Autonomic Nervous System (ANS) – Study Notes

1. Nervous System Overview

• CNS: Brain + Spinal cord

• PNS: Nerves outside CNS

  • Sensory (afferent) – to CNS

  • Motor (efferent) – from CNS

    • Somatic motor: voluntary, skeletal muscles

    • Visceral motor (ANS): involuntary, smooth muscle, glands, heart, GI

2. ANS Divisions

1. Sympathetic → "Fight or flight"

2. Parasympathetic → "Rest and digest"

3. Enteric → GI control (semi-independent)

3. Somatic vs Autonomic Pathways

• Somatic

  • 1 neuron (alpha motor neuron)

  • Origin: anterior horn

  • NT: ACh → nicotinic receptors on skeletal muscle

• Autonomic

  • 2-neuron chain:

    • Preganglionic (CNS → ganglion)

    • Postganglionic (ganglion → effector)

  • NT varies by system

4. Sympathetic Nervous System (SNS)

• Function: energy use, stress, fight/flight

• Outflow: Thoracolumbar (T1–L2)

• Preganglionic: short

• Postganglionic: long

Ganglia Types

1. Paravertebral (chain ganglia)

   • Along vertebral column

   • Pathways: synapse at same/other level or pass through → splanchnic nerves

   • Targets: pilomotor (hair), vasomotor (vessels), sudomotor (sweat → exception: ACh not NE)

2. Prevertebral (collateral ganglia)

   • Anterior to aorta (celiac, superior/inferior mesenteric)

   • Fibers via splanchnic nerves → abdominal/pelvic viscera

3. Adrenal medulla

   • Direct preganglionic synapse

   • Releases E + NE into blood

Neurotransmitters

• Preganglionic: ACh

• Postganglionic: NE (except sweat glands → ACh)

5. Parasympathetic Nervous System (PNS)

• Function: conserve energy, rest & digest

• Outflow: Craniosacral (CN III, VII, IX, X + S2–S4)

• Preganglionic: long

• Postganglionic: short (near/within organ)

Ganglia

• Terminal (intramural) ganglia – close to/within effector

Neurotransmitters

• Preganglionic: ACh

• Postganglionic: ACh

6. Sympathetic vs Parasympathetic – Comparison

Feature	Sympathetic (SNS)	Parasympathetic (PNS)
Outflow	T1–L2 (thoracolumbar)	CN III, VII, IX, X + S2–S4
Preganglionic	Short	Long
Postganglionic	Long	Short
NT at effector	NE (adrenergic)	ACh (cholinergic)
Function	Fight/Flight	Rest/Digest

7. Key Terms

• Preganglionic neuron – CNS → ganglion

• Postganglionic neuron – ganglion → target organ

• Cholinergic – ACh release

• Adrenergic – NE/E release

• Ganglion – PNS neuron cell body cluster.

• Splanchnic nerves – carry autonomic fibers to viscera.