Which feature best distinguishes sympathetic nervous system activation from parasympathetic nervous system activation in a patient presenting with acute stress?

Explanation

## Distinguishing Sympathetic from Parasympathetic Activation ### Key Physiological Differences **Key Point:** Sympathetic and parasympathetic divisions produce opposing effects on most organs. The discriminating feature lies in the constellation of cardiovascular, pupillary, and GI responses. ### Comparative Features | Feature | Sympathetic | Parasympathetic | |---------|-------------|------------------| | **Heart Rate** | Increased (β₁ receptors) | Decreased (M2 receptors) | | **Pupils** | Dilated (mydriasis) via α₁ | Constricted (miosis) via M3 | | **GI Motility** | Decreased (α & β receptors) | Increased (M3 receptors) | | **Salivation** | Scant, viscous | Copious, watery | | **Bronchi** | Dilated (β₂) | Constricted (M3) | | **Sweat** | Increased (cholinergic) | Minimal | | **Blood Pressure** | Increased | Decreased | ### Why Option B is Correct **High-Yield:** The combination of **increased heart rate + mydriasis (dilated pupils) + decreased GI motility** is pathognomonic for sympathetic activation. This triad is the most reliable discriminator in acute stress states. **Clinical Pearl:** In acute stress or "fight-or-flight" response, the sympathetic nervous system prepares the body for action: the heart pumps faster, pupils dilate to improve vision, and digestion is suppressed to redirect blood to skeletal muscles. ### Mechanism - **Tachycardia:** Noradrenaline binds β₁-adrenergic receptors on sinoatrial node → increased cAMP → faster depolarization - **Mydriasis:** Noradrenaline binds α₁-adrenergic receptors on radial muscle of iris → contraction → pupil dilation - **Decreased GI motility:** α₁ and β₂ receptors on smooth muscle → relaxation; α₂ receptors on parasympathetic neurons → inhibition **Mnemonic:** **SLUDGE** = Salivation, Lacrimation, Urination, Defecation, GI upset, Emesis — these are **parasympathetic** signs. The opposite pattern (dry mouth, no tears, constipation, dilated pupils, tachycardia) = **sympathetic**.