Abstract
Our previous report of the decreased mechanical withdrawal threshold in the eccentrically contracted muscle of rats, and the increased number of c-Fos immunoreactive neurons in the superficial dorsal horn of the spinal cord at L4 after compression to that muscle demonstrated the existence of mechanical hyperalgesia (tenderness) in the exercised muscle of rats. The purpose of this study is to examine whether the mechanical response of C-fiber sensory receptors of the muscle is altered after exercise. Rats were given eccentric contraction (ECC) by electrical stimulation of the common peroneal nerve innervating the extensor digitorum longus muscle 2 days before the electrophysiological experiment. Activities of single nerve fibers (mechanically sensitive but not stretch sensitive ones whose conduction velocity was less than 2.0 m/s) were recorded from the muscle-nerve preparations in vitro. Net discharge rate of C-fiber receptors during a ramp mechanical stimulation (0-20 g in 10 sec) was significantly higher in the ECC preparation than the control (CTR) (ECC (n=33): 5.5 ± 3.2 Hz (mean ± SD), vs CTR (n=13): 3.0 ± 2.6 Hz, p<0.01, un-paired t test). In addition, the threshold of the mechanical response was significantly lower in the ECC preparation (ECC: 5.2 ± 3.1 g, CTR: 8.7 ± 4.8 g, p<0.05). These changes in muscle afferents might be related to the tenderness in delayed onset muscle soreness after exercise. [Jpn J Physiol 54 Suppl:S174 (2004)]
