Sympathetic vs Parasympathetic Nervous System
Sympathetic Nervous System vs Parasympathetic Nervous System
The autonomic nervous system has two divisions with opposing effects: the sympathetic "fight or flight" system and the parasympathetic "rest and digest" system. Understanding their anatomical differences (thoracolumbar vs craniosacral outflow) and physiological effects is essential for pharmacology and clinical medicine.
Comparison Table
Key Differences
- →Outflow: Thoracolumbar (sympathetic) vs Craniosacral (parasympathetic)
- →Fiber lengths: Short pre/long post (sympathetic) vs Long pre/short post (parasympathetic)
- →Postganglionic neurotransmitter: Norepinephrine (sympathetic) vs Acetylcholine (parasympathetic)
- →General effect: Mobilizes energy vs Conserves energy
Clinical Relevance
- •Beta-blockers (β1): Decrease heart rate and blood pressure (block sympathetic)
- •Atropine (muscarinic antagonist): Increases heart rate, dilates pupils (blocks parasympathetic)
- •Horner's syndrome: Sympathetic lesion causing miosis, ptosis, anhidrosis
- •Anticholinergics side effects: "Dry as a bone, red as a beet, blind as a bat, mad as a hatter"
Study Tips
- ✓Sympathetic: "Fight or Flight" - dilated pupils, fast heart, bronchodilation, blood to muscles
- ✓Parasympathetic: "Rest and Digest" - constricted pupils, slow heart, increased GI activity
- ✓"Craniosacral" for parasympathetic: Cranial nerves + Sacral spinal nerves
- ✓Remember SLUDGE for parasympathetic excess: Salivation, Lacrimation, Urination, Defecation, GI upset, Emesis
FAQs
Common questions about this comparison
Sympathetic preganglionic neurons exit the spinal cord and synapse quickly in the paravertebral (sympathetic chain) or prevertebral ganglia, which are close to the spinal cord. This creates a short preganglionic fiber. The postganglionic fiber then travels a longer distance to reach the target organ.
Sweat glands are embryologically derived from ectoderm and retain acetylcholine receptors. While they are innervated by sympathetic postganglionic fibers (anatomically sympathetic), these particular fibers release acetylcholine instead of norepinephrine - they are "sympathetic cholinergic" fibers. This is an exception to the rule.