Sino-Atrial Node
சைனோ-ஏட்ரியல் கணு (SAN)
The heart does not wait for the brain to tell it to beat. The Sino-Atrial Node (SAN) is myogenic, meaning it generates its own electrical impulses independently. If you think the heart is controlled by the CNS for its basic rhythm, you have already lost a mark.
The idea
The SAN is a specialized patch of nodal tissue located in the right upper corner of the right atrium. It acts as the natural pacemaker of the heart. It generates the maximum number of action potentials, ranging from 70 to 75 per minute. This rhythm initiates the contractile activity of the entire heart.
Why it matters
To act as the primary rhythm generator that sets the pace for the entire heart's contraction cycle.
NEET lens
NEET examiners love to test the 'myogenic' nature of the heart. Remember: SAN = 70-75 bpm, AVN = 40-60 bpm. Do not confuse the location; it is the right upper corner of the right atrium, not the left. Expect questions comparing the impulse generation rates of different nodal tissues.
Clinical significance
When the SAN fails, the heart's rhythm becomes erratic or dangerously slow, a condition known as sick sinus syndrome. In such cases, the heart cannot maintain cardiac output, leading to syncope or fainting. Surgeons must then implant an artificial electronic pacemaker to restore the rhythm.
Introduction
The Sino-Atrial (SA) node is a small mass of specialized cardiac muscle cells in the right atrium wall near the opening of the superior vena cava — the primary pacemaker of the heart, generating 70–75 spontaneous impulses per minute.
Core concept
Location: right atrium, near superior vena cava opening. Size: ~15 mm × 3 mm. Impulse generation: spontaneous depolarization due to funny current (If) — Na⁺ slowly enters → threshold reached → action potential → spreads across both atria → atrial systole. Fastest spontaneous rate in heart → sets the pace for all other nodal tissue. ANS modulation: Sympathetic (norepinephrine) → increases rate (positive chronotropy). Parasympathetic/vagus (acetylcholine) → decreases rate (negative chronotropy).
Go deeper
SA node action potential is different from regular cardiomyocytes: slow upstroke (Ca²⁺ mediated, not Na⁺), no true resting potential (slowly drifts toward threshold), no plateau phase. This slow Ca²⁺-dependent depolarization is why Ca²⁺ channel blockers (verapamil) slow heart rate.
From Class 10
The heart's natural pacemaker is a tiny cluster of cells in the right atrium. It sets the rhythm for the entire heart.