Question

Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R
Assertion A: Cardiac excitation normally begins in the sinoatrial (SA) node.
Reason R: SA node cells do not have a stable resting potential. They depolarize to threshold spontaneously.

• A. Both A and R are correct and R is the correct explation of A
• B. Both A and R are correct but R is NOT the correct explation of A
• C. A is correct but R is not correct
• D. A is not correct but R is correct

Hint:

SA Node = Primary Pacemaker of the heart.
Mechanism: Unstable resting potential (Pacemaker Potential driven by $I_f$ "funny" $\text{}^+$ currents) $\to$ Spontaneous intrinsic depolarization.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question explores the cardiac conduction system, focusing on why the sinoatrial (SA) node functions as the primary tural pacemaker of the heart.

Step 2: Key Formula or Approach:
Autorhythmic cardiac cells initiate spontaneous action potentials without exterl neural stimulation due to an unstable resting membrane potential known as the pacemaker potential (prepotential).

Step 3: Detailed Explation:

• Alysis of Assertion A: The sinoatrial (SA) node, situated in the right atrium wall, turally initiates each heartbeat, establishing the cardiac sinus rhythm. Thus, Assertion A is correct.

• Alysis of Reason R: Unlike contractile cardiac myocytes, SA node autorhythmic cells lack a constant, stable resting membrane potential. Inward hyperpolarization-activated cyclic nucleotide-gated channels ($I_f$ "funny" currents) and T-type $\text{Ca}^{2+}$ channels cause gradual, spontaneous depolarization toward threshold potential ($\sim -40\ \text{mV}$). Thus, Reason R is correct.

• Causal Relationship: Because SA node cells depolarize spontaneously faster than any other autorhythmic region (generating 70–80 action potentials/minute), they reach threshold first and drive the entire cardiac conduction system. Therefore, Reason R directly explains why excitation begins in the SA node.

Step 4: Fil Answer:
Both A and R are correct, and R is the correct explation of A.