Autonomic Nervous System

ANS Overview - Dual Control Central

• Autonomic Nervous System (ANS): Involuntary control; maintains homeostasis.
• Divisions:
  • Sympathetic: "Fight or flight" (e.g., ↑HR, pupil dilation).
  • Parasympathetic: "Rest and digest" (e.g., ↓HR, pupil constriction).
  • Enteric: GI tract's intrinsic system.
• Dual Innervation: Most organs: dual input (sympathetic/parasympathetic), often antagonistic.
• Central Control Hierarchy:
  • Hypothalamus: Main ANS control.
  • Brainstem (Pons, Medulla): Reflex centers (vital functions).
  • Spinal Cord: Preganglionic neuron origin.

⭐ The enteric nervous system is often called the 'second brain' and can function independently of the CNS.

Sympathetic System - Adrenaline Rush

• Origin: Thoracolumbar outflow (T1-L2/L3 spinal segments).
• Ganglia:
  • Paravertebral (sympathetic chain)
  • Prevertebral (e.g., celiac, superior mesenteric)
• Neurotransmitters:
  • Preganglionic: Acetylcholine (ACh) → Nicotinic receptors (Nn).
  • Postganglionic: Norepinephrine (NE) → Adrenergic receptors (α, β).
    • Exception: Sweat glands (ACh → Muscarinic).
  • Adrenal Medulla: Modified ganglion; releases Epinephrine (Epi, 80%) & NE (20%) into blood.
• Key Actions (Fight or Flight): 📌
  • Cardiovascular: ↑ Heart rate (β1), ↑ contractility (β1), vasoconstriction (α1) → ↑ BP.
  • Respiratory: Bronchodilation (β2).
  • Ocular: Mydriasis (pupil dilation, α1).
  • Metabolic: ↑ Glycogenolysis, ↑ gluconeogenesis (β2, α1), ↑ lipolysis (β3).
  • GI/GU: ↓ Motility & secretions, sphincter contraction, ejaculation (α1).

⭐ Exam Favourite: Sweat glands are anatomically sympathetic but functionally cholinergic, releasing Acetylcholine (ACh) which acts on Muscarinic receptors (M).

Parasympathetic System - Chill Vibes Only

• Origin: Craniosacral (CN III, VII, IX, X; S2-S4).
• Ganglia: Long preganglionic, short postganglionic fibers. Ganglia near/in effector organs (terminal/intramural).
• Neurotransmitters (NTs) & Receptors:
  • Preganglionic NT: Acetylcholine (ACh) → Nicotinic (Nn) receptors on postganglionic neuron.
  • Postganglionic NT: Acetylcholine (ACh) → Muscarinic (M1-M5) receptors on effector organs.
• Function: "Rest & Digest"; conserves energy, promotes anabolic activities.
• Key Actions: 📌 (Think "SLUDD" + Heart/Lungs/Eyes)
  • Salivation, Lacrimation, Urination, Digestion (↑motility/secretions, sphincter relaxation), Defecation.
  • Heart (M2): ↓ Heart rate (bradycardia), ↓ atrial contractility, ↓ AV node conduction.
  • Lungs (M3): Bronchoconstriction, ↑ bronchial secretions.
  • Eye (M3): Miosis (pupil constriction via sphincter pupillae), accommodation (via ciliary muscle).
  • Genitals: Erection (vascular engorgement).
• ACh Breakdown: Rapidly by Acetylcholinesterase (AChE) in synaptic cleft.

⭐ The vagus nerve (CN X) accounts for approximately 75% of all parasympathetic outflow.

ANS Pharmacology - Chemical Messengers

• Primary Neurotransmitters:
  • Acetylcholine (ACh): All preganglionic neurons (SNS & PNS); PNS postganglionic neurons; SNS postganglionic neurons innervating sweat glands.
  • Norepinephrine (NE): Majority of SNS postganglionic neurons.
  • Epinephrine (Epi): Released from adrenal medulla; acts on adrenergic receptors.
• Receptor Subtypes & Locations:
  • Cholinergic (ACh):
    • Nicotinic (N): N_N (autonomic ganglia, adrenal medulla); N_M (neuromuscular junction). Ligand-gated ion channels.
    • Muscarinic (M): M₁-M₅ (PNS effector organs; CNS). G-protein coupled receptors (GPCRs).
  • Adrenergic (NE, Epi): GPCRs.
    • Alpha (α): α₁ (smooth muscle contraction - vasoconstriction); α₂ (↓ NE release, ↓ insulin release).
    • Beta (β): β₁ (↑ heart rate & contractility); β₂ (smooth muscle relaxation - bronchodilation, vasodilation); β₃ (lipolysis).

⭐ Beta-1 receptors are primarily found in the heart ('1 heart'), while Beta-2 receptors are in the lungs ('2 lungs'). (Mnemonic: B1 - 1 heart, B2 - 2 lungs)

• Pharmacological Modulation: Drugs can act by:
  • Mimicking or blocking neurotransmitter actions at receptors (agonists/antagonists).
  • Altering neurotransmitter synthesis, storage, release, or degradation/reuptake.

High‑Yield Points - ⚡ Biggest Takeaways

• Sympathetic (Thoracolumbar T1-L2): Fight-or-flight; NT Norepinephrine (NE), except ACh at sweat glands.
• Parasympathetic (Craniosacral CN III,VII,IX,X; S2-S4): Rest-and-digest; NT Acetylcholine (ACh).
• Adrenal Medulla: Modified sympathetic ganglion; secretes Epinephrine & Norepinephrine.
• Ganglionic Transmission: ACh acts on Nicotinic (Nn) receptors in both systems.
• Key Adrenergic Receptors: α1 (vasoconstriction), β1 (↑heart rate/contractility), β2 (bronchodilation/vasodilation).
• Key Muscarinic Receptors: M2 (heart: ↓rate), M3 (smooth muscle contraction, gland secretion).