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  4. Assertion: Sino-atrial node (SAN) is called the pacemaker. Reason: SAN generates the maximum number of action potentials and is responsible for initiating and maintaining the rhythmic contractions of the heart

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Q. Assertion : Sino-atrial node $(SAN)$ is called the pacemaker.
Reason : $SAN$ generates the maximum number of action potentials and is responsible for initiating and maintaining the rhythmic contractions of the heart

Body Fluids and Circulation

Solution:

In the myogenic heart, contraction is initiated by specialised muscles. The $SAN$ can generate the maximum number of action potentials, i.e., $70-75\, min^{-1}$, and is responsible for initiating and maintaining the rhythmic contractile activity of the heart. Therefore, it is called the pacemaker. The $SAN$ controls the beat of the heart because its rate of rhythmical discharge is faster than that of any other part of the heart. Therefore, the $SAN$ is virtually always the pacemaker of the normal heart.