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The stimulation of sympathetic cardiac nerves increases the rate of action potential generation from the sinoatrial (SA) node of the heart. The following statements suggest the mechanism of this action:

A. The depolarizing effect of ‘h’ current (Ih) is decreased by sympathetic stimulation.
B. Norepinephrine secreted by the sympathetic endings binds to β₁ adrenoceptors, resulting in an increase in intracellular cAMP.
C. The increased intracellular cAMP facilitates the opening of long-lasting (L) Ca²⁺ channels.
D. The Ca²⁺ current (Ica) due to the opening of voltage-gated L-type Ca²⁺ channels is decreased.

Correct option is B

The sympathetic nervous system increases heart rate (positive chronotropic effect) by stimulating the sinoatrial (SA) node through norepinephrine release, leading to enhanced action potential generation. Let’s analyze each statement:

Statement A: "The depolarizing effect of ‘h’ current (Ih) is decreased by sympathetic stimulation."  (Incorrect)

• The "h" current (Ih), also known as the funny current, is a key pacemaker current in SA node cells.
• Sympathetic stimulation actually increases the funny current (Ih), leading to faster depolarization and increased firing of SA node action potentials.
• This statement is incorrect because Ih is increased, not decreased.

Statement B: "Norepinephrine secreted by the sympathetic endings binds to β₁ adrenoceptors, resulting in the increase of intracellular cAMP." (Correct)

• Norepinephrine (NE) binds to β₁-adrenergic receptors in pacemaker cells.
• This activates the G-protein (Gs) pathway, leading to an increase in cyclic AMP (cAMP).
• Increased cAMP enhances pacemaker activity, making the heart beat faster.
  

Statement C: "The increased intracellular cAMP facilitates the opening of long-lasting (L) Ca²⁺ channels."  (Correct)

• cAMP enhances calcium influx by activating protein kinase A (PKA), which phosphorylates L-type Ca²⁺ channels.
• This allows more Ca²⁺ to enter, increasing the strength of contraction and depolarization.
  

Statement D: "The Ca²⁺ current (Ica) due to the opening of voltage-gated L-type Ca²⁺ channels is decreased." (Incorrect)

• Sympathetic stimulation increases, not decreases, the Ca²⁺ current (Ica).
• More Ca²⁺ entry enhances depolarization and contraction, helping in faster heart rate.
• This statement is incorrect because Ica is increased, not decreased.

Information Booster

Here are some key concepts related to sympathetic regulation of the SA node:

1. Pacemaker Activity in the SA Node:

   • The SA node generates spontaneous action potentials that set the heart rate.
   • Funny current (Ih) and Ca²⁺ currents contribute to pacemaker activity.

2. Sympathetic Stimulation and Heart Rate:

   • Norepinephrine binds to β₁-adrenergic receptors.
   • This activates adenylyl cyclase, increasing cAMP.
   • cAMP enhances Ih (funny current) and Ca²⁺ currents, leading to faster depolarization.

3. Role of cAMP and PKA:

   • cAMP directly enhances Ih, making the pacemaker potential reach threshold faster.
   • Protein kinase A (PKA) phosphorylates L-type Ca²⁺ channels, increasing Ca²⁺ influx.
   • Overall effect: Increased heart rate (positive chronotropic effect).

4. Effect on Ion Currents:

   • Increased Ih (funny current) → Faster pacemaker potential.
   • Increased Ica (Ca²⁺ current) → Stronger depolarization and contraction.

5. Comparison with Parasympathetic Effects:

   • Parasympathetic (vagus nerve) stimulation decreases cAMP, reducing Ih and Ica, leading to slower heart rate.