Biology · Body Fluids and Circulation · NEET
NCERT clearly says all components of the nodal tissue (SAN, AVN, bundle of His, Purkinje fibres) can fire on their own. The SAN is called the pacemaker only because it has the highest rate of depolarisation. It fires fastest (70-75 times/min), so its impulse spreads to the AVN and ventricles before the slower parts can fire on their own. The slower parts get 'reset' by the SAN each beat, so they follow the SAN's rhythm instead of their own. This is exactly the reasoning NEET tests.
The SAN starts each heartbeat, not the brain. The heart is myogenic, meaning the beat begins inside the heart's own nodal muscle, not from a nerve. That is why a frog's heart keeps beating even after it is removed from the body. The brain (medulla oblongata, through the autonomic nervous system) only speeds up or slows down the rate; it does not create the beat.
Yes. Because the AVN is also auto-excitable, it can act as a backup pacemaker if the SAN fails. But the AVN fires slower (about 40-60/min), so the heart rate drops. This is why the SAN is called the primary/normal pacemaker and the AVN is a secondary pacemaker. NEET may ask why AVN is only a backup: because its rate of depolarisation is lower than the SAN's.
About 70-75 action potentials per minute in a healthy resting human. This matches the normal resting heart rate of about 72 beats per minute, because each SAN action potential leads to one heartbeat. Remember this number, it is a direct NCERT line and a common fill-in-the-blank.
In the right upper corner of the right atrium (a patch of nodal tissue). Do not confuse it with the AVN, which sits in the lower left corner of the right atrium, near the atrio-ventricular septum. NEET often swaps these two locations to trap you.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Because it has the highest rate of depolarisation and generates the maximum number of action potentials (70-75 per minute), so it sets the pace for the whole heart. The other nodal parts follow its rhythm.
The AVN is auto-excitable and can act as a backup (secondary) pacemaker if the SAN fails, but it fires slower (about 40-60/min). The SAN is the normal primary pacemaker.
Because the heartbeat starts within the heart's own nodal muscle (SAN), not from a nerve. NCERT says the heart is auto-regulated by specialised muscles (nodal tissue), hence myogenic.
About 72 beats per minute at rest, matching the 70-75 action potentials the SAN generates each minute.
Yes. The medulla oblongata, through the autonomic nervous system, can speed up (sympathetic) or slow down (parasympathetic) the SAN, but it does not start the beat.
