Which anatomical feature best distinguishes the corticospinal tract from the spinothalamic tract in terms of their decussation pattern and clinical presentation?

## Decussation Patterns: Key Discriminator **High-Yield:** The level of decussation is the most clinically relevant feature distinguishing these two major descending and ascending tracts. ### Corticospinal Tract (CST) 1. Originates from motor cortex (Brodmann area 4) 2. Descends through internal capsule and cerebral peduncle 3. **Decussates at the pyramids of the medulla** (~90% of fibres) 4. Crosses as pyramidal decussation (visible anatomical landmark) 5. Uncrossed fibres (10%) descend as ventral CST and cross at spinal cord level ### Spinothalamic Tract (STT) 1. Originates from dorsal horn neurons (substantia gelatinosa) 2. **Decussates immediately at the spinal cord segment of entry** (within 1–2 segments) 3. Ascends contralaterally to thalamus 4. Carries pain and temperature sensation ### Clinical Correlation | Feature | Corticospinal Tract | Spinothalamic Tract | |---------|-------------------|---------------------| | **Decussation site** | Medullary pyramids | Spinal cord (1–2 segments above entry) | | **Level of lesion effect** | Contralateral motor loss below lesion | Contralateral pain/temp loss at/below lesion | | **Medullary syndrome** | Ipsilateral motor loss (above decussation) | Contralateral pain/temp loss | | **Spinal cord lesion** | Ipsilateral motor loss below | Contralateral pain/temp loss below | **Key Point:** A medullary lesion (e.g., Wallenberg syndrome) causes **ipsilateral motor loss** (CST hasn't crossed yet) but **contralateral pain/temperature loss** (STT already crossed at spinal cord level). This dissociation is pathognomonic. **Clinical Pearl:** In Brown-Séquard syndrome (hemisection of spinal cord), the **ipsilateral motor loss** and **contralateral pain/temperature loss** reflect the different decussation levels of these two tracts. [cite:Snell's Clinical Neuroanatomy Ch 4]