Tonic suppression of reflex transmission in low spinal cats.

Transmission in various spinal hindlimb reflex pathways arising from muscle, cutaneous and joint afferents, was investigated in acute low spinal (Th10) cats before and after lesions at more caudal levels in the lumbar segments. Lesions of the ipsilateral dorsolateral funiculus (DLF) resulted in increased mono- and polysynaptic ventral root discharges. With conditioning of monosynaptic reflexes it was demonstrated that the DLF lesion enhanced transmission in reflex pathways from group II and III muscle afferents. The DLF lesion also resulted in increased dorsal root potentials from cutaneous, joint and group III muscle afferents. These reflex enhancements could not be obtained with lesions rostral to L2, but developed with lesions in L2 and to some extent in L3, but no further effect was obtained by adding lesions caudal to L3. Ventral extension of the DLF lesion gave hardly any increase of reflex transmission. It is postulated that the investigated reflex pathways may be tonically inhibited in the acute low spinal state by propriospinal neurones with cell bodies in the L2-L3 segments and with axons descending in the dorsolateral funiculus.