Influence of the pontine and medullary reticular formation on synchrony of gamma motoneurone discharge in the cat.

Discharges of gamma motoneurones were recorded from cut filaments of the nerve to the gastrocnemius medialis muscle in the cat decerebrated at an intercollicular level. Gamma motoneurones exhibited a background discharge in the absence of intentional stimulation, or could be made to discharge by continuous, innocuous stimulation of the skin of the heel. The discharges were periodic and regular (low coefficient of variation of interspike intervals), and no correlation was observed between the discharges of pairs of individual gamma efferents. Electrolytic lesion of the ipsilateral pontine and medullary reticular formation in the nucleus subcoeruleus, the nucleus reticularis gigantocellularis or the nucleus reticularis magnocellularis, invariably decreased regularity of discharge and resulted in short term synchrony. Lesions of the peri-aqueductal grey, the nucleus raphe dorsalis or the midline raphe nuclei did not induce synchrony. Surgical lesions in the locus coeruleus caused irregular firing and synchrony only when the lesion extended into the adjacent nucleus subcoeruleus. We conclude that monoaminergic neurones of the nucleus subcoeruleus, or a closely associated tegmental field, with axons descending through the gigantocellularis and magnocellularis fields, are the most likely origin of the bulbospinal control of synchronizing influences on gamma motoneurone discharge.