Cerebellar regulation of sensorimotor activity in brown trout

Evoked field potentials were recorded from the mesencephalic ('optic') tectum, cerebellar corpus, midline rhombencephalon, and spinal cord of decerebrated brown trout in response to single electrical shocks given to an optic nerve. Evoked responses were also recorded from the rhombencephalon and spinal cord following stimulation (singly and with trains) to the optic tectum and to the cerebellar corpus. The potentials recorded from the tectum in response to optic nerve stimulation were similar in form to those reported by other workers from other species of teleost. The rhombencephalic responses to optic nerve and tectal stimulation were complex and comprised presumed pre- and post-synaptic events. Cerebellar stimulation evoked no detectable responses in these brain regions, but when given prior to tectal stimulation (by up to 10 ms), tectally-evoked spinal cord responses were reduced in amplitude by as much as 85%. After cerebellar ablation, there was no difference from controls in the latency, form or amplitude of any response, even when tested with paired pulse stimulation. However, when the cerebellum was ablated, rhombencephalic and spinal responses to optic nerve and tectal stimulation were markedly enhanced (by 200-300%). These clear-cut complimentary consequences of cerebellar ablation or stimulation emphasize the gain-setting role of the cerebellum and indicate, at least in relation to optically evoked motor activity, that cerebellar regulation acts at the level of the supraspinal drive to spinal motor circuits and not within the sensory centers.