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CNRS URA 1448, Université René Descartes, Paris, France.

1. Clarke's column neurons of the dorsal spinocerebellar tract (DSCT) were recorded intracellularly in anaesthetized cats during weak sustained contractions of triceps surae (TS) produced by direct electrical stimulation of the muscle. 2. Of 145 DSCT neurons, 77 (53%) were contraction sensitive suggesting that information about weak contraction of a limited number of muscles is widely distributed among DSCT neurons. Four types of effects were observed in individual neurons during TS contractions. 3. In the first group of 11 DSCT neurons (14% of the contraction-sensitive cells), the effect was excitation persisting throughout the duration of contractions. These responses were ascribed to actions of afferents from contraction-activated tendon organs. 4. In a second group of 15 neurons (20% of the contraction-sensitive cells), quickly declining excitatory potentials were recorded during sustained TS contractions. By analogy with previous observations of contraction-induced effects in motoneurons, the decline of excitation might be explained by contraction-induced presynaptic inhibition of group I afferents in Clarke's column. 5. Declining inhibitions, resembling those previously observed in homonymous and synergic motoneurons, were recorded in 49% of contraction-sensitive DSCT neurons. This appears in keeping with the fact that interneurons mediating Ib inhibition to motoneurons project axon collaterals to DSCT neurons. Presynaptic inhibition of Ib fibres might therefore cause parallel reductions of inhibitory potentials in motoneurons and in DSCT neurons. 6. In a final group of 13 neurons, mixed excitatory and inhibitory effects were observed during TS contractions. Such DSCT neurons might monitor the excitability of Ib interneurons by integration of information about input to and output from these neurons. 7. The non-uniform patterns of DSCT responses to TS contractions suggest complex processing of information on ankle extensor activity in cerebellum. Phasic signalling of contraction onset is observed in many DSCT neurons while others carry messages about duration and strength of contraction.

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