Recurrent actions on gamma‐motoneurones mediated via large and small ventral root fibres in the cat.

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Abstract
Effects on single lumbar .gamma.-motoneurons, mediated via fibers running in the ventral roots, were studied by micro-electrode recording in cats anesthetized with chloralose. Graded electrical stimulation of ventral roots or of peripheral nerves was used. The cells were identified as .gamma.-motoneurons by antidromic stimulation and by measurement of their axonal conduction velocity. Some of the cells were classified as static or dynamic. The findings confirm the existence of low-threshold, presumed recurrent, inhibition of both static and dynamic .gamma.-motoneurons. Strong evidence for the occurrence of high-threshold recurrent inhibition of .gamma.-motoneurons is also presented. Excitatory effects on .gamma.-cells, also mediated via fibers in the ventral roots, are described. The low-threshold effects from ventral root fibers are attributed to recurrent .alpha.-collateral activity and the high-threshold effects to .gamma.-collateral activity. The significance of recurrent inhibition of .gamma.-motoneurons is discussed in relation to the gain regulator concept proposed by Hultborn et al. (1979).