Complex spikes in Purkinje cells in the lateral vermis (b zone) of the cat cerebellum during locomotion.

1. Complex spikes (c.s.s) due to climbing fibre input were recorded from forty-one Purkinje cells in the lateral part of the vermis (i.e. the b zone) of lobule V of the cerebellum in cats walking on a moving belt or a horizontal ladder. Most cells were near the tips of the folia making up the lobule and some were shown by antidromic invasion to project to the ipsilateral lateral vestibular nucleus. In all cells c.s.s. could be evoked through mechanical stimuli delivered manually to the neck and/or trunk and/or the limb girdles and/or the proximal parts of the limbs. 2. During walking c.s.s. occurred at rates which ranged in different cells from 0.8 to 2.55/s (i.e. ca. 0.8 to 2/step). When activity was averaged across many successive steps the probability of c.s. occurrence was never completely constant throughout the step cycle, but no tendency was detected for c.s.s to recur at any precisely fixed time during the cycle. 3. When ladder locomotion was perturbed because a rung underwent an unexpected 2 cm descent when stepped on, some cells generated a c.s. at short latency in a proportion of trials. Such responses were well time-locked to the onset of rung movement but not to its cessation (which they often preceded). 4. For perturbations of either forelimb the earliest displacement-related c.s. occurred in different cells between 40 and 64 ms after the onset of rung movement. In different cells c.s.s occurred in from one out of five to three out of four perturbed steps (mean ca. two out of five steps). Eight out of seventeen cells responded to perturbation of the forelimb ipsilateral to the cell and five out of ten responded to contralateral perturbations. 5. Perturbation of the ipsilateral hind limb was accompanied by c.s.s in four out of nine cells and latency was usually longer (by ca. 30-40 ms). One cell showed a decrease in the probability of c.s. occurrence. Insufficient data were obtained for a systematic study of responsiveness to perturbation of the contralateral hind limb. 6. Cells showed different patterns of limb specificity, responding to perturbation of one, two or all of the three limbs studied. In total, c.s.s accompanied perturbation of at least one limb in thirteen out of twenty cells studied (65%).(ABSTRACT TRUNCATED AT 400 WORDS)