Group II-activated lumbosacral interneurones with an ascending projection to midlumbar segments of the cat spinal cord.

1. In anaesthetized cats, single-unit microelectrode recordings were made in the lateral funiculus at L6, from the axons of lumbosacral interneurones discharged by hindlimb group II muscle afferents. 2. The level of the ascending projection of these interneurones was investigated by antidromic activation of their axons in the lateral funiculus from different spinal levels. The majority of units encountered were found to have an ascending projection to at least the L4 level and, of these, most (85%) did not project beyond the L4 or L3 segments of the cord. 3. The axons studied were discharged by group II afferents primarily from knee extensor muscles. Some units were discharged in addition by cutaneous and/or joint afferents. 4. The implications of this ascending projection are discussed.