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Overview of the Sympathetic Nervous System

Apr 19, 2025

Sympathetic Nervous System

Instructor Information

Learning Outcomes

  • Describe sympathetic supply to the head & neck.
  • Identify Horner syndrome.
  • Demonstrate sympathetic supply to the thorax.
  • Explain sympathetic supply to the abdomen.
  • Describe sympathetic supply to the pelvis.
  • Clarify Orbelli phenomenon.

Autonomic Nervous System

  • Definition: Part of the peripheral nervous system regulating involuntary or visceral functions.
  • Divisions:
    • Sympathetic nervous system (thoraco-lumbar division).
    • Parasympathetic nervous system (cranio-sacral division).

Sympathetic Nervous System

Origin & Relay

  • Origin: Lateral horn cells (LHCs) of the first and second thoracic segments.
  • Relay: Superior cervical ganglion (SCG).

Functions

  • Eye:
    • Contraction of dilator pupillae muscles: Pupil dilation (mydriasis).
    • Contraction of smooth muscles in eyelids: Widening of palpebral fissure.
    • Contraction of Müller's muscles (exophthalmos).
    • Vasoconstriction of blood vessels in lacrimal glands and trophic secretion.
  • Salivary Glands:
    • Vasoconstriction of blood vessels.
    • Trophic secretions: Little, viscous, enzyme-rich.
  • Skin:
    • Vasoconstriction of blood vessels.
    • Hair erection via piloerector muscle contraction.
    • Sweat secretion.
  • Cerebral Blood Vessels:
    • Mild vasoconstriction.

Horner Syndrome

  • Definition: Results from interruption of sympathetic supply to the head and neck.
  • Causes: Lesions in T1, T2 segments, or SCG.
  • Site: Same side as the lesion.
  • Signs:
    • Ptosis: Drooping of upper eyelids.
    • Miosis: Pupil constriction.
    • Anhydrosis: Absence of sweat secretion.
    • Enophthalmos: Eye sinking into orbit.
    • Vasodilatation of skin blood vessels.

Sympathetic Supply to the Thorax

  • Origin: LHCs of upper 4 or 5 thoracic segments.
  • Relay: 3 cervical ganglia, upper 4 thoracic ganglia.
  • Functions:
    • Heart:
      • Increases heart rate, force of contraction, and conductivity.
    • Coronary Vessels:
      • Direct vasoconstriction, indirect vasodilatation.
    • Lungs:
      • Bronchodilatation, inhibition of mucus secretion, vasoconstriction of pulmonary vessels.

Sympathetic Supply to the Abdomen

  • Origin: LHCs of T6-12 segments (splanchnic nerves).
  • Relay: Collateral (prevertebral) ganglia (celiac, superior mesenteric, inferior mesenteric).
  • Functions:
    • GIT: Relaxation of walls, contraction of sphincters, inhibition of digestion.
    • Liver: Stimulation of glycogenolysis.
    • Gallbladder: Relaxation of wall, contraction of sphincter of Oddi.
    • Spleen: Contraction of smooth muscles, blood release.
    • Pancreas: Inhibition of endocrine and exocrine secretion.
    • Kidneys: Stimulates renin secretion, decreases blood flow and urine output.
    • GIT Vessels: Vasoconstriction.

Supra Renal Medulla (SRM)

  • Origin: LHCs of T10 and 11 segments.
  • Relay: Direct with SRM cells (chromaffin cells).
  • Functions:
    • Releases adrenaline (80%) and noradrenaline (20%).
    • Works with SNS during stress.

Sympathetic Supply to the Pelvis

  • Origin: LHCs of L1, L2, and L3 segments.
  • Relay: Inferior mesenteric ganglia.
  • Functions:
    • Urinary Bladder: Relaxation and contraction for urine retention.
    • Rectum: Relaxation and contraction for fecal retention.
    • Sex Organs:
      • Male: Contraction of seminal vesicle, vas deferens for ejaculation.
      • Female: Vasoconstriction of external sex organ vessels, variable effects on uterus.

Orbelli Phenomenon

  • Definition: Sympathetic stimulation accelerates chemical reactions in muscles.
  • Effects:
    • Better muscle contraction.
    • Delayed fatigue.
    • Early recovery.

References

  • Guyton AC, and Hall JE (2021); Textbook of Medical Physiology, 14th ed., W.B. Saunders Company.
  • Sembulingam, K., & Sembulingam, P. (2022); Essentials of Medical Physiology (9th ed.). Jaypee Brothers Medical Publishers.