Autonomic Nervous System

ANS Overview - Body's Autopilot

• ANS: Regulates involuntary bodily functions, crucial for homeostasis.
• Divisions: Sympathetic (fight-or-flight), Parasympathetic (rest-and-digest), Enteric (gut brain).

⭐ Dual innervation is key, but some structures (e.g., adrenal medulla, sweat glands, most blood vessels) receive only sympathetic innervation.

Sympathetic System - Fight-Flight Frenzy

• Origin: Thoracolumbar outflow (segments T1-L2/L3 of spinal cord). 📌 T1-L2/L3.
• Ganglia:
  • Paravertebral: Form sympathetic chain flanking vertebral column.
  • Prevertebral (collateral): Near major abdominal arteries (e.g., celiac, mesenteric).
• Neurotransmitters:
  • Preganglionic fibers: Release Acetylcholine ($ACh$).
  • Postganglionic fibers: Release Norepinephrine ($NE$), except those innervating sweat glands ($ACh$).
• Receptors: Adrenergic receptors on target organs (α and β subtypes).
• Functions: Mobilizes body for "Fight or Flight" response. 📌 Fight or Flight.

⭐ The adrenal medulla is a modified sympathetic ganglion; its chromaffin cells release epinephrine (80%) and norepinephrine (20%) directly into the bloodstream.

Parasympathetic System - Rest-Digest Repose

• Function: "Rest and Digest"; conserves energy.
• Origin: Craniosacral outflow.
  • Cranial: CN III, VII, IX, X. 📌 Mnemonic: "3, 7, 9, 10 make you feel fine".
  • Sacral: S2-S4. 📌 Mnemonic: "S2,3,4 keep P's (Pee, Poo, Point) off the floor".
• Ganglia: Terminal/intramural (near/in effector organs); long preganglionic, short postganglionic fibers.
• Neurotransmitters:
  • Preganglionic: Acetylcholine (ACh).
  • Postganglionic: Acetylcholine (ACh).
• Receptors: Muscarinic (M1-M5).

⭐ The Vagus nerve (CN X) provides ~75% of parasympathetic outflow, innervating thoracic and most abdominal viscera.

ANS Neurotransmitters/Receptors - Chemical Commandoes

• Messengers:
  • ACh: Preganglionic; Parasymp. postgang.; Symp. sweat (muscarinic). Degraded by AChE.
  • NE: Most Symp. postgang. Reuptake; MAO/COMT degradation.
• Receptor Actions:

  Type	Subtype	Location Highlights	Key Action(s)
  Cholinergic	Nn	Autonomic ganglia	Ganglionic transmission
  	Nm	NMJ	Muscle contraction
  	M1	CNS, Glands	↑Memory, ↑Secretions
  	M2	Heart	↓HR
  	M3	Smooth muscle, Glands	Contraction, ↑Secretions
  Adrenergic	α1	Vasc. smooth muscle	Vasoconstriction
  	α2	Presynaptic terminals	↓NE release
  	β1	Heart (1♥)	↑HR, ↑Contractility
  	β2	Lungs (2L), Smooth muscle	Bronchodilation, Relaxation
  	β3	Adipose tissue	Lipolysis

• 📌 Adrenergic: α1 (Excite), α2 (Inhibit presynaptic). Muscarinic: M1/M3 (Excite/Secrete), M2 (Heart↓).

⭐ All preganglionic ANS neurons (both sympathetic and parasympathetic) are cholinergic, releasing ACh that acts on Nicotinic (Nn) receptors on postganglionic neurons. Postganglionic sympathetic neurons to sweat glands are also cholinergic (muscarinic).

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ANS Clinicals - Autopilot Glitches

• Horner's Syndrome: Sympathetic trunk damage. 📌 PAM Horney: Ptosis, Anhidrosis, Miosis.

  Feature	Description
  Ptosis	Drooping eyelid
  Anhidrosis	↓ Sweating
  Miosis	Constricted pupil

  ⭐ Classic triad: unilateral miosis, ptosis, anhidrosis from sympathetic trunk damage.

• Autonomic Dysreflexia: Spinal cord injury (typically ≥T6). Symptoms: ↑BP, headache, sweating.

• Shy-Drager Syndrome: Multiple system atrophy with autonomic failure.

• Key Drug Classes:

  • Sympathomimetics: Mimic sympathetic action (e.g., adrenaline).
  • Sympatholytics: Block sympathetic action (e.g., beta-blockers).
  • Parasympathomimetics: Mimic parasympathetic action (e.g., pilocarpine).
  • Parasympatholytics: Block parasympathetic action (e.g., atropine).

High‑Yield Points - ⚡ Biggest Takeaways

• Sympathetic system: Thoracolumbar outflow (T1-L2/L3); primarily norepinephrine (NE).
• Parasympathetic system: Craniosacral outflow (CN III, VII, IX, X; S2-S4); acetylcholine (ACh).
• Adrenal medulla is a modified sympathetic ganglion releasing epinephrine & NE.
• Horner's syndrome (ptosis, miosis, anhydrosis) indicates sympathetic chain lesion.
• Key neurotransmitters: ACh (cholinergic) and NE (adrenergic).
• Enteric nervous system acts as the "second brain" for GI motility and secretion control independently of CNS input but modulated by ANS input