Decussation of pyramids

Corticospinal Tracts - The Body's Puppeteers

The primary descending motor pathway for voluntary movement.

• Origin: Upper Motor Neurons (UMNs) in the primary motor cortex.
• Pathway: Descend via internal capsule and brainstem.
• Decussation: At the medullary pyramids, ~90% of fibers cross over.
  • Lateral Corticospinal Tract (LCST): Decussating fibers controlling contralateral limbs.
  • Anterior Corticospinal Tract (ACST): Uncrossed fibers for axial/proximal muscles.

⭐ A lesion above the decussation (e.g., stroke) causes contralateral paralysis. A spinal cord lesion below it causes ipsilateral paralysis.

The Motor Pathway - From Cortex to Medulla

• Origin: Upper motor neuron (UMN) cell bodies in the primary motor cortex (precentral gyrus).
• Descent: Axons descend via the corona radiata, converging in the posterior limb of the internal capsule.
• The tract then traverses the midbrain's cerebral peduncle and the ventral pons.
• In the medulla, these fibers form two prominent ventral bulges: the pyramids.

⭐ The internal capsule is a common site for lacunar strokes. Due to the dense concentration of motor fibers here, a small lesion can cause a "pure motor" stroke, producing contralateral weakness of the face, arm, and leg.

Decussation of Pyramids - The Great Crossover

• Location: Caudal medulla oblongata.
• Function: The anatomical crossover point for the majority of corticospinal tract fibers.
• The Split:
  • ~90% of fibers decussate (cross) to form the Lateral Corticospinal Tract (LCST) → controls contralateral limbs.
  • ~10% of fibers remain uncrossed, forming the Anterior Corticospinal Tract (ACST) → controls ipsilateral/bilateral axial & trunk muscles.

⭐ A lesion above the decussation (e.g., stroke) causes contralateral hemiplegia. A lesion in the spinal cord below the decussation causes ipsilateral motor deficits.

Clinical Correlations - Lesions & Locations

• Rule of Thumb: The site of decussation is the key to predicting the laterality of motor deficits.
• Lesions manifest with Upper Motor Neuron (UMN) signs: spastic paralysis, hyperreflexia, and a positive Babinski sign.

⭐ Brown-Séquard Syndrome (Spinal Cord Hemisection): A classic example of a lesion below the decussation, causing ipsilateral UMN paralysis and contralateral loss of pain/temperature.

High‑Yield Points - ⚡ Biggest Takeaways

• The decussation of the pyramids is where most corticospinal tract fibers cross the midline.
• This crucial event occurs at the caudal medulla, at the cervicomedullary junction.
• About 85-90% of fibers cross to form the lateral corticospinal tract, controlling distal limb muscles.
• Lesions above the decussation (e.g., stroke) result in contralateral motor deficits.
• Lesions below the decussation (e.g., spinal cord injury) cause ipsilateral weakness.
• The anterior spinal artery supplies this critical region.