The X zone and CX subzone of the cerebellum in the rat.

The existence of an X zone (lateral to the A zone) and a CX subzone (lateral to the C1 subzone) was documented within the anterior lobe and lobule VI in cats and primates. On the basis of their respective efferent and climbing fibre (CF) afferent connections, delineation of these two cortical subdivisions has been investigated here, in the rat, using small injections of WGA-HRP in the cerebellar cortex. We observe that both X and CX are "fractured" into a rostral and a caudal compartment. The rostral compartment of the X zone extends over lobules IV, V and VI and its caudal compartment over lobules VIII, IX and X. The rostral compartment of the CX subzone seems to be restricted to lobules V and VI, its caudal compartment cannot be topographically distinguished, over lobules IX and X, from the caudal compartment of the X zone. The olivary afferents to the X zone and the CX subzone arise from the horizontal and vertical lamellae of the medial accessory olive: subnucleus a projects into the rostral compartment and lobule VIII of the X zone. Subnuclei b and c project into the rostral compartment of both X and CX. The dorsomedial cell column selectively projects onto the caudal compartment of both X and CX over the vestibulo-cerebellum. The corticonuclear projections of the X zone have been found within the junctional region between the nucleus medialis and the nucleus interpositus (NI), here defined as the interstitial cell groups (icg), the corticonuclear projections of the CX subzone within the medial NI. It is suggested that the icg correspond to clusters of neurones dissociated from the medial aspect of the NI. We therefore consider the X zone and CX subzone of the rat, on the basis of their corticonuclear efferents, as "medial C1" and "lateral C1" subzone, respectively, although both may be regarded as part of the A zone on the basis of their olivary afferents.