S.A. node is pacemaker in heart because
**Core Concept:**
The sinoatrial (SA) node is a specialized cell group located in the right atrium of the heart, responsible for generating the electrical impulses that initiate the cardiac cycle. It acts as the primary pacemaker of the heart, responsible for initiating the conduction of electrical signals through the atria and ventricles.
**Why the Correct Answer is Right:**
The SA node is the right atrial pacemaker because it possesses unique properties that make it superior to other regions in generating the heart's intrinsic rhythm. Its cells possess a higher resting membrane potential (-40 mV to -50 mV) and a slower sodium leak (slow I-Na) compared to other cardiac cells, ensuring a longer refractory period and allowing time for atrial depolarization before the impulse reaches the SA node. Additionally, the SA node's cells maintain a high resting membrane potential, which helps in generating a rapid and regular impulse.
**Why Each Wrong Option is Incorrect:**
A. The atrioventricular (AV) node is the secondary pacemaker and acts as the conduction system's integrator, regulating the rate and rhythm of the heart. The AV node is not the primary pacemaker because its membrane potential and properties are not as distinct as those of the SA node.
B. The atrial myocytes are essential for atrial contraction but are not the pacemaker. They follow the electrical impulse initiated by the SA node, not generating it.
C. The ventricular myocytes are responsible for ventricular contraction but are not the pacemaker. They follow the electrical impulse generated by the SA node and transmitted via the AV node.
D. The sinus node artery supplies oxygenated blood to the SA node, but it does not determine the pacemaker function of the SA node. The SA node's unique properties are responsible for its role as the primary pacemaker.
**Core Concept:**
The SA node is part of the Hodgkin-Huxley model, which describes the electrical properties of cardiac cells. This model includes the repolarization phase, action potential duration, and the action potential waveform.
**Why the Correct Answer is Right:**
The Hodgkin-Huxley model is based on the study of cardiac myocytes' electrical properties, including the repolarization phase, action potential duration, and action potential waveform. The SA node's unique properties, as described by the Hodgkin-Huxley model, make it the primary pacemaker of the heart.
**Why Each Wrong Option is Incorrect:**
A. The atrioventricular (AV) node is part of the Hodgkin-Huxley model, but it is not the primary pacemaker due to its lower resting membrane potential and slower action potential duration compared to the SA node.
B. The ventricles' myocytes follow the SA node's electrical impulse, but they are not the primary pacemaker because of their lower resting membrane potential and slower action potential duration compared to the SA node.
C. The atrioventricular (AV) bundle is an extension of the SA node but is not the primary pacemaker due to