The activity of gamma efferent units in lumbar ventral roots of the cat was investigated during conditioning procedures in which the CS (1,000 cycle/sec tone) was paired with a US (shock) applied to either forepaw or to the contralateral hind paw. Naive cats exposed to the conditioning procedure only during the terminal experiment were used as well as cats trained before the terminal experiment to give an overt conditioned flexion of the forepaw or contralateral hind paw. Conditioned responses of gamma units were recorded in ventral roots distant and contralateral from the spinal outflow innervating the “conditioned limb”. These widespread responses appeared during initial learning and were also present after the conditioned flexion response had been established. However, such conditioned activity did not represent a generalized excitation of the gamma motor system, as both conditioned excitation and conditioned inhibition were exhibited by different units. The conditioned response of a particular unit reflected the discharge of that unit to the US. When the unconditioned response of the unit was changed, as from a slowed to an accelerated pattern of discharge, the conditioned discharge pattern of the unit likewise changed. Such widespread yet specific conditioned responses of gamma neurons may subserve the total conditioned movement involving not only leg flexion but also contralateral extension and postural adjustments.
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