The Journal of Neurophysiology Vol. 87 No. 2 February 2002, pp. 712-720
Copyright ©2002 by the American Physiological Society

Acute Effect of an Incision on Mechanosensitive Afferents in the Plantar Rat Hindpaw

 ¹Department of Pharmacology and  ²Department of Anesthesia, College of Medicine, The University of Iowa, Iowa City, Iowa 52242

Hämäläinen, Minna M., G. F. Gebhart, and Timothy J. Brennan. Acute Effect of an Incision on Mechanosensitive Afferents in the Plantar Rat Hindpaw. J. Neurophysiol. 87: 712-720, 2002. The purpose of this study was to examine which primary afferent fibers are sensitized to mechanical stimuli after an experimental surgical incision to the glabrous skin of the rat hindpaw. Afferent fibers teased from the L₅ dorsal root or the tibial nerve were recorded in anesthetized rats. The mechanical response properties of each fiber were characterized before and 45 min after an incision (or sham procedure) within the mechanical receptive field. Sensitization is characterized by an expansion of the mechanical receptive field, an increase in background activity, an increase in response magnitude, or a decrease in response threshold. After incision, the background activity and response properties of A-fibers (n = 9) to mechanical stimuli were unchanged. Four of 13 mechanosensitive A-fibers exhibited sensitization after the incision; response threshold decreased, response magnitude increased, or receptive field size increased. Background activity of A-fibers was not increased by the incision. Sensitization was observed in 4 of 18 mechanosensitive C-fibers 45 min after the incision. Background activity of C-fibers was not increased by the incision. In a group of mechanically insensitive afferent fibers (MIAs), 3 of 7 A-fibers and 4 of 10 C-fibers sensitized 45 min after incision. Response threshold was decreased in only 2 of 17 MIAs; receptive field size increased in 7 of 17 MIAs. A-fibers did not sensitize after the incision, and only 8 of 31 (26%) mechanosensitive A- and C-fibers gave evidence of sensitization. In a group of MIA A- and C-fibers, a greater percentage of 17 fibers studied (41%) were sensitized after incision. In this model, the principal effect of an incision, when examined 45 min after the insult, is an increase in receptive field size of the afferents, particularly those characterized as MIAs. To the extent that the mechanical hyperalgesia characterized in the same model is initiated in the periphery, it would appear that spatial summation of modestly increased response magnitude is important to the development of hyperalgesia.