J Neurophysiol (February 1, 2003). 10.1152/jn.00624.2002
Submitted on Submitted 31 July 2002; accepted in final form 19 October 2002

Encoding of Compressive Stress During Indentation by Group III and IV Muscle Mechano-Nociceptors in Rat Gracilis Muscle

Department of Biomedical Engineering, State University of New York, Stony Brook, New York 117941-8181

Ge, Weiqing and Partap S. Khalsa. Encoding of Compressive Stress During Indentation by Group III and IV Muscle Mechano-Nociceptors in Rat Gracilis Muscle. J. Neurophysiol. 89: 785-792, 2003. The mechanical state encoded by group III and IV muscle afferents, putative mechano-nociceptors, during indentation was examined using an isolated muscle-nerve preparation in a rat model. Gracilis muscle and its intact innervation were surgically removed from the medial thigh of the rat hindlimb and placed in a dish containing rodent synthetic interstitial fluid. The tendons of the muscle were coupled to an apparatus that could stretch and apply compression to the muscle. Using a standard teased-nerve preparation, the neural responses of single mechanically sensitive group III or IV afferents were identified. Afferents were classified as mechano-nociceptors on the basis of their graded response to noxious levels of compressive stress (or strain) as well as, in some cases, their polymodal response to noxious thermal stimuli. Mechano-nociceptors (n = 13) were stimulated using controlled compressive stress (10-30 kPa) or strain (40-80%) while simultaneously measuring displacement and force by compressing the muscle between a flat cylinder and a hard platform. Linear regression was used to evaluate the relationships between neural response and mechanical stress, force, strain, and displacement. The mean neural response (threshold: 1.1 ± 0.4 kPa; sensitivity: 0.5 ± 0.1 Hz/kPa; means ± SE) was significantly and substantially more highly correlated with compressive stress than force, strain, or displacement. The data from this study support the hypothesis that muscle nociceptors stimulated by indentation encode compressive stress rather than force, strain, or displacement.