Brain Research, 116 (1976) 139-144 139 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands Impulses reflected from dorsal root ganglia and from focal nerve injuries JOHN F. HOWE, WILLIAM H. CALVIN and JOHN D. LOESER Department of Neurological Surgery, University of Washington School of Medicine, Seattle, Wash. 98195 (U.S.A.) (Accepted July 19th, 1976) The middle of an axon is not usually a site of impulse generation; it is a region of impulse replication, where impulses originating elsewhere are faithfully reproduced in a one-for-one fashion. Nonetheless, it is not so specialized that it cannot generate impulses upon compressionX0,11. For example, because of rudimentary mechanosen- sitivity of axons, compression of the ulnar nerve at the elbow produces paresthesia. This ectopic generation of nerve impulses appears to operate in the manner of most mechanoreceptors: a generator potential is developed 11, and the repetitive firing patterns which result are those expected from the pacemaker-like rhythmic firing mode 1° in which depolarization is converted into firing rate. Although repetitive firing in normal peripheral nerves and dorsal roots is usually quite transient even with sustained compressionllJ 9, dorsal root ganglion cells and chronically injured axons are capable of producing sustained repetitive firing to sustained compression, as we have described elsewhere 11. This report considers a special form of ectopic impulse generation, the reflected impulse 6. On theoretical grounds, regions of spatial inhomogeneity (sudden changes in axon size or physiological properties) may allow an impulse to propagate through, but at the same time generate an extra impulse which travels backwards in the same axon a,la,18. This occurs because the duration of the impulse becomes prolonged at the site of transition, so that the impulse is still in progress when the refractoriness of the nearby membrane is over. Thus the recovering membrane is re-excited and an impulse propagates backwardsa, 16. These 'reflections' are thought to be one basis for the extra impulse mode of repetitive firing, characterized by short stereotyped intervals between two recorded nerve impulses, commonly observed in many types of neu- rons z-5,7,17. Because segmentally demyelinated axons might be expected to possess electrical inhomogeneities similar to those in regions of sudden enlargement (as we have discussed elsewhere 8 in conjunction with a theory for the pain mechanism of tic douloureux), we have tested this prediction by looking for reflected impulses in a preparation characterized by segmental demyelination. Because Tagini and Camino 17 observed ectopic impulse initiation near the dorsal root ganglion (DRG) in frogs, and because the spatial inhomogeneities there suggest a substrate for the reflection mechanism, we have similarly examined cat DRG cells for reflected