Excitability changes of terminal arborizations of single Ia and Ib afferent fibers produced by muscle and cutaneous conditioning volleys

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Abstract
In anesthetized cats, recordings were made from dorsal root ganglion (DRG) cells having a peripheral process in the gastrocnemius-soleus (GS) nerve. The GS nerve was left in continuity with the muscle to allow identification of group Ia and Ib fibers. Most Ia units, but no Ib units, could be activated antidromically by stimulation within the motor nucleus, whereas Ib units were antidromically activated from the intermediate nucleus. The central processes of the Ia units had slower conduction velocities than did the peripheral processes. The thresholds to electrical stimulation of the Ia and Ib fibers overlapped. Conditioning volleys in the biceps-semitendinosus nerve increased the excitability of the central processes of Ia and Ib DRG cells. Stimulation of low threshold cutaneous afferents increased the excitability of 4 Ia fibers, decreased the excitability of two and had no effect on four. This mixture of effects could explain the apparent lack of excitability change produced in Ia fiber populations by cutaneous volleys. An increased excitability of Ib afferents was observed following cutaneous volleys.