Answer: Funny current (If, HCN channels) carrying Na+/K+ inward current during diastole, plus T-type Ca 2+ channels near threshold.
- A A constitutive sodium leak through voltage-insensitive channels that never inactivate rather than the time-dependent gating actually shown by HCN-type funny current channels
- B Funny current (If, HCN channels) carrying Na+/K+ inward current during diastole, plus T-type Ca<sup>2+</sup> channels near threshold
- C Continuous potassium influx through inward-rectifying channels during diastole rather than the net inward cation current that actually drives diastolic depolarization
- D The complete absence of any stable resting membrane potential in nodal tissue despite nodal cells reliably oscillating between a defined maximum and threshold potential
Correct answer: B. Funny current (If, HCN channels) carrying Na+/K+ inward current during diastole, plus T-type Ca<sup>2+</sup> channels near threshold
Explanation: SA node pacemaker potential: If (funny current through HCN channels) causes slow diastolic depolarization. T-type Ca<sup>2+</sup> channels then activate near threshold, followed by L-type Ca<sup>2+</sup> channels for action potential upstroke.
A single ECG cycle: the P wave marks atrial depolarisation (contraction signal), the large QRS complex marks ventricular depolarisation, and the T wave marks ventricular repolarisation (recovery) before the next cycle begins.
Concept context
Blood, lymph, the heart, the cardiac cycle, blood pressure, and ECG. High NEET weightage.