On the basis of mechanism of generation of action potential in pace maker tissue (SA/AV node), the following statement were proposed from the above figure
A. ‘funny’ channels are activated at ‘b’
B. Outward flow of K⁺ occurs at ‘a’
C. T-Ca⁺⁺ channels are closed at ‘c’
D. Fast Na⁺ channels are opened at ‘d’
E. The upward phast at ‘d’ is largely due to inward movement of Na⁺
Which one of the following combinations represent correct statements?
(1) A and B (2) B and C
(3) C and D (4) D and E

The cardiac pacemaker cells in the sinoatrial (SA) and atrioventricular (AV) nodes generate spontaneous action potentials that regulate heart rhythm. Unlike typical cardiac muscle cells, these pacemaker cells exhibit unique ionic currents responsible for their automaticity. Understanding the sequence and nature of these currents at various phases of the pacemaker action potential is key to grasping heart rate control.

Key Ionic Events in the Pacemaker Action Potential

The pacemaker action potential features distinct phases involving specialized ion channels:

• Phase ‘a’ (Repolarization): Outward flow of potassium ions (K⁺) through potassium channels causes the membrane potential to hyperpolarize.

• Phase ‘b’ (Pacemaker Potential Initiation): The hyperpolarization triggers activation of ‘funny’ (If) channels — hyperpolarization-activated cyclic nucleotide-gated (HCN) channels.

• Phase ‘c’ (Late Pacemaker Potential): Transient calcium (T-type Ca²⁺) channels open to allow Ca²⁺ influx, contributing to membrane depolarization.

• Phase ‘d’ (Upstroke Depolarization): In pacemaker cells, this phase is primarily due to inward calcium through long-lasting L-type Ca²⁺ channels, as fast sodium channels are largely absent or inactive.

• Phase ‘e’ (Repolarization): Potassium channels open again allowing outward K⁺ to restore resting membrane potential.

Evaluating the Given Statements

• A. ‘funny’ channels are activated at ‘b’
  Correct. The funny current (I_f) carried by HCN channels is activated by hyperpolarization during phase ‘b’ and initiates the slow depolarization (pacemaker potential).jennysjamjar+1youtube

• B. Outward flow of K+ occurs at ‘a’
  Correct. The repolarization phase ‘a’ corresponds to potassium efflux repolarizing the membrane.youtubejennysjamjar

• C. T-Ca++ channels are closed at ‘c’
  Incorrect. At phase ‘c’, transient T-type calcium channels are actively open, allowing calcium influx that advances depolarization.litflyoutube

• D. Fast Na+ channels are opened at ‘d’
  Incorrect for pacemaker cells. Fast sodium channels are either absent or inactive in the SA/AV node pacemaker cells; upstroke at phase ‘d’ is mainly due to L-type calcium channels.jennysjamjar+1

• E. The upward phase at ‘d’ is largely due to inward movement of Na+
  Incorrect. The ‘upstroke’ depolarization in pacemaker cells is caused by calcium, not sodium, influx.litfl+1

Correct Combination of Statements

The only fully correct statements based on pacemaker action potential physiology are:

• A and B

Thus, the correct option is:

(1) A and B.

Summary Table

Additional Insights

Pacemaker cells differ significantly from ventricular myocytes in ionic currents:

• They lack a stable resting potential; the membrane potential slowly depolarizes after each action potential because of the funny current.

• Upstroke (phase 0) depends on calcium influx rather than sodium.

• Potassium channels control repolarization and help restart the cycle.

Conclusion

For the pacemaker action potential in SA/AV nodal tissue:

• Funny channels activate at phase ‘b’ initiating depolarization,

• Outward potassium flow repolarizes the membrane at phase ‘a’,

• T-type calcium channels are open (not closed) at phase ‘c’,

• Fast sodium channels do not contribute at phase ‘d’, which is dominated by calcium influx.

Therefore, the correct choice describing ion channel involvement is (1) A and B.