Which structural feature best distinguishes the genu of the internal capsule from the posterior limb?

Explanation

## Anatomical Organization of Internal Capsule The internal capsule is a compact white matter tract that carries both ascending and descending fibres. Its three main divisions have distinct fibre compositions and functional roles. ### Genu of Internal Capsule **Key Point:** The genu contains corticobulbar (corticonuclear) tract fibres and thalamocortical fibres projecting to the frontal lobe, particularly the prefrontal and premotor cortices. - Carries motor fibres to muscles of face, tongue, pharynx, and larynx - Receives thalamocortical fibres from mediodorsal (MD) nucleus of thalamus - Damage results in facial droop, dysarthria, and dysphonia ### Posterior Limb of Internal Capsule **High-Yield:** The posterior limb is the most clinically significant division and contains: 1. **Anterior two-thirds:** Corticospinal tract (motor fibres to limbs and trunk) 2. **Posterior one-third:** Sensory fibres from VPL nucleus (proprioception, touch, pain) and visual radiations from lateral geniculate nucleus (LGN) ### Comparative Table | Division | Primary Fibres | Thalamic Input | Clinical Deficit on Damage | | --- | --- | --- | --- | | **Genu** | Corticobulbar | Mediodorsal (MD) nucleus | Facial weakness, dysarthria | | **Anterior Limb** | Frontopontine fibres | Dorsomedial nucleus | Cognitive/behavioural changes | | **Posterior Limb (anterior 2/3)** | Corticospinal tract | VPL nucleus | Contralateral limb weakness | | **Posterior Limb (posterior 1/3)** | Sensory + visual radiations | VPL + LGN | Contralateral sensory loss + homonymous hemianopia | **Clinical Pearl:** A lacunar stroke affecting the genu produces facial weakness with preserved limb power (facial-sparing pattern), whereas posterior limb involvement causes contralateral hemiplegia with or without sensory loss depending on the exact location. **Mnemonic:** **GENU = Bulbar** (Genu contains fibres to muscles innervated by cranial nerves — bulbar muscles). Posterior limb = **Spinal** (Corticospinal tract). [cite:Snell's Clinical Neuroanatomy Ch 7]