Cerebellar lesions produce ipsilateral ataxia because the cerebellum:

• A Sends its output through crossed pathways that re-cross in the brainstem before reaching the spinal cord ✓
• B Controls only ipsilateral cranial nerve nuclei
• C Has direct uncrossed projections to spinal motor neurons via the rubrospinal tract
• D Communicates exclusively through the posterior limb of the internal capsule

Explanation

Cerebellar output from the deep cerebellar nuclei (dentate, interposed) crosses in the superior cerebellar peduncle (brachium conjunctivum) to reach the contralateral red nucleus and thalamus. The corticospinal pathway then crosses again in the medullary pyramids. This double decussation results in the cerebellum ultimately coordinating the same (ipsilateral) side of the body. Hence, a cerebellar hemispheric lesion produces ipsilateral signs such as dysmetria, intention tremor, and ataxia.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.