Correct option is D
Correct Answer: (4) D and E
Concept:
The vagus nerve (parasympathetic) releases acetylcholine (ACh), which slows the heart rate by acting on the SA node. It affects pacemaker cells by:
Increasing K⁺ permeability (via G-protein gated K⁺ channels)
Decreasing cAMP levels, which affects funny current (Ih) and Ca²⁺ channel activity
Reducing the slope of pacemaker potential, hence slowing depolarization and decreasing heart rate
Statement-wise analysis:
A. "The nodal cell membrane becomes depolarized."
Incorrect
Cholinergic vagal stimulation hyperpolarizes (not depolarizes) the SA node membrane due to increased K⁺ conductance.
B. "The slope of the pacemaker potential is increased."
Incorrect
Parasympathetic stimulation decreases the slope of the pacemaker potential → slows down rate of impulse generation.
C. "The K⁺ conductance of nodal cell membrane is decreased."
Incorrect
Actually, K⁺ conductance increases due to ACh-activated G-protein gated K⁺ channels (IKACh).
D. "The depolarizing effect of ‘h’ current (Ih) on the membrane potential is slowed down due to the opening of G protein gated K⁺ channels."
Correct
Increased K⁺ outflow via G-protein gated channels opposes the depolarizing effect of the Ih current, thereby slowing pacemaker depolarization.
E. "The opening of Ca²⁺ channels are slowed down due to the decreased cAMP level in the nodal cells."
Correct
Vagal stimulation → ↓ cAMP → ↓ activation of L-type Ca²⁺ channels → Slower depolarization.
