Hind leg ataxia of cervical origin and cervico-lumbar spinal interactions with a supratentorial pathway.

1. Unilateral section of the nerves to an extensor muscle of the head, biventer cervicis, leads to an ataxia of the hind legs in cats.2. In chloralose anaesthetized cats, shocks to the central end of the cut biventer cervicis muscle nerves leads to direct discharges over lumbosacral ventral roots, to a facilitation of monosynaptic reflexes in flexor and extensor muscles and to a prolonged inhibition of spino-bulbo-spinal reflexes. Such effects are not unique, but follow stimulation of skin and muscle nerves of the forepaw.3. Supratentorial pathways are involved in descending spinal interactions in the chloralose anaesthetized cat, but are more critical for interactions taking origin in biventer cervicis nerve than in forepaw nerves.4. In non-anaesthetized spinal cats showing interactions from forepaw nerves, no interactions could be found taking origin in biventer cervicis nerves. Descending interactions from forepaw nerves could be reduced or abolished by anaesthetic doses of chloralose.5. The rich pathway from biventer cervicis nerve to the superior colliculus does not appear to be involved in the descending interactions reported here as neither stimulation nor ablation of the superior colliculi has any effect on descending interactions.