Sympathetic Nervous System

Instructor Information

• Contact: Dr. Zienab Helmy Eldken & Dr. Eman Hamdy Basha
• Department: Medical Physiology
• Email:
  • [email protected]
  • [email protected]

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.