Spinal termination of fine afferent fibers from muscle mediates mechanical sensation, including local pressure and pain sensation. Thinly my-elinated (group III) and unmyelinated (C or group IV) fibers are known to be a major component of muscle afferent fibers (Mense 1993). A few reports have dealt with central terminals of C-afferent fibers from muscles, but information in this regard is still fragmentary (Hirakawa et al. 1992; Torre et al. 1995). Neck and forelimb muscle afferents terminate in laminae I and V (Abraham and Swett 1986), while dense projections from the gastrocnemius and soleus muscle are reported to terminate in the substantia gelatinosa (Brushart et al. 1981) or diffusely terminate in laminae I-V (Kalia et al. 1981). Other studies report that muscle afferents terminate clearly in laminae I and V, but not in lamina II (Mense and Craig 1988). Intra-axonal labeling with horseradish peroxidase (HRP) of group III afferents from muscle or deep tissues demonstrates that the axons terminate in lamina I only or in laminae I, IV, and V (Hoheisel et al. 1989). By contrast, unmyelinated (C) afferent fibers from the skin terminate mainly in laminae I and II of the spinal cord of guinea pigs (Sugiura et al. 1986, 1989) and monkeys (Alvarez et al. 1993). On the other hand, visceral C-afferent axons terminate in laminae I, V, and X (Sugiura et al. 1989, 1993).
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