Conducting System of the Heart

00:01 Well, let’s look at the conducting system of the heart.

00:06 The conducting system of the heart consists of the sinoatrial node, the atrioventricular node, and then these bundles of conducting fibres named in various ways, but I don’t want to get into those namings in this particular lecture. In the middle section, labeled is the atrioventricular bundle. It’s embedded with connective tissue. It's cardiac muscle cell. And on the right-hand side, you can see the endocardium. And directly under the endocardium if you look very carefully, often, you find evidence of these conducting fibres called Purkinje fibres. Now, the atrioventricular bundle is just a large bundle of Purkinje fibres.

00:55 Those Purkinje fibres are then branched out and head down through the myocardium, generally, very closely associated with the subendocardial layer. And that’s where you could locate them histologically. Let me stress that these conducting fibres of the heart are not nerves. They’re specialized cardiac muscle. So if you look at the diagram of a heart, the blue circular structure and the blue lines running down through the myocardium represents these conducting fibres. And the white circular structure up at the top that’s actually lying right next to the entry of the superior vena cava is the sinoatrial node. And it has processes Purkinje fibres or conducing fibres radiating from that sinoatrial node. The sinoatrial node is the pacemaker of the heart.

02:04 Cardiac muscle, when it developed, has quite an inherent ability to contract, that contracts without any stimulation except the impulses emanating from the sinoatrial node. However, that sinoatrial node can be influenced by the autonomic nervous system. The firing rate of the impulse can be increased or decreased by external factors, not just nerves from the autonomic nervous system, but by other factors as well. So you can increase or decrease the rate of the heart by influencing the rate of firing initiated from that sinoatrial node. But essentially, those conducting fibres are cardiac muscle, but they are just specialized. And you can tell the difference between these conducting fibres from normal cardiac muscle. Have a look very carefully at the central section of the atrioventricular bundle, or even have a look at the Purkinje fibres labeled just under the endocardium. Both these labels or both these fibres rather have clear types of components within them. The cytoplasm is less duct stained, and sometimes, it’s even very, very pale. That’s because these cardiac muscles are conducting fibres. They’re not contractile. So they don’t contain all the contractile proteins, all the contractile factory within the cell. They don’t need it. So they don’t take up stain because they’re not there. It’s as simple as that.

03:54 Another way you can tell the difference between the Purkinje fibre, particularly, and surrounding cardiac muscle fibres is that Purkinje fibres often have two nuclei. Remember, cardiac muscle only has one nucleus.