PAD and PAH response patterns of group Ia- and Ib-fibers to cutaneous and descending inputs in the cat spinal cord.

The characteristics of the primary afferent depolarization (PAD) of Ia- and Ib-fibers generated by segmental and descending inputs have been analyzed in the spinal cord of anesthetized cats. The PAD was inferred from the changes produced by conditioning inputs on the intraspinal stimulus current required to produce a constant antidromic firing of single group I afferent fibers from the gastrocnemius (GS) or posterior biceps and semitendinosus (PBSt) nerves. Group I GS and PBSt fibers ending in the intermediate nucleus could be classified in three different types according to their PAD patterns in response to stimulation of cutaneous nerves and of descending fibers. In one set of group I fibers stimulation of cutaneous nerves and of the ipsilateral brain stem reticular formation, or the contralateral red nucleus, produced no PAD, but was able to inhibit the PAD generated by stimulation of group I fibers from flexors (type A PAD pattern). PBSt nerve fibers with this PAD pattern had peripheral thresholds and conduction velocities between 1.01 and 1.56 times threshold and 76.3 to 118 m/s, respectively. A second set of group I fibers was found to be depolarized by cutaneous nerves as well as by stimulation of rubrospinal and reticulospinal fibers (type B PAD pattern). The peripheral thresholds and conduction velocities of PBSt afferent fibers with a type B PAD pattern were of 1.66-2.03 times threshold and 71-83 m/s, respectively. We found a third set of group I fibers that were also depolarized by reticulospinal and rubrospinal inputs, but not by cutaneous nerves that instead inhibited the PAD elicited by group I volleys in flexor nerves (type C PAD pattern). All PBSt afferent fibers with a type C PAD pattern, with the exception of two, had peripheral thresholds and velocities between 1.46 and 2.16 times threshold and between 72 and 89 m/s, respectively. Stimulation of the Deiter's nucleus was found to depolarize the intraspinal terminals of a small fraction of group I GS fibers with a type A PAD pattern and of all group I GS and PBSt fibers with type B and C PAD patterns. The PAD produced by vestibulospinal stimulation in fibers with type A and C PAD patterns could be inhibited by conditioning volleys applied to cutaneous nerves. It is suggested that group I afferent fibers from flexors and extensors with a type A PAD pattern are group Ia, and that most fibers with type B and type C PAD patterns are group Ib.(ABSTRACT TRUNCATED AT 400 WORDS)