Recurrent inhibition of fusimotor neurones exhibiting background discharges in the decerebrate and the spinal cat.

1. Discharges of functionally single fusimotor neurones have been recorded from the nerves to hamstring and triceps surae muscles in the cat. Background discharges were examined for both muscle groups in decerebrate animals but only for hamstrings in spinal preparations.2. Background discharges in forty-six out of sixty-seven triceps and twenty-one out of twenty-five flexor hamstring fusimotor neurones were inhibited by an antidromic volley in other efferent axons. One semimembranosus neurone was similarly inhibited.3. The inhibition was due to impulses in alpha motoneurone axons. It had a mean central delay of 2.3 +/- 0.2 msec and 2.7 +/- 0.2 msec for triceps and hamstring muscles respectively and was detectable for up to 100 msec, although shorter durations of the order of 10 msec were common.4. It was possible for a fusimotor neurone in one ventral root to be inhibited by an antidromic volley in an adjacent ventral root. The origin of the inhibition was not confined to a unit's own muscle nerve.5. The actions of eserine and strychnine on the inhibition were similar to their effects on recurrent inhibition of alpha motoneurones. Eserine potentiated and strychnine reduced the inhibition following an antidromic volley.6. The inhibition was found in spinal and in decerebrate cats but it was depressed following spinal section. L-Dopa in the spinal animal depressed the background discharge of some neurones and potentiated that of others. Both sets of neurones could be inhibited by antidromic volleys.7. It is concluded that antidromic inhibition of some fusimotor neurones does occur and that it is highly likely to be mediated via recurrent collaterals and Renshaw interneurones. The evidence from the use of L-Dopa indicates that the inhibition is present to both dynamic and static fusimotor neurones.