Makindo Medical Notes

Corticospinal Tract

The corticospinal tract is the principal descending motor pathway controlling voluntary movement, particularly fine, skilled movements of the distal limbs. It links the cerebral cortex to spinal motor neurons and is therefore central to neurological localisation. Damage anywhere along this tract produces a predictable pattern of weakness and upper motor neuron signs.

📍 Origin – Motor Cortex

Corticospinal fibres arise predominantly from layer V pyramidal cells (Betz cells) in the primary motor cortex of the precentral gyrus. Additional contributions come from premotor and supplementary motor areas, integrating planning with execution. The motor homunculus explains why cortical lesions produce disproportionate weakness of the hand and face.

• Primary motor cortex (precentral gyrus)
• Somatotopic organisation (leg medial, face lateral)
• Large pyramidal neurons generate fast-conducting fibres

🧭 Course Through the Brain

From the cortex, fibres descend through the corona radiata and converge tightly within the posterior limb of the internal capsule. This anatomical bottleneck explains why small capsular strokes can cause dense contralateral weakness. The tract then continues through the cerebral peduncles of the midbrain and the ventral pons.

• Corona radiata → posterior limb of internal capsule
• Midbrain: crus cerebri
• Pons: ventral (basis) pons

🔁 Pyramidal Decussation

At the caudal medulla, approximately 85–90% of corticospinal fibres cross to the opposite side at the pyramidal decussation. These fibres form the lateral corticospinal tract, which controls distal limb muscles. The uncrossed fibres descend as the anterior corticospinal tract, mainly influencing axial and proximal muscles.

• Decussation in lower medulla
• Lateral corticospinal tract (contralateral control)
• Anterior corticospinal tract (bilateral axial control)

🦴 Spinal Cord Termination

Corticospinal fibres synapse either directly onto anterior horn cells or via interneurons. Direct monosynaptic connections are especially important for fine finger movements. Lesions below the decussation cause ipsilateral weakness; lesions above cause contralateral weakness.

⚠️ Upper vs Lower Motor Neuron Lesions

The corticospinal tract defines the concept of the upper motor neuron (UMN). Damage above the anterior horn cell produces UMN signs due to loss of inhibitory cortical control. This distinction is fundamental in UK exams and bedside neurology.

• UMN signs: Weakness, spasticity, hyperreflexia, clonus, extensor plantar response
• LMN signs: Weakness, wasting, fasciculations, hyporeflexia

🩺 Clinical Correlation

A pure motor stroke affecting face, arm, and leg equally strongly suggests an internal capsule lesion. Brainstem lesions often produce crossed signs (ipsilateral cranial nerve palsy with contralateral limb weakness). In spinal cord disease, corticospinal involvement explains the combination of weakness and increased tone below the level of the lesion.

🧠 Teaching Pearl

Always ask two localisation questions: is the weakness ipsilateral or contralateral to the lesion, and are UMN signs present? Once you know where the fibres cross, corticospinal localisation becomes logical rather than memorised. This is one of the highest-yield pathways in clinical neurology.