Correct option is C
Explanation:
(1): Incorrect. M5 receptors are primarily found in the brain, not in the nodal tissue of the heart. These receptors are involved in central nervous system functions and are not primarily responsible for heart rate regulation.
(2): Incorrect. NM receptors (Nicotinic Muscular receptors) are located at neuromuscular junctions and are involved in muscle contraction. They are not involved in the heart's nodal tissue.
(3): Correct. The M2 receptors are muscarinic receptors found in the nodal tissue of the heart (specifically in the SA node and AV node). These receptors play a role in decreasing the heart rate by inhibiting the action of adenylate cyclase and reducing cAMP production.
(4): Incorrect. M4 receptors are also muscarinic receptors, but they are more associated with the central nervous system rather than the nodal tissue of the heart.
Information Booster:
M2 muscarinic receptors are primarily responsible for slowing down the heart rate (negative chronotropy) and are located in the heart's pacemaker cells (SA node and AV node).
The M2 receptor in the heart is activated by acetylcholine, which is released by the parasympathetic nervous system during rest or relaxation.
Muscarinic receptors are G-protein-coupled receptors, and activation of M2 receptors results in a decrease in heart rate by reducing the activity of the pacemaker cells.
M5 receptors are associated with the brain, and NM receptors are involved in skeletal muscle activation, both not related to the heart's nodal tissue.
