A lesion at the T10 spinal cord level on the right side damages the corticospinal tract. At which level below the lesion would motor weakness be expected?

Explanation

## Corticospinal Tract: Decussation and Clinical Correlation **Key Point:** The corticospinal tract (CST) is a **long-segment crossed tract** that decussates at the **pyramids of the medulla** (90–95% of fibers), not in the spinal cord. Therefore, a spinal cord lesion damages the **already-crossed fibers**, causing **ipsilateral weakness below the lesion**. ### Anatomical Pathway ```mermaid flowchart TD A[Motor cortex]:::outcome --> B[Corticospinal tract descends<br/>through internal capsule & brainstem]:::action B --> C{Pyramidal decussation<br/>at medulla}:::decision C -->|90-95% of fibers| D[Crossed to contralateral side]:::action C -->|5-10% of fibers| E[Remain ipsilateral<br/>ventral CST]:::action D --> F[Descend in lateral CST<br/>contralateral spinal cord]:::action E --> G[Descend in ventral CST<br/>ipsilateral spinal cord]:::action F --> H[Synapse on contralateral<br/>lower motor neurons]:::outcome G --> I[Synapse on ipsilateral<br/>lower motor neurons]:::outcome ``` ### Clinical Correlation: Spinal Cord Lesion at T10 (Right Side) | Level of Lesion | Fibers Affected | Motor Deficit | |-----------------|-----------------|---------------| | **Spinal cord (T10 right)** | Already-crossed CST fibers on right side | **Right-sided weakness below T10** (ipsilateral) | | **Medulla (right)** | CST before decussation | **Left-sided weakness below medulla** (contralateral) | | **Cerebral hemisphere (right)** | CST before any crossing | **Left-sided weakness** (contralateral) | **High-Yield:** The **level of the lesion determines the pattern of weakness**: - **Spinal cord lesion** → ipsilateral weakness (fibers already crossed) - **Brainstem/cerebral lesion** → contralateral weakness (fibers not yet crossed) **Clinical Pearl:** A patient with a right-sided spinal cord infarct at T10 will have right-sided paraplegia below T10, but a right-sided medullary stroke will cause left-sided hemiplegia — a classic dissociation that localizes the lesion. **Mnemonic:** **"Spinal = Same side; Brain = Opposite side"** — the lesion location determines whether weakness matches the lesion side (spinal cord) or opposes it (brain). [cite:Snell's Neuroanatomy Ch 4]