Sympathetic Supply of Thorax and Abdomen

Study Notes

Origin: Lateral horn cells (LHCs) of the upper 4 thoracic segments (T1-T4)
Relay: Preganglionic fibers relay in all cervical ganglia and upper 4 thoracic ganglia. Postganglionic fibers arise from these ganglia to supply the following structures:
- Heart
- Lungs
Function:
- Heart:
  - Increases heart rate
  - Increases force of contraction
  - Coronary vasodilatation leading to increased blood supply to the cardiac muscle (indirect effect through accumulation of metabolites)
- Lungs:
  - Bronchodilatation (widening of air passages) due to relaxation of wall smooth muscle
  - Vasoconstriction of pulmonary blood vessels
  - Decreases mucous secretion in the air passages

Origin: LHCs of 6-12 thoracic segments (T6-T12)
Relay:
- Celiac ganglion
- Renal ganglion
- Superior mesenteric ganglion
Function:
- Relaxes the wall of the stomach, small intestine, and proximal part of the large intestine
- Contracts sphincters (leading to food retention)
- Liver: Stimulates glycogenolysis leading to increased blood glucose
- Spleen: Contracts splenic capsule, releasing stored blood to the general circulation (about 200 ml)
- Suprarenal Medulla: Secretory to 80% adrenaline and 20% noradrenaline
- Vasoconstriction (VC) of blood vessels of the stomach, small intestine, proximal part of the large intestine, liver, kidney, and pancreas

Origin: LHCs of the 12th thoracic segment and the upper 3 lumbar segments. The preganglionic fibers form right and left lesser splanchnic nerves which are joined to form the pre-sacral nerve.
Relay: Inferior mesenteric ganglion.
Function:
- Rectum:
  - Relaxation of the wall
  - Contraction of the internal anal sphincter leading to retention of stool
- Urinary Bladder:
  - Relaxation of the wall
  - Contraction of the internal urethral sphincter leading to retention of urine
- Genital Organs:
  - Ejaculation of semen due to contraction of the vas deferens, seminal vesicle and prostatic muscles
  - VC of blood vessels of pelvic viscera including the external genital tract leading to shrinkage of the penis.

Origin:
- Upper Limb: From LHCs of the (5-9) thoracic segments (T5-T9)
- Lower Limb: From LHCs of the (10-12) thoracic segments (T10-T12)
Relay:
- Upper Limb: Middle and inferior cervical ganglia and upper 2 thoracic ganglia.
- Lower Limb: Lumbar, sacral
Function: (In both upper and lower limbs)
- Skeletal Muscle Blood Vessels:
  - Vasodilatation - causing better contraction, delayed fatigue, and rapid recovery (Orbelli phenomenon)
    - Mechanism:
      - Increase in blood flow due to vasodilatation, this helps to supply more oxygen and nutrients and easy removal of waste products
      - Increase sensitivity of the motor end plate to acetylcholine
      - Activation of phosphorylase enzyme which helps glycogen breakdown and release of energy in the muscle
- Skin:
  - Vasoconstriction
  - Contraction of piloerector muscles causing erection of hair
  - Secretory to sweat glands