⚖️ Nervous System Lesson

Fight or Flight vs Rest and Digest: the autonomic nervous system

Two divisions of the autonomic nervous system produce essentially opposite effects on nearly every organ in the body — and even use different neurotransmitters to do it.

Symp.

Fight/flight

Parasymp.

Rest/digest

📖 Full Breakdown

Opposite effects, opposite neurotransmitters, one helpful acronym

SLUDD captures the parasympathetic system's effects on secretions and digestion in one memorable acronym.

Sympathetic — Fight or Flight

Prepares the body for action

Increases heart rate, dilates pupils, inhibits digestion, increases blood pressure, and dilates bronchioles. Uses norepinephrine (adrenergic) as its neurotransmitter.

Parasympathetic — Rest and Digest

Promotes recovery and maintenance functions

Decreases heart rate, constricts pupils, stimulates digestion, decreases blood pressure, and constricts bronchioles. Uses acetylcholine (cholinergic) as its neurotransmitter.

SLUDD

A parasympathetic memory aid

Salivation, Lacrimation, Urination, Digestion, Defecation — all specifically STIMULATED by the parasympathetic system, capturing its broad "housekeeping" role during rest.

Antagonistic organ effects

Nearly every organ receives both inputs

Most organs receive both sympathetic and parasympathetic innervation, with each division producing essentially opposite effects — this dual, opposing control allows fine-tuned regulation depending on the body's current needs.

🩺 Clinical / Exam Application

A patient given a medication that blocks acetylcholine receptors develops dry mouth, blurred vision, rapid heart rate, and constipation, all at once. Because acetylcholine is specifically the parasympathetic neurotransmitter, and the parasympathetic system normally stimulates salivation, near vision focusing, slower heart rate, and digestion (as captured by SLUDD), BLOCKING this system produces the exact opposite of all these effects simultaneously. This single mechanism — anticholinergic blockade — explains an entire cluster of seemingly unrelated side effects from one shared cause.

⚠️ Exam Alert

The neurotransmitter pairing — sympathetic uses norepinephrine, parasympathetic uses acetylcholine — is one of the most frequently tested facts in pharmacology and physiology, since it directly predicts how many drugs (like beta-blockers or anticholinergics) will affect the body.

🚧 Common Trap

Don't assume sympathetic activation is always "bad" or parasympathetic is always "good." Both are essential, complementary systems — sympathetic activation is life-saving during genuine emergencies, while excessive, chronic sympathetic activation (as in chronic stress) can be harmful. Neither system is inherently better than the other.

✅ Quick Check

A medication increases heart rate, dilates pupils, and inhibits digestion. Which autonomic division is this medication likely mimicking, and which neurotransmitter is involved?

📝 Exam Prep

Common Exam Questions

❓ What is the difference between the sympathetic and parasympathetic nervous systems?

✅ The sympathetic nervous system triggers "fight or flight" responses — increased heart rate, pupil dilation, and inhibited digestion — using norepinephrine. The parasympathetic nervous system triggers "rest and digest" responses — decreased heart rate, pupil constriction, and stimulated digestion — using acetylcholine.

❓ What does the SLUDD mnemonic represent?

✅ SLUDD represents effects stimulated by the parasympathetic nervous system: Salivation, Lacrimation, Urination, Digestion, and Defecation.