Mossy fiber projections to the cerebellar flocculus from the extraocular muscle afferents

Inputs from extraocular muscles to the cerebellar flocculus were studied in anesthetized rabbits by recording neural responses either to electric stimulation to the nerve of the extraocular muscle or to the extraocular muscle stretch. The mossy fiber (mf) origin of the responses was identified by the similarity in the laminal profiles of these responses to those of the mossy fiber responses due to eighth nerve stimulation, the capacity to follow repetitive stimulation and the responses of simple spike discharges of Purkinje cells. The latency of mf responses evoked by electric stimulation of the afferent nerve from the superior oblique muscle was about 4.5-8.0 msec. Ramp displacements applied either to an individual extraocular muscle or to the whole eyeball also produced mf responses. Simple spike discharges of flocculus Purkinje cells were modulated by ramp displacements of extraocular muscles. Forty cells out of 47 cells examined responded with excitation and in 7 cells only inhibition was detected. Phasic, tonic and phasictonic excitations were obtained during ramp displacements. Responses of simple spike discharges were classified as direction-specific, plane-specific and non-specific types depending on the convergence of afferent sources of muscles producing excitation. Loci in the flocculus responding to extraocular muscle stimulation were studied histologically and it was revealed that mf responses from extraocular muscles were obtained in extensive parts of the flocculus.