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

The dorsal root reflex (DRR) is a special manifestation of primary afferent interaction which can be used to further examine and extend our understanding of cutaneous fiber interactions derived from studies of the dorsal root potential (DRP) and intracellular recording of primary afferent depolarization (PAD). The presynaptic interactions of mammalian cutaneous afferents, for example, are thought by some s,9 to be mediated by a group of spinal interneurons, thus accounting, in part, for the observed 3 ~, msec intraspinal delay of the DRP 1,16 and DRR 7,27,28. This hypothesis is supported by intra-axonal and intraand extracellular recordings showing appropriately timed interneuronal discharges and primary afferent depolarizationa,L However, others have suggested, on the basis of morphological 28 and physiological dataZ, 29 that more direct interaction among afferent fibers might underlie some components of the DRR and DRP. We wished to obtain an independent estimate of the intraspinal delay for the DRR. Since the DRR is composed of propagated action potentials, it is possible to collide orthodromic and antidromic activity within the spinal cord and provide another estimate of the delay interposed between primary afferent interactions. The DRR can also be used to reveal types of interacting fibers. Recent estimates of the average conduction velocities of fibers composing the cutaneous DRR have ranged from 14.2 m/sec in the phalanger 18 to 50 m/sec in the cat 3. Toennies 27, on the basis of collision experiments in the cat, concluded that both large (A-alpha) and small diameter (A-delta) myelinated fibers were included in the reflex discharge. As regards the afferent limb of this reflex, there is general agreement that a full DRR can be elicited by stimulation of large myelinated fibers. J~inig and Zimmermann lz have reported occasional long-latency single fiber DRRs following the addition of C fibers to an A fiber volley, but we wished to determine the efficacy of input volleys strictly limited to either A-delta or C fibers. Additional details of the organization of the DRR should be revealed by studying the action of the different spinal afferents influencing DRR excitability. Pro-