Corticospinal Tract
The corticospinal tract (CST) — also called the pyramidal tract — is the most clinically important descending motor pathway in the nervous system. It controls voluntary movement of the extremities and is the pathway whose lesions produce the classic UMN syndrome.
1. Origin
Fibres arise from multiple cortical areas:
| Area | Contribution |
|---|
| Primary motor cortex — Brodmann area 4 (precentral gyrus) | ~30% |
| Premotor + supplementary motor cortex — Brodmann area 6 | ~30% |
| Somatosensory cortex — Brodmann areas 3, 1, 2, 5, 7 (postcentral gyrus + parietal lobe) | ~40% |
Neurons contributing to the CST are found mainly in cortical layer V. About 3% are giant Betz cells (~60 µm diameter), found only in the primary motor cortex — these are the largest neurons in the human nervous system. Their fibres conduct at ~70 m/sec. Each tract contains >1 million fibres total.
2. Course — Level by Level
Cerebral Cortex → Internal Capsule
- Axons enter the corona radiata (fan-shaped white matter)
- Converge into the posterior limb of the internal capsule (between caudate nucleus and putamen)
Internal Capsule → Brainstem
- Pass through the basis pedunculi (crus cerebri) of the midbrain
- Scatter into the longitudinal fascicles of the pons (broken up by pontine nuclei)
- Reconverge at the medulla to form the pyramids of the medulla oblongata (giving the tract its alternative name — pyramidal tract)
Pyramidal Decussation (caudal medulla)
- ~85–90% of fibres cross to the contralateral side in an interdigitated manner → descend as the lateral corticospinal tract in the lateral funiculus of the spinal cord
- ~10–15% do not cross → descend ipsilaterally as the anterior (ventral) corticospinal tract in the ventral funiculus; most of these eventually cross at their segmental level via the ventral white commissure (in cervical and upper thoracic segments)
3. Termination in the Spinal Cord
| Division | Location in cord | Termination | Function |
|---|
| Lateral corticospinal tract | Lateral funiculus | Laminae IV–VII and IX; extends throughout all cord levels | Controls distal limb muscles — fine voluntary movement |
| Anterior corticospinal tract | Ventral funiculus | Lamina III, cervical + upper thoracic only (after crossing) | Controls axial/bilateral postural muscles |
- Most fibres synapse on spinal interneurons in the intermediate zone
- Some synapse on sensory relay neurons in the dorsal horn
- A very few synapse directly on anterior motor neurons (alpha motor neurons) — these direct monosynaptic connections are especially important for fine finger movements
4. Somatotopic Organization
The CST maintains somatotopy throughout:
- In the motor cortex: face → hand → arm → trunk → leg (lateral to medial), with disproportionately large representation for hands and speech (Penfield homunculus)
- In the internal capsule: fibres in the posterior limb are arranged anterior (face) → posterior (leg)
- In the spinal cord lateral columns: cervical fibres are most medial; sacral fibres most lateral
5. UMN vs. LMN Lesions
| Feature | UMN Lesion (CST damage) | LMN Lesion |
|---|
| Weakness | Yes | Yes |
| Tone | Increased (spasticity) | Decreased (flaccidity) |
| Reflexes | Hyperreflexia | Hyporeflexia / areflexia |
| Babinski sign | Present | Absent |
| Atrophy | Mild (disuse) | Severe + early |
| Fasciculations | Absent | Present |
Note: In acute UMN lesions (e.g., stroke, spinal shock), flaccid paralysis and hyporeflexia may initially occur — spasticity develops over hours to months. Spasticity is thought to arise not from CST damage itself, but from concurrent damage to descending inhibitory pathways that run alongside the CST.
6. Clinical Correlate
A lesion above the pyramidal decussation (e.g., internal capsule stroke) → contralateral hemiplegia/hemiparesis with UMN signs.
A lesion below the decussation (e.g., lateral spinal cord) → ipsilateral UMN weakness below the lesion level.
The classic example: a 74-year-old woman waking with sudden right-sided face, arm, and leg paralysis with brisk reflexes and a Babinski sign on the right — pointing to a left-sided lesion above the pyramidal decussation (e.g., left internal capsule or motor cortex).
— Neuroanatomy through Clinical Cases, 3rd Ed. | Localization in Clinical Neurology, 8e | Guyton & Hall Medical Physiology