The dentato-rubro-thalamic tract (DRTT) decussates in which structure before reaching the contralateral thalamus?

• A Pyramid of the medulla
• B Superior cerebellar peduncle (brachium conjunctivum) at the level of the inferior colliculus ✓
• C Middle cerebellar peduncle
• D Posterior commissure

Explanation

Efferent fibres from the dentate nucleus travel through the superior cerebellar peduncle (brachium conjunctivum), which decussates in the tegmentum of the caudal midbrain at the level of the inferior colliculus, then synapses in the contralateral red nucleus (forming the dento-rubro tract) before ascending to the contralateral VL thalamus. This explains why cerebellar hemisphere lesions produce ipsilateral limb ataxia (the DRTT decussates, projects to contralateral thalamus which drives contralateral cortex, which in turn controls ipsilateral limbs via the corticospinal decussation — net effect is ipsilateral). Middle cerebellar peduncle carries afferents from pontine nuclei; posterior commissure subserves upgaze.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

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