rhythmic cells that have an intrinsic ability to spontaneously generate an action potential. And so these auto-rhythmic cells function as the pacemaker of the heart, generating the action potential that then spreads through the heart to stimulate contraction. There are auto-rhythmic cells throughout the intrinsic conduction system of the heart, which is shown in yellow in this illustration. So all of the auto-rhythmic cells are capable of spontaneous depolarization to produce an action potential. However, the SA node contains auto-rhythmic cells that have the fastest rate of spontaneous depolarization. Therefore, the SA node normally functions as the pacemaker of the heart. The SA node is located in the right atrium at the entrance of the superior vena cava. The sinoatrial node or SA node generates an action potential, and that action potential will spread from the auto-rhythmic cells into the contractile cells because the cells of cardiac muscle are connected by gap junctions. These gap junctions allow the action potential to spread from cell to cell, and so when the SA node fires an action potential, that action potential will spread throughout the myocardium in the atria, stimulating atrial contraction. Atrial contraction is also known as atrial systole. So the word systole refers to contraction of a heart chamber, and the atrial systole will occur before ventricular systole. So the atria will contract and the force blood down through the AV valves into the ventricles. And then the electrical signal that was generated by the SA node, it spreads into another region of auto-rhythmic cells known as the AV node or atrioventricular node. The AV node functions to slow down the spread of the action potential as the action potential is relayed down to the ventricles through the interventricular septum. So there is a atrioventricular bundle of auto-rhythmic cells that relays the action potential from the AV node down into the interventricular septum, and then the atrioventricular bundle, also known as the bundle of his, branches into a right bundle branch and a left bundle branch. The bundle branches then relay the action potential into the right and left ventricular myocardium. And there are smaller branches of the intrinsic conduction system that come off of the bundle branches. These are known as Purkinje fibers, and the Purkinje fibers branch many times throughout the myocardium of the ventricles enabling the action potential to rapidly spread all through the ventricles. This slide shows us the pattern of electrical and mechanical activity through the cardiac cycle. We start with the heart in the resting phase known as diastole when the heart muscle is not contracting. Electrical activity is at the resting membrane potential, so the heart has become repolarized from its last action potential. And then blood is flowing into the atria from the veins. Once enough blood enters the atria that the pressure of the atria is greater than the ventricles, the AV valves open and blood flows down into the ventricles during the ventricular filling phase of the cardiac cycle. But then an action potential is generated in the SA node and spreads through the atria, and this atrial depolarization stimulates atrial systole, the contraction that forces blood from the atria into the ventricles. Then as the atria repolarize, the action potential spreads through the AV bundle from the AV node down to the bundle branches and Purkinje fibers through the ventricular myocardium, and this ventricular depolarization stimulates ventricular systole. As the ventricles contract, the pressure of blood in the ventricles increases and forces the AV valves closed during the isovolumetric contraction phase of the early ventricular systole. Once the ventricular pressure is higher than the pressure in the arteries, the semi-lunar valves are forced open and ventricular ejection occurs during the late ventricular systole, and this then will be followed by ventricular repolarization as the ventricles enter diastole and relax during the early ventricular diastole. The AV and semi-lunar valves are both closed. This is known as the isovolumetric relaxation phase, but once the pressure inside of the ventricles falls lower than the pressure in the atria, then the AV valves will open and will return to the ventricular filling phase of the cardiac cycle. Changes in blood pressure are what stimulates the opening and closing of the heart valves. During late ventricular diastole, the blood pressure inside of the atria is higher than the blood pressure inside of the ventricles, so the AV valves open allowing blood to flow from the atria down into the ventricles, beginning the ventricular filling phase of the cardiac cycle. As the pressure inside of the ventricles is lower than the pressure inside of the arteries, the semi-lunar valves remain closed. During ventricular systole, when the pressure of blood inside of the ventricles becomes higher than the pressure of blood inside of the atria, the AV valves are forced closed. When the pressure of blood inside of the ventricles becomes higher than the blood pressure inside of the arteries, the semi-lunar valves are forced open and blood flows out of the ventricles into the arteries during the ventricular ejection phase of the cardiac cycle. While the SA node of the heart functions as the pacemaker of the heart, generating the action potentials that stimulate contraction independent from input of the nervous system, the nervous system can increase or decrease the heart rate. In particular, it's the autonomic efferent fibers that regulate the heart. Parasympathetic efferent fibers release acetylcholine in the SA node and AV node, leading to a decreased heart rate. Sympathetic efferent fibers release norepinephrine throughout the heart, leading to increased heart rate as well as increased force of contraction.