It has recently been demonstrated that conduction velocities of cerebellar climbing fibre afferents in the rat are tuned according to fibre length such that conduction time between their origin in the inferior olive and their target cortical Purkinje cells is constant. Here we have examined the situation in the cat, where individual climbing fibres are substantially longer. Complex spike responses of Purkinje cells located at various depths in the vermis (zones a and b) were evoked by electrical stimulation of olivocerebellar fibres close to their origin and were recorded either extra- or intracellularly. 2. The onset latencies of directly evoked complex spikes ranged from 2.6 to 6.9 ms. A consistent trend in each electrode penetration was that the complex spike latencies were longer for the superficially encountered cells (where olivocerebellar fibre length is greatest) and shorter for deeper cells (where olivocerebellar fibre length is shorter). 3. Linear regression analysis suggests that conduction time in olivocerebellar fibres in the cat is not fixed but varies linearly with conduction distance. Our findings would be consistent with a uniform conduction velocity in olivocerebellar fibres of about 6.6 m s-1.
