## Sympathetic Nervous System Effects — Receptor Mechanisms ### Correct Sympathetic Effects **Key Point:** The sympathetic nervous system uses catecholamines (noradrenaline, adrenaline) acting on α and β adrenergic receptors to produce widespread physiological responses. | Organ System | Receptor | Effect | Mechanism | |---|---|---|---| | Heart | β₁ | ↑ HR, ↑ contractility | Increased cAMP, Ca²⁺ influx | | Liver | β₂ | Glycogenolysis | Activates phosphorylase kinase | | Skeletal muscle | β₂ | Glycogenolysis | Mobilizes glucose for fight-or-flight | | Bronchi | β₂ | Bronchodilation | Smooth muscle relaxation | | Eye (pupil) | α₁ | **Mydriasis** (dilation) | Iris dilator muscle contraction | | Blood vessels | α₁ | Vasoconstriction | Increased peripheral resistance | ### Why Option 2 (Pupillary Constriction) Is Wrong **High-Yield:** Sympathetic activation causes **pupillary dilation (mydriasis)**, NOT constriction. This occurs via α₁-adrenergic activation of the **iris dilator muscle** (radial muscle), which is innervated by sympathetic fibers. **Clinical Pearl:** Pupillary constriction (miosis) is a **parasympathetic** effect — mediated by muscarinic M₃ receptors on the iris sphincter muscle via the oculomotor nerve (CN III). **Mnemonic:** **SLUDGE** = Parasympathetic excess (Salivation, Lacrimation, Urination, Defecation, GI upset, Emesis) — includes **miosis**. Sympathetic does the opposite: **mydriasis**. ### Sympathetic vs. Parasympathetic Pupil Effects ```mermaid flowchart TD A[Pupil control] --> B[Sympathetic pathway]:::action A --> C[Parasympathetic pathway]:::action B --> D[α₁ receptors on iris dilator]:::outcome D --> E[Mydriasis - pupil dilates]:::outcome C --> F[M₃ receptors on iris sphincter]:::outcome F --> G[Miosis - pupil constricts]:::outcome ``` [cite:Guyton & Hall Physiology 14e Ch 51]
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