Rhythmic activity of spinal interneurons in harmaline-treated cats. A model for olivo-cerebellar influence at the spinal level.

After harmaline administration, rhythmic discharges at 8-12 c/s were obtained by intracellular recording from interneurons in the lumbo-sacral cord of decerebrate paralyzed cats. Unitary extracellular recordings were also made from cerebellar Purkinje cells, showing a rhythmic complex spike activity. Simultaneous recordings of cerebellar and spinal interneurons reveal a precise time relation between rhythmic activities recorded at these two levels. Following each rhythmic cerebellar signal one can note an interruption of interneuron activity. It is also noted that sustained high frequency tonic firing of interneurons coincides with cessation of cerebellar rhythmic activity. These facts are related to the impingement of a disfacilitatory supraspinal influence on spinal interneurons. A model of the mechanisms of signal transfer along the olivo-cerebello-bulbo-spinal system is proposed, according to which tremorogenic supraspinal influences might act on motoneurons by disfacilitating excitatory or inhibitory interneurons.