Unmedullated Fibers Originating in Dorsal Root Ganglia

Abstract

Information about the physiology of C fibers has been based largely upon studies of visceral nerves; and the properties revealed have, therefore, been those of the unmedullated axons of the postganglionic components of the sympathetic nervous system. An indication that the C fibers in mammafian skin nerves might have properties different from those of their sympathetic analogues first appeared in 1938 (Grundfest and Gasser), when it was observed that the curve of recovery of excitability of C fibers in the saphenous nerve of the cat was different from the one found in the examination of visceral nerves. Subsequent investigation has brought to light additional points of difference, to such an extent that it is now necessary to divide the C group of fibers into two subgroups. Much further work will be required to supply a full description of these fibers. Consequently, in connection with the principal subject of this paper, which is the form of the compound action potential and its relationship to the fibers which produce it, it will be necessary to bring forward in a preliminary way mention of characteristics still awaiting more complete analysis. Information about these characteristics is required both for the interpretation of the action potential and for support of the thesis that there is a group of unmedullated fibers, afferent in function, specialized in their properties, and systematized in the same sense in which the medullated fibers in mammalian skin nerves are known to he systematized.To meet the demands of the problem two separate lines of approach to it were required: a study of the functional behavior of the fibers, and a study of their morphology. The results of the two studies will be described in the order mentioned; and finally there will be pointed out the parts of the two studies which can be brought into relationship. SECTION I THE ACTION POTENTIALMet~ds.--The principal technical difficulty arose in the maintenance of a steady base line, the need for which will become apparent as soon as the records are examined. Three imposed experimental conditions converged toward the production of unsteadiness: the strong shocks required for stimulation, the high gain required for recording,

The excitatory action of acetylcholine on cutaneous non‐myelinated fibres

“…Some of them may be efferent fibres, for according to Heinbecker, O'Leary & Bishop (1933) about 20 % of the non-myelinated fibres in the cat's saphenous nerve are sympathetic motor fibres. These might be the fibres responsible for the smaller C2 elevation, although Gasser (1950) believes that they are not distributed in any single group to give rise to a discrete elevation but contribute to all the elevations of the C potential. Studies on axon reflexes (Coon & Rothman, 1940) have suggested that sympathetic motor fibres in the skin may indeed be excited by ACh.…”

“…It has already been suggested that the C2 fibres or many of them may not be sensory at all, but may be sympathetic motor fibres. While the evidence suggests that no more than a small proportion of the nonmyelinated fibres in the saphenous nerve can be specific pain fibres, it must be remembered that C fibres are so numerous in this nerve (according to Gasser (1950) they outnumber all the A fibres several times) that even a small fraction of them would still be a large number.…”

“…The first possibility is that the after-potentials were secondary to the burst of repetitive activity set up by the ACh exciting the nerve endings, for it is known that repetitive activity may give rise to very large afterpotentials in C fibres (e.g. Gasser, 1950;Brown & Holmes, 1956;Ritchie & Straub, 1956). Such after-potential changes, however, have been demonstrated previously only with electrical stimulation at high rates, and there has been some doubt whether changes in after-potentials can occur in the absence of such electrical stimulation (Douglas & Ritchie, 1957d).…”

The temporal course of the effects of postganglionic axotomy on the inferior mesenteric ganglion of the cat

“…When he cut the sciatic nerve in the dog, at a distance of several centimetres from the dorsal-root ganglion, however, few of the cells died, but again the cells having non-myelinated axons gave evidence of severe axonal reaction (Ranson, 1912). Since physiological differences exist between dorsal-root C-fibres and sympathetic C-fibres (Gasser, 1950), it is safer for the question at hand to rely upon evidence in sympathetic ganglia. Unfortunately, anatomical studies of the axonal reaction in this tissue have been scanty.…”

Effects of phenol on nervous conduction

“…This was obtained by both stimulating and recording from a posterior root. The distance that we found to be optimal in pilot experiments is about half that which Gasser (1950) found suitable for recording the C action potential in peripheral nerves. A possible reason for the difference is that the diameter of the C fibres of the root central to the ganglion is half that of the fibres peripheral to the ganglion, a fact shown by Gasser (1955); the consequent decrease in conduction velocity would result in greater temporal dispersion over the longer distance.…”