Atropine blocks muscarinic receptors. In a patient given IV atropine 0.6 mg, the heart rate increases from 60 to 90 bpm. This tachycardia occurs because atropine removes:

• A Sympathetic beta-1 stimulation of the SA node
• B Alpha-1 adrenergic inhibition of the AV node
• C Parasympathetic vagal tone that normally slows the SA node pacemaker ✓
• D Nicotinic receptor activation at the SA node

Explanation

The resting heart rate is determined by the balance between sympathetic and parasympathetic tone, with vagal (parasympathetic) tone normally dominant at rest. Acetylcholine released from vagal terminals activates M2 muscarinic receptors on SA nodal cells, increasing K+ conductance (IKACh channel), hyperpolarizing the membrane, and slowing spontaneous depolarization. Atropine blocks M2 receptors, removing this inhibitory vagal tone and allowing the intrinsic SA node rate (approximately 90–100 bpm) to be expressed, causing tachycardia.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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