Electrical Conduction System of the Heart (Nursing)

00:01 So now let's Look at the electrical events of the heart.

00:06 The heart depolarizes and contracts without nervous system stimulation although the rhythm can be altered by the autonomic nervous system when necessary.

00:20 So remember that the heart contains gap junctions which are going to allow for the heart to function as a syncytium.

00:30 The heart contains an intrinsic cardiac conduction system which includes a network of non-contractile, autorhythmic cells that are going to initiate and distribute impulses to coordinate depolarization and subsequent contraction of the heart.

00:51 An action potential is going to be initiated by these pacemaker cells or these are autorhythmic cells and these are going to have an unstable resting membrane potential and we sometimes refer to this unstable potential as the pacemaker potential or the prepotential.

01:13 So during this process, there are three main parts.

01:17 First, we have our pacemaker potential or our prepotential and during this point, the potassium channels are closed but the slow sodium channels are open.

01:29 This causes the interior of the membrane to become slightly more positive or to begin to depolarize.

01:39 Second, we have what's known as the depolarization where now, calcium channels are going to open.

01:47 This happens around -40 millivolts and allows a huge influx of calcium ions to come into the cell which is now going to lead to a rising phase of the action potential as the membrane is going to depolarize even more.

02:06 After this peak, we're going to get a repolarization where now, we're going to open our potassium channels.

02:15 The opening of these channels allows the potassium to exit the cell and thus we're losing these positive ions and the cell becomes more negative.

02:29 So now let's look at how this works in the actual heart.

02:33 So our cardiac pacemaker cells are gonna pass impulses in the following order across the heart and this process is going to take about .22 seconds.

02:46 So first, we start with the pacemaker cells in the sinoatrial node.

02:53 From there, we're going to send impulses to the next node known as the atrioventricular node.

03:02 From there, we send an impulse down a chord of neurons known as the atrioventricular bundle.

03:10 The atrioventricular bundle will then separate into the right and left bundle branches and from there it goes to the sub endocardial conducting network or the Purkinje fibers of the heart.

03:26 So starting at the first step or the sinoatrial node, this is what's known as the pacemaker of the heart in the right atrial wall.

03:37 This one is going to depolarize the fastest out of all of the nodes in the myocardium.

03:44 It generates an impulse about 75 times per minute and this is what's referred to as the sinus rhythm.

03:54 It also has an inherent rate of about 100 times per minute that can be tampered by extrinsic factors such as the autonomic nervous system.

04:06 From this node, the impulse is going to spread across the first atria, the right atria to the AV node.

04:17 The AV node or the atrioventricular node is gonna be found in the inferior interatrial septum.

04:26 It is going to delay impulses for approximately .1 seconds And because these fibers are smaller in diameter, they also have fewer gap junctions and this allows for an atrial contraction prior to a ventricular contraction.

04:45 The inherent rate of this node is 50 times per minute in the absence of the SA node input so if we did not have step 1, this would happen on its own about 50 ptimes per minute.

05:01 From the AV node, we go to the AV bundle.

05:05 The AV bundle is located in the superior portion of the interventricular septum.

05:11 This is going to be the only electrical connection between the atria and the ventricles.

05:17 Just note that the atria and the ventricles are not connected by way of gap junctions so this bundle is how they are connected.

05:27 From the AV bundle, we are now going to the go and split into the right and left bundle branches and these two pathways are also found in that interventricular septum between the two ventricles.

05:43 This is going to carry the impulse from the AV bundle down to the apex or that bottom part of the heart.

05:52 Lastly we have our subendocardial conducting network or our Purkinje fibers.

05:59 These are going to complete the pathway through the interventricular septum into the apex and then into our ventricular walls.

06:09 Because of the process that the left side of the heart has to go through, the Purkinje fibers are more elaborate on the left side of the heart.

06:19 And the AV bundle and the subendocardial conducting network are going to depolarize at about 30 times per minute in the absence of the previous node - the AV node input.

06:35 Ventricular contraction is also going to be immediately followed from the apex toward the atria.

06:42 In the process from initiation at the SA node all the way to the end to the Purkinje fibers is going to take about .22 seconds.

06:56 So while the heart does not rely on our nervous system in order to contract, the heartbeat can be modified by the autonomic nervous system by way of cardiac centers found in the medulla oblongata.

07:14 There's the cardioacceleratory center which is gonna send signals through the sympathetic trunk to increase the rate and the force of the contraction of the heart and it does this by stimulating the SA and the AV nodes, as well as the heart muscles and the coronary arteries.

07:36 The second center found in the medulla oblongata is the cardioinhibitory center which is going to send parasympathetics signals by way of the vagus nerve in order to decrease the rate of the heart.

07:51 It does this by inhibiting the SA and AV nodes via the vagus nerve.