Intraspinal latency, cutaneous fiber composition, and afferent control of the dorsal root reflex in cat

Abstract

The intraspinal delay, fiber composition, and excitability of the cutaneous dorsal root reflex (DRR) was studied in pentobarbital-anesthetized cats.A maximal cutaneous DRR is elicited by a volley of cutaneous fibers with conduction velocities above the A-delta range. The addition of A-delta and C fibers does not increase DRR amplitude or duration; nor does an isolated A-delta or C fiber volley, delivered during anodal polarization block of the larger fibers, elicit a DRR. Collision experiments reveal that the initial phase of the cutaneous DRR recorded from nerves is composed of activity in large myelinated fibers; the later phase is due to active A-delta fibers. Thus, the observations reveal that A-delta fibers carry, but do not elicit, the DRR.A collision technique shows that the minimum delay for the inter-fiber interaction producing the DRR may be as short as 1.5 msec, a value compatible with DRR mediation by one interneuron.Prolonged DRR depression follows mechanical stimulation of the skin and single shock or repetitive electrical stimulation of the A-alpha cutaneous afferents capable of DRR excitation; neither large (A-alpha) fiber volleys below DRR thresh-old nor isolated A-delta volleys depress the DRR. In the case of cutaneous nerves, this depression affects unconditioned nerves, but the effects of dorsal rootlet tetany are restricted to DRRs elicited by or recorded from the conditioned rootlet.The results are summarized in a model in which A-alpha cutaneous afferent fibers activate interneuronal systems mediating DRR excitation and depression.