Topic overview

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The ANS adjusts visceral activity without conscious control. Its sympathetic division has a thoracolumbar origin and is associated with “fight or flight,” while the parasympathetic division has a craniosacral origin and is associated with “rest and digest.” The enteric division provides GI-specific control. Sympathetic and parasympathetic pathways differ in the relative lengths of their preganglionic and postganglionic fibers, and many organs receive input from both divisions. Receptor actions help explain these effects: adrenergic α₁ receptors support vasoconstriction, β₁ receptors affect heart rate and contractility, and β₂ receptors promote bronchodilation. Cholinergic responses depend on receptor type; nicotinic receptors are excitatory, while muscarinic effects vary by subtype and location. The lesson also relates selected receptor actions to clinical pharmacology.