Learn Cardiac Conduction System:

What tells the human heart to contract rhythmically and continuously throughout one's life? In this article, we will try to answer this question. In the end, we will discuss the most expected questions from this topic for the MDCAT exam.

Introduction to the electrical conducting system of the heart:  

We know that the bulk of the heart is made of cardiac muscle tissue. Some part of this myocardial tissue has developed the capacity to undergo depolarization at a specific rate.Thus generating impulse for the myocardium to undergo contraction rhythmically 72 times per minute. The electrical conduction system of heart lies within in the heart and is not nervous tissue in nature but modified myocardial tissue that is responsible for regulating the activity of the heart. This modified myocardial tissue is spread inside the heart thus spreading the impulse throughout the heart for the heart to beat.
Components Of the Electrical Conduction System Of The Heart:
The myocardial tissue modified for the generation of electrical impulses for the heart to beat has the following components:
1. Primary pacemaker of the heart/ Sinoatrial node: 
Location: It is located in the right upper corner of the right atrium near the entry of the superior vena cava.
The sinoatrial node is the primary pacemaker because its depolarization rate is the fastest. It is this primary pacemaker that is regulating the heart beat, enabling the heart to beat 72 times per minute.The SA node actual depolarization rate is 100 times per minute but is limited by the parasympathetic nervous system to 72 times per minute so the heart beats 72 times in 60 seconds.
2.AV node(atrioventricular node)/ Pacesetter/relay node:
Location: The AV node is located in the left lower corner in the base of the right atrium near the coronary sinus.
The AV nodes depolarization rate is slower than the SA node that is 60 to 70 times per minute. The nerve impulse from the sinoatrial node after passing through internodal pathways is collected by the AV node. It is also called the relay station because it relays the impulse  received from the SA node for 0.1 second so that ample time is provided for the blood to enter the ventricles from the auricles.
If for some reasons the SA node stop working the AV node will control the heart beat but the heart will beat at slower rate because its depolarization rate is slower than SA node. In such condition artificial pacemaker is used to enable the heart to beat at its normal rate.
3.Bundle of HIS: From the AV node bundles of cardiomyocytes carries the nerve impulse through the interventricular septum into the ventricles from where two bundle branches split and supply the nerve impulse to each ventricle thus spreading the nerve impulse in the ventricular musculature.
4. Subendocardial fibers/ Purkinje fibers: These are thin fibers that spread the nerve impulse inside the ventricular musculature. Thus there job is to induce ventricular contraction.

Pathway through which the nerve impulse spread in the myocardium:

SA node👉Internodal pathways👉AV node👉Bundle of HIS👉Bundle branches👉Purkinje fibers
                     The nerve impulse from SA node through internodal pathways causes atrial contraction. When the impulse reaches the AV node there is 0.1 sec delay. After which the nerve impulse passes to the ventricular musculature through the bundle of HIS and its branches and finally through the Purkinje fibers in the whole ventricular musculature causing ventricular contraction.
Heart will continue to beat after taken out of the Thoracic cavity: Now we know that the beating of the heart is regulated intrinsically(through modified myocardial tissue) and is independent of external nerve innervation. Therefore, the heart will be able to beat for some time even after taken out of the thoracic cavity. This is because the SA node is continuously generating impulses for the heart to beat.

Expected questions for the MDCAT:

1. Electrical conducting system of the heart comprises of which tissue.
a. Nervous tissue
b. Muscular tissue
c. Connective tissue
d. Epithelial tissue
2. Which node has the fastest depolarizing rate:
a. SA node
b.AV node 
c. None
3. Purkinje fibers are located in:
a.auricles 
b. ventricles
c. aorta
d. none