A 58-year-old man with hypertension presents with acute onset right-sided weakness and facial droop. MRI brain shows a lacunar infarct in the left internal capsule. Which anatomical feature best distinguishes the genu of the internal capsule from the posterior limb?

Explanation

## Anatomical Distinction: Genu vs Posterior Limb of Internal Capsule ### Structural Organization **Key Point:** The internal capsule is a V-shaped white matter tract with three distinct segments: anterior limb, genu, and posterior limb. Each segment carries functionally distinct fibre tracts. ### Comparative Features | Feature | Genu | Posterior Limb | |---------|------|----------------| | **Primary fibres** | Corticobulbar (motor) | Corticospinal (motor) + Thalamocortical (sensory) | | **Destination** | Brainstem motor nuclei (CN V, VII, XII) | Spinal cord (pyramidal tract) + Thalamus | | **Clinical deficit if damaged** | Facial weakness, dysarthria, dysphagia | Contralateral limb weakness, sensory loss | | **Position** | Between anterior and posterior limbs | Posterolateral, between thalamus and putamen | ### Why the Genu is Distinctive **High-Yield:** The genu contains **corticobulbar fibres exclusively**, which project from the motor cortex (Brodmann area 4) to the motor nuclei of cranial nerves in the brainstem. This is the critical distinguishing feature. **Clinical Pearl:** Lesions of the genu produce: - Facial droop (CN VII) - Dysarthria (CN XII) - Dysphagia (CN IX, X) - Preserved limb strength (posterior limb intact) ### Posterior Limb Composition The posterior limb is a "mixed" segment containing: 1. **Corticospinal tract** — motor fibres to limbs and trunk 2. **Thalamocortical radiations** — sensory fibres (VPL and VPM nuclei) ascending to sensory cortex 3. **Thalamocortical fibres** — visual and auditory radiations in the retrolenticular and sublenticular portions **Mnemonic:** **SCENT** — Sensory, Corticospinal, Eye (visual), Nuclei (auditory), Thalamic radiations in the posterior limb. ### Why Other Options Fail Option 1 (genu = corticobulbar) is the **correct discriminator** because it is the **only** fibre type unique to the genu. The posterior limb does NOT carry corticobulbar fibres. [cite:Snell's Clinical Neuroanatomy Ch 7]