STRUCTURE OF THE CONDUCTING SYSTEM
The atria are separated from the ventricles by a ring of fibrous tissue. This fibrous demarcation acts as an insulator, thereby barricading the entrance of electrical activity from the atria into the ventricles. The functional structure of the heart requires it to fill the atria with blood before the ventricles. Therefore, it is important for the atrial musculature to contract first and force blood into the ventricles before the ventricles undergo contraction. After a time lapse of milliseconds, electrical activity in both the ventricles causes them to contract and pump blood to the pulmonary circuit and to the rest of the body.
Electrical activity in the cardiac muscle is initiated by the sinoatrial node (SAN), located near to the opening of superior vena cava. The SA node is comprised of a collection of modified cardiac cells that have the potential to generate electrical signals.Multiple pathways, originating from the SAN, carry electrical signals to the atrial muscles. Similarly, one of the pathways stimulates the atrioventricular node (AVN) present at the base of right atrium and limited by the coronary sinus, the atrial septum and the tricuspid valve (Koch’s triangle). Bundles of fibers arise from the AV node, pierce the atrioventricular septum and enter the ventricles. In the ventricles, the fibers recollect and divide into two bundles that run down the interventricular septum. These bundles are known as the bundle of His. Upon reaching the apex of the heart, the bundles of His give rise to Purkinje fibers that ascend along the ventricular walls in a fashion which allows for spread of electrical activity in an ascending fashion (from the apex to upwards).
TYPES OF CELLS PRESENT IN THE CARDIAC SYSTEM
The cells of the cardiac system are primarily classified as follows:
- Functional Cells
- Electrical Cells
- Functional cells of the heart are further divided into contractile cells and conducting cells. Contractile cells are basically muscular cells that are joined together at their ends forming intercalated discs. Gap junctions between cells within the intercalated discs allow movement of ions, thereby causing the heart to work as a syncytium (single unit). Hence, the contractile cells show properties of conduction as well. Conducting cells, on the other hand, are modified to allow passage of electrical current only and do not exhibit any contractile properties.
- Electrical cells are sub-classified into fast and slow fibers depending upon the conduction velocity and speed of depolarization which they exhibit. The rate at which an action potential travels through a conducting fiber depends on its diameter and the presence of various channels on its cell membrane. Purkinje fibers have the largest diameter relative to the rest of the fibers in the conducting system. Hence, Purkinje fibers have the highest conduction velocity. In general, the conducting fibers in the heart also possess fast sodium channels on their cell membranes. The number of these fast sodium channels helps determine the rate of depolarization and hence, the conduction velocity.
The two nodes (SAN & AVN), on the other hand, have slow calcium channels present on their cell membranes. Movement of ions through these channels results in a gradual increase in the membrane potential leading to slow depolarization of the nodes.
Another determinant affecting conduction velocity is the presence of gap junctions between cardiac cells. Gap junctions facilitate movement of ions from one cell to another. Higher the number of these gap junctions, higher would be the conduction velocity as more cells get depolarized at a time. The AV node and its emergent fibers have the least number of gap junctions and hence, the conduction velocity is slowest.
DIRECTION OF IMPULSE TRAVEL
Before moving on, it should be noted that the two nodes (SAN & AVN) and all the conducting fibers in the heart muscle have an intrinsic ability to undergo depolarization. This explains why even if the heart is isolated from the rest of the body, it will resume its pacemaker activity and continue to beat on its own. An isolated SA node has the highest frequency of impulse generation, i.e. 100 beats/min. This intrinsic rhythm of the SAN is regulated down to 72 beats/min under the influence of the autonomic nervous system. Similarly, the AV node has the ability to depolarize at a rate of 60 beats/min. SA node, having a considerably higher frequency of depolarization, overrides the pace maker activity of the AV node. This causes the AV node to generate action potentials at a rate similar to SA node. Upon cessation of high frequency impulses from SA node, as happens during bundle blocks, the AV node is shown to beat at its own inherent frequency. There is a respective decrease in the frequency of depolarization as we move along the bundle of His and the Purkinje fibers. This relative difference in the intrinsic rhythm of different parts of the cardiac conduction system allows a uni-directional flow of impulses across the entire conducting system.
It’s important to remember that, AV bundle is the only pathway which allows electrical transmission of impulses from atria to the ventricles. Moreover, the impulses are prevented from propagating in retrograde direction (back into the atria from the ventricles) by the fibrous atrioventricular septum (insulating layer). If the impulses were allowed to travel back into the atria through accessory pathways, the heart would lose its rhythmical beating and arrhythmias may ensue.
PART OF THE CONDUCTING SYSTEM
INTRINSIC RHYTHM (BPM)
|SAN (Pacemaker of Heart)||100 (Normal HR is 72 BPM under the influence of ANS)|
|AV Bundle of His||40-50|
DURATION OF IMPULSES
Duration of impulses is important as it dictates periodic filling of the atria and ventricles and ejection of blood effectively. The duration of an impulse travelling along the conducting system can be briefly described as follows:
- It takes 0.03 seconds for an impulse to travel from SA node to the AV node.
- The speed is substantially reduced as impulses reach AV node because of its smaller diameter and fewer gap junctions. As a consequence, a time lapse of 0.09 seconds occurs within the AV node. Since, impulse transmission is the slowest within the AVN; it’s a site for various drugs that affect the heart rate.
- Travelling further, it takes 0.04 seconds for the impulse to move along the AV bundle and the bundle of His.
- It takes another 0.06 seconds for the impulse to spread throughout the ventricular muscles as the fast Purkinje fibers allow rapid and instantaneous conduction of nerve impulses. This is made possible due to the large diameter of Purkinje fibers and presence of numerous gap junctions. The latter allows the ventricles to work as a syncytium, resulting in simultaneous contraction of the entire ventricular musculature.