The field potentials evoked in the cerebellar cortical c1 zone by single-pulse, non-noxious stimulation of the superficial radial nerve have been recorded with tungsten-in-glass microelectrodes in awake cats. Responses that were due to transmission in the spino-olivocerebellar pathways (SOCPs), which terminate in the cortex as climbing fibres, were identified and studied while the cat walked on a moving belt. 2. The size of the climbing fibre-evoked potentials varied systematically during the step cycle. They were invariably largest in mid- to late swing of the ipsilateral forelimb and, at most recording sites (5/6), they were smallest during the first half of stance. 3. With low stimulus strength, the probability of evoking a measurable response also varied. The probability was greatest in mid- to late swing and least in early stance. 4. Similar variations were shown to occur when the analysis was restricted to responses evoked by a single functionally homogenous SOCP, the dorsal funiculus SOCP. 5. It is proposed that these variations reflect the operation of a gating mechanism which modulates the excitability of the SOCPs and prevents them transmitting self-generated tactile inputs to the cerebellum while facilitating transmission when unexpected inputs are most likely to arise. 6. The present data are compared with those from a similar study of the c2 zone SOCPs (Apps, Lidierth & Armstrong, 1990) and are discussed in relation to a study of the effects of unexpected mechanical perturbations of stepping (Andersson & Armstrong, 1987).
