The internal capsule is traversed by descending motor fibers from the motor cortex. Which of the following best describes the somatotopic organization of these corticospinal tract fibers within the posterior limb of the internal capsule?

## Somatotopic Organization of the Internal Capsule **Key Point:** The posterior limb of the internal capsule contains descending corticospinal tract fibers with a precise somatotopic arrangement: **upper limb fibers occupy the medial portion, while lower limb fibers occupy the lateral portion**. ### Anatomical Basis The internal capsule is a V-shaped white matter structure containing both ascending and descending fiber tracts. The posterior limb specifically carries motor fibers from the primary motor cortex (Brodmann area 4) and premotor cortex (area 6). According to **Snell's Clinical Neuroanatomy** and **Kandel's Principles of Neural Science**, the corticospinal fibers within the posterior limb are somatotopically organized from medial to lateral as: face → upper limb → trunk → lower limb. ### Somatotopic Arrangement in Posterior Limb | Region | Fiber Content | Location | |--------|---------------|----------| | Medial portion | Face and upper limb (arm) fibers | Medial | | Middle portion | Trunk fibers | Central | | Lateral portion | Lower limb (leg) fibers | Lateral | **High-Yield:** This arrangement mirrors the medial-to-lateral progression seen in the motor homunculus of the cortex as it projects through the internal capsule. Upper limb fibers travel medially and lower limb fibers travel laterally within the posterior limb. **Clinical Pearl:** A lacunar infarct affecting the lateral posterior limb (supplied by lenticulostriate arteries) preferentially causes contralateral lower limb weakness, while a medial posterior limb lesion causes contralateral upper limb and facial weakness. This somatotopic knowledge is essential for correlating imaging findings with clinical deficits. ### Why Option C is Incorrect Option C states "lower limb fibers are medial, upper limb fibers are lateral" — this is the **reverse** of the correct arrangement. Standard neuroanatomy references (Snell, Kandel, Gray's Anatomy) consistently place lower limb fibers **laterally** and upper limb fibers **medially** in the posterior limb of the internal capsule. ### Clinical Correlation The internal capsule is a common site of lacunar infarction due to hypertensive lipohyalinosis of penetrating lenticulostriate arteries (branches of the middle cerebral artery). Understanding the somatotopic organization allows clinicians to predict the pattern of motor deficit based on lesion location on MRI.