SUMMARY

The conclusions portion of this results chapter will begin with a separate consideration of connections of the paramedian lobule and crus II. The final analysis will comprise a comparison of the two cerebellar areas.

As the summary will discuss the injection sites in terms of the zonal subdivisions of the cortex, descriptions of the injection sites will focus on their rostro-caudal position within the folial chain.

A. Paramedian lobule

The olivocerebellar projection

Three of the four paramedian lobule cases (GR-95, Co5 and m2-961eft) were all situated in the lateral folia of the lobule, at the border with the medial edge of crus II. These cases therefore were positioned rostrally with respect to the folial chain.

PMD2, on the other hand lay ventromedially, in a folium adjacent to the vermis, and thus

caudally in the paramedian folial chain.

All of the paramedian lobule cases involved injections into cortical zone D l, as evidenced by retrogradely labelled olivary cells in the dorsal leaf and lateral bend. There was evidence of a more detailed topography in the olivocerebellar projection: for the cases in the rostral levels of the chain (in lateral paramedian folia), the olivary label was situated at mid-to-rostral levels, and mid-mediolateral distance, focusing around the two invaginations within dorsal lamella. The caudal case, adjacent to the vermis, however, contained dorsal lamella and thus still D l label, but at extreme medial and rostral levels. There was also some indication of a very detailed olivocerebellar topography within the rostral cases, as in spite of very similar injection sites and olivary label, there were slight variations in the mediolateral or rostrocaudal positions of the olivary label. However, it was impossible to discern a consistent topography, and more cases would be required to satisfactorily suggest a pattern.

The cortico-nuclear projection

A difference between the rostral and caudal paramedian folial chain (lateral and medial folia) was also apparent in the cortico-nuclear projection. All the rostral cases projected preferentially to the caudal dentate (zone D l), with additional smaller projections to the posterior interposed. Instead, the caudal area projected to the most rostral levels of the dentate (zone D2), with additional projections to the anterior and posterior interposed. Nuclear projections from the caudal and rostral levels of the paramedian folial chain did not overlap in either the dentate or the interposed.

The terminal fields in the caudal dentate were consistently striped. The strips of terminal label ran mediolaterally from the hilus of the nucleus, with the interdigitating areas being free from label.

The olivocorticonuclear system in the paramedian lobule did not follow the parasagittal zonation precisely. A number of cases (e.g. cases PMD2 and M2-961eft) contained olivary label appropriate to particular zones which were then not represented in the nuclear terminal field, while others (e.g. GR-95) contained nuclear projections into a zone which was not represented at the olivary level. Therefore both convergence and divergence from the parasagittal system occurs in the paramedian lobule. A particularly powerful example of the convergence in the olivocorticonuclear system was evident in the dentate nucleus. All of the cases in the rostral paramedian chain (lateral folia) contained olivary label in zone D l and projected to the caudolateral Dl zone in the dentate. However, case M2- 961eft which contained olivary label in both the dorsal and ventral lamella simultaneously (and therefore included zones Dl and the D2), contained only one focus of striped terminal field - it was located in the caudal half of the dentate, a nuclear area associated with D l. Thus, this case demonstrated “intra-zonal” convergence as cortical D l and D2 zones converged onto a single nuclear zone.

The NRTP/pontocerebellar projection

Due to the (limited) bilaterality of the pontocerebellar projection, the data from case M2- 961eft will not be discussed.

All paramedian lobule cases received input from the pontine nuclei, with no marked variation between the cases. GR-95 was unusual in having very sparse label, due to the superficiality of the injection site, with restricted label of the granular layer. The dorsolateral pontine nucleus contained fewer cells than the vermal cases, and although no nucleus was consistently over-represented, medial and ventral cell groups tended to dominate.

The clusters of labelled pontine cells covered the entire rostrocaudal length of the pons. Although the majority of cells were seen contralaterally, when ipsilateral label was found, it was usually a mirror-representation of the medial cell groups seen on the contralateral side.

The NRTP does not project to the paramedian lobule.

The nucleo-cortical pathway

The nucleo-cortical projection to the paramedian lobule is relatively sparse. The sparseness in some cases could be attributed to superficial injection sites, although correlation with the amount of pontine label, which arises as retrograde transport from the same cortical layer, would suggest that the projection is indeed small. There was no evidence of topography in the projection, and labelled cells were found both within and beyond terminal fields. The nucleo-cortical projection to the paramedian lobule therefore has reciprocal and non-reciprocal components, with no consistent morphological differences between them.

The nucleo-cortical projection has no contralateral component in the paramedian lobule.

B. CRUS II

The olivocerebellar projection

Both cases of crus II injections contained labelled cells restricted to the lateral bend of the principal olive, corresponding to zone D l. Olivary afferents to the more caudal areas of the crus II chain (case M2-96right) began at more caudal levels of the olive than those to the more rostral case (sb6). There was a small amount of overlap between the two at the rostral levels of m2-96right and caudal levels of sb6 olivary label.

The cortico-nuclear projection

Both cases contained terminal fields in the posterior interposed and the dentate nuclei. The terminal fields overlapped completely in the posterior interposed, but there was no overlap of dentate terminal fields: terminal fields in case m2-96right (caudal chain) began more rostrally than those in sb6 (rostral chain). When there was overlap in the rostrocaudal dimension, the terminal fields in the two cases were separated dorsoventrally until the most caudal levels of the nucleus. As in the paramedian lobule cases, the terminal fields in the dentate were striped.

The crus II cases provided another example of divergence from the parasagittal zonation of the olivocorticonuclear projection. Both cases lacked olivary label for the C2 zone (rostral medial accessory), but contained strong, overlapping terminal fields in the C2 nuclear zone (posterior interposed nucleus). There was no indication of convergence of two cortical D zones onto a single nuclear zone.

The NRTP/pontocerebellar projection

Case m2-96right will not be discussed here, due to the bilaterality of the pontocerebellar projection.

The majority of retrogradely labelled pontine cells were seen contralaterally, forming a ring around the fascicles of the cortico-spinal tract. A few cells were seen near or inside the dorsolateral pontine nucleus, but on the whole, they were focused in ventral and dorsomedial nuclear groups. The ipsilateral component was modest and in general mirrored the medial groups on the contralateral side.

The NRTP projects to the crus II, albeit moderately. Labelled cells were found in the central areas of the contralateral nucleus, with a relatively strong ipsilateral component.

The nucleo-cortical projection

Both cases in crus II showed evidence of a purely ipsilateral nucleo-cortical pathway. The projection from the deep nuclei is both reciprocal and non-reciprocal to the cortico-nuclear projection, with no evidence of topography. There was no evidence of morphological differences between reciprocal and non-reciprocal cortical afferents.

c. COMPARISON OF PARAMEDIAN LOBULE AND CRUS II

Although all paramedian and crus II cases contained olivary label in the dorsal leaf and lateral bend, often situated close together, there were no instances of complete overlap in olivary afferents to these separate cortical areas. The crus II and paramedian cases with the most similar olivary label were Co5 (lateral paramedian lobule) and sb6 (medial crus II) which lay either side of the prepyramidal fissure and contained label in the lateral bend of the principal olive. In terms of the folial chain, case Co5 lay caudal to sb6, and its olivary label began at more caudal levels than that from sb6, indicating a rostrocaudal topography in the olivocerebellar pathway across the hemisphere.

The similarity, but segregation of olivary afferents to these cortical areas is a testament to the precision of the olivocerebellar pathway.

The cortico-nuclear projection

Projections from both crus II and neighbouring paramedian lobule focus onto the caudal dentate. Caudal levels of the paramedian folial chain project more rostrally in the dentate. The stripes inherent to the terminal fields were arranged similarly in both areas, but it is impossible to say from these single tracer studies whether they overlapped or interdigitated with one another. Projections from the crus II and paramedian lobule to the caudal posterior interposed showed clear overlap at mid-rostrocaudal levels of the nucleus. Both the crus II and paramedian lobule had connections which converged and diverged from the classical parasagittal zonation of the olivocorticonuclear projection.

The NRTP/pontocerebellar projection

Pontine labelling was similar following injections into the crus II and paramedian lobule, but it is impossible to determine from single-tracer studies whether the pontocerebellar projections to the crus II and paramedian lobule overlap or interdigitate.

There is, however, a clear dichotomy in projections from the NRTP to the paramedian lobule and crus II. There are no NRTP projections to the paramedian lobule, but moderately strong ones to crus II.

The nucleo-cortical projection

Both crus II and paramedian lobule received projections from the deep nuclei, both reciprocally and non-reciprocally. There was no evidence of overlap in the retrogradely labelled cell populations between the crus II and paramedian lobule cases. No symmetrical projections were encountered.