The topographic organization of cerebellar cortical efferent fibers from the posterior lobe of the albino rat was studied by silver impregnation methods. The corticonuclear fibers from the vermis of the posterior lobe project to the caudomedial and middle parts of the medial cerebellar nucleus (MN) with medio-lateral localization; fibers from the medial portion of the vermis project to the caudomedial part of the MN (MNcm) and those from the lateral portion project to the middle part of the MN (MNm). Corticovestibular fibers originate in the caudal vermis lateral to the area projecting to the MNcm and MNm, and terminate in the dorsal part of the lateral vestibular nucleus (LVN). The origins of corticonuclear fibers to the dorsolateral protuberance of the MN (MNdlp), the posterior interpositus nucleus (PIN) and the anterior interpositus nucleus (AIN) are located latero-medially in the intermediate cortex. Fibers from the area adjacent to the vermis and rostral to the prepyramidal fissure project to the MNdlp, while those from the area lateral to the origin of the corticovestibular fibers and caudal to the fissure project to the medial AIN. Together, these areas comprise a medial portion of the intermediate cortex. Corticonuclear fibers to the PIN and lateral AIN originate from the lateral portion of the intermediate cortex. The corticonuclear fibers to the dorsolateral hump and lateral nucleus originate from the medial and lateral portions of the lateral cortex, respectively.

The topographic organization of the corticonuclear and corticovestibular fibers from the pyramis and copula pyramidis in the albino rat was investigated by the autoradiographic orthograde tracing technique. Cortical efferent fibers from the medial part of the pyramis project to the caudoventral part of the caudomedial and middle subdivisions of the medial cerebellar nucleus, whereas those arising from the lateral part of the pyramis project to the caudomedial part of the posterior interpositus nucleus and also to the dorsal part of the lateral vestibular nucleus via the juxtarestiform body. Fibers from the medial part of the copula pyramidis project to the medial part of the anterior interpositus nucleus. On the other hand, fibers from the medial portion of the lateral part of the copula pyramidis project to the posterior interpositus nucleus, while those from the lateral portion, including the lateral limit of the copula pyramidis, project to the rostroventral part of the lateral cerebellar nucleus. These results indicate that the cortical efferent projections from the pyramis and the copula pyramidis are clearly oriented mediolaterally in the albino rat.

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