Thoracolumbar Outflow
The thoracolumbar outflow refers to the anatomical origin of the sympathetic division of the autonomic nervous system, which arises from the thoracic and lumbar segments of the spinal cord.
Definition
The thoracolumbar outflow comprises the preganglionic sympathetic neurons located in the intermediolateral cell column (IML) of the spinal cord segments T1 to L2/L3.
Pathway
- Preganglionic neurons originate in the IML (lateral horn)
- Their axons exit the spinal cord through ventral roots
- They pass through white rami communicantes to reach:
- Paravertebral ganglia (sympathetic chain)
- Or prevertebral ganglia (e.g., celiac, superior mesenteric)
- Postganglionic fibers then project to:
- Viscera (heart, lungs, GI tract)
- Blood vessels
- Sweat glands
- Pupillary dilator muscles
Functional Roles
- ↑ Heart rate and cardiac output
- ↑ Pupil dilation (mydriasis)
- Bronchodilation
- ↓ Gastrointestinal motility
- Vasoconstriction in skin and viscera
- Sweat secretion
Comparison
| Division | Origin | Key Features |
|---|---|---|
| Sympathetic (Thoracolumbar) | T1–L2/L3 | “Fight or flight” responses |
| Parasympathetic (Craniosacral) | Cranial nerves III, VII, IX, X + S2–S4 | “Rest and digest” responses |
Clinical Relevance
- Lesions affecting T1–L2 segments can impair sympathetic function
- Disruption may cause:
- Orthostatic hypotension
- Thermoregulatory dysfunction
- Bladder/bowel dysfunction