A DORSO-LATERAL SPINAL PATHWAY MEDIATING INFORMATION FROM MESENCEPHALON TO DYNAMIC FUSIMOTOR NEURONES

In cats anesthetized with halothane, the activity of primary muscle spindle afferents from the long digital flexor muscle of the hind limb was recorded. Repetitive electrical stimulation in a restricted part of the mesencephalon in the region of the red nucleus caused a rather selective increase in the sensitivity of the spindles to length changes (dynamic sensitivity). During single shock stimulation in the same region, a response consisting of two positive waves could be recorded in the ventral medulla or in the dorsal part of the lateral funiculus of the spinal cord. By making restricted cord lesions it could be shown that the pathway transmitting the mesen‐cephalic influence to dynamic fusimotor neurones was also located in the dorsal part of the lateral funiculus. The two positive waves were shown to represent activity in rubro‐spinal and cortico‐spinal tracts, the last mentioned being activated by the mesencephalic stimulus via the cerebral cortex. Destruction of rubro‐and cortico‐spinal pathways at a medullary level resulted in the disappearance of the double wave cord response, but did not interfere with the mesencephalic action on dynamic fusimotor neurones. The tract mediating this influence is therefore still unidentified, although its spinal course is now known. © 1969 Scandinavian Physiological Society