The heart has a special conduction system for the following purpose
- Generating rhythmical electrical impulses to cause rhythmical contraction of the heart muscle.
- Conducting these impulses rapidly through the heart.
Automaticity means the ability of the cell to undergo depolarization spontaneously causing the production of electrical impulses.
Rhythmicity means that spontaneous depolarization occurs at regular intervals that is in a rhythmic manner.
What is a Cardiac Impulse?
The action potential in the heart is called the cardiac impulse and like action potential in the nerve fibers it can travels.
What are the components of the Conductive System
- SA node
- Internodal pathways
- A-V node
- A-V bundle
- Right and Left bundle branches
SA node is called the pacemaker of the heart. Normally SA node is responsible for generating the cardiac electrical impulses that bring about the mechanical activity that is contraction of the heart.
SA node has the fastest rate of autorhythmicity.
Where is the location of the SA node?
- SA node is a small, ellipsoid strip of specialized cardiac muscle about 3mm wide, 15 mm long, and 1mm thick. This is located in the superior posterolateral wall of the right atrium immediately below and slightly lateral to the opening of the superior venacava in to the right atrium.
- SA nodal fibers are connected directly with the atrial muscle fibers.
The cardiac impulse after it’s origin in the SA node spreads through out the atrial muscle through two routes
1.Ordinary Atrial muscle fibers
2. Specialized anterior, middle and posterior conducting bundles
- Anterior internodal bundle of Bachman
- Middle internodal bundle of Wenkebach
- Posterior internodal bundle of Thoral
Specialized anterior, middle and posterior conducting bundles are called the internodal pathway that is given above.
- These inter nodal pathways conduct the impulses at a faster rate than the ordinary atrial muscle fibers.
- The cause of rapid conduction in these bundles is the presence of specialized conduction fibers.
- These inter nodal pathways conduct the impulses at a faster rate than the ordinary atrial muscle fibers.
- The cause of rapid conduction in these bundles is due to the presence of specialized conduction fibers.
- The velocity of conduction in most atrial muscle is about 0.3m/sec.
- In the specialized internodal pathways the conduction velocity may reach upto 1m/sec.
- The impulse after leaving SA node takes 0.03 sec to reach the AV node
The AV node is located in the posterior wall of the right atrium immediately behind the tricuspid valve.
Cause of Slow Conduction in the A-V Node
The cause of slow conduction is mainly due to reduced number of gap junctions between the successive cells in the conducting pathways. Becauses of this there is great resistance to conduction of excitatory ions from one conducting fiber to the next.
What is the significance of AV nodal delay?
The cardiac impulse does not travel from the atria to the ventricles too rapidly.
This delay allows time for the atria to empty their blood into the ventricles before ventricular contraction begins. This increases the efficiency of the pumping action of the heart.
It is primarily the AV node and it’s adjacent fibers that delay this transmission into the ventricles
AV Bundle or Bundle of His
- From the AV node arises a special conducting pathway named the bundle of His. Except for the very small part which penetrates through the AV fibrous tissue and has low conduction velocity, the bundle of His is made up of purkinje fibers which possess maximum conduction velocity in the heart
- Purkinje fibers are very large fibers and they transmit action potentials at a velocity of 1.5 to 4.0 m/sec.
- The rapid transmission of action potentials through the Purkinje fibers is believed to be caused by a very high level of permeability of gap junctions at the intercalated discs between the successive cells of Purkinje fibers.
- The rapid conduction through the purkinje fibers ensures that different parts of ventricles are excited almost simultaneously; this greatly increases the efficiency of heart as a pump
- Normally the Bundle of His is the only conducting mass between the atrial and ventricular musculature and it transmits the cardiac impulses from the AV node to the ventricles.
- After penetrating the fibrous tissue between the atrial and ventricular muscle, the distal portion of the A-V bundle passes downward in the ventricular septum for 5 to 15 mm toward the apex of the heart.
- Then the bundle of His splits into two branches which are called right and left bundle branches that lie on the respective sides of the ventricular septum
- Each branch spreads downward toward the apex of the ventricle, progressively dividing into smaller branches.
- These branches inturn course sidewise around each ventricular chamber and back toward the base of heart.
- The ends of Purkinje fibers penetrate about one third of the way into muscle mass and finally become continuous with cardiac muscle fibers
- From the time the cardiac impulse enters the bundle branches until it reaches the terminations of Purkinje fibers , the total elapsed time averages only 0.03 sec.
- A special characteristic of the A-V bundle is it’s inability, except in the abnormal states , of action potentials to travel backward from the ventricles to the atria.
- It will prevents re-entry of cardiac impulse by this route from the ventricles to the atria.
- The atrial muscle is separated from the ventricular muscle by a continuous fibrous barrier that acts as an insulator to prevent the passage of cardiac impulse between the atrial and ventricular muscle through any other route besides forward conduction through A-V bundle itself.
- Once the impulse reaches the ends of the Purkinje fibers it is transmitted through the ventricular muscle mass by the ventricular muscle fibers themselves.
- For transmission of the cardiac impulse from the endocardial surface to the epicardial surface recquires another 0.03 sec.
- Thus the total time for transmission of cardiac impulse from the initial bundle branches to the last of the ventricular muscle fibers in the normal heart is about 0.06 sec
