Messages conveyed by spinocerebellar pathways during scratching in the cat. I. Activity of neurons of the lateral reticular nucleus.

(1) Signals transmitted to the cerebellum by the spino-reticulocerebellar pathway (SRCP) during scratching were studied. For this purpose, the activity of neurons of the lateral reticular nucleus (LRN), which are the last-order neurons of the SRCP, was recorded during scratching in thalamic cats. Scratching was evoked by stimulation of the pinna. LRN neurons were identified antidromically by stimulation of the hindlimb area in the cerebellar anterior lobe. In most experiments, animals were immobilized with Flaxedil, and stimulation of the pinna resulted in fictitious scratching, i.e., in periodical reciprocal activity of flexor and extensor motoneurons typical of actual scratching. (2) During both actual and fictitious scratching, the discharge frequency of LRN neurons was rhythmically modulated in relation with the scratch cycle. Most LRN neurons fired in short high-frequency bursts of spikes which coincided (completely or partly) with the extensor phase of the cycle. In this respect the SRCP differs from the ventral spinocerebellar tract (VSCT) which is maximally active in the flexor phase of the cycle. (3) The firing pattern of LRN neurons during fictitious scratching was similar to that during actual scratching. Therefore, the rhythmical burst firing of LRN neurons is determined mainly by the central mechanisms and not by the rhythmical sensory input. (4) Rhythmical modulation of LRN neurons disappeared after transection of the ipsilateral lateral funiculus of the spinal cord in which spinoreticular fibers are located. On the other hand, considerable reduction of rhythmical activity in descending brainstem-spinal pathways after contralateral hemisection of the spinal cord did not affect the discharge pattern of LRN neurons. These two facts indicate that the SRCP conveys mainly messages about activity of the central spinal mechanisms, and that influences of supraspinal motor centers.on LRN neurons and on spinoreticular neurons are of minor importance. (5) Axonal terminations of LRN neurons are distributed rather evenly over the hindlimb area in the anterior lobe of the cerebellum. Therefore, messages about the events, which happen within the spinal cord in the vicinity of the extensor phase of the cycle, arrive at every point of the hindlimb area.