Activation of cerebellar climbing fibres to rat cerebellar posterior lobe from motor cortical output pathways

1. The activation of climbing fibres projecting to the posterior lobe cereb ellar cortex by focal stimulation of the cerebral corticofugal pathway was investigated in anaesthetised rats. Large climbing fibre responses were evo ked in parts of crus II and paramedian lobule by stimulation of corticofuga l fibres. Lesions of the pyramidal tract just rostral to the inferior olive substantially reduced these responses, suggesting that they were. not medi ated by relays in the rostral brainstem. 2. By comparison of latencies of climbing fibre responses evoked from diffe rent locations in the corticofugal pathway, the conduction velocities of th e corticofugal fibres that mediate the responses were estimated to be 1.9 /- 0.3 m s(-1) (mean +/- The fastest conducting corticofugal fibres were es timated to conduct significantly faster (18.7 +/- 2.3 m s(-1)). 3. Climbing fibre responses with similar form and cerebellar distribution w ere evoked from sites in the pyramidal tract rostral and caudal to the infe rior olive. This suggests that at least a proportion of the fibres that act ivate climbing fibres are corticospinal fibres. 4. Lesions of the dorsal column nuclei did not affect the climbing fibre re sponses evoked in crus II, and produced a relatively small reduction of the responses in the paramedian lobule. This implies that the climbing fibre r esponses were not exclusively mediated via the dorsal column nuclei. 5. Corticofugal evoked climbing fibre responses were mapped across the cere bellar hemisphere. At some sites they were co-localised with responses evok ed by limb afferents. On the basis of limb afferent inputs and other work, these zones were tentatively identified as being functionally equivalent to the c1, c2 and d zones described in the cat.