Muscular nociceptors.

The discharge characteristics of various types of muscle afferents in response to mechanical and chemical painful stimulation were studied in cats anaesthetized with chloralose. In a few experiments, heat was used as an additional stimulus. The results indicate that only among the fibre groups III and IV, units are regularly found that have high thresholds on mechanical and thermal stimulation, and that response to administration of algesic agents in pain-producing doses. The response behaviour of individual group III and group IV afferent units to the various stimuli was very similar, but when the fibre populations are compared, the group III afferents appeared to be more sensitive to the mechanical and chemical stimuli of this study. It is concluded that both the fibre groups III and IV include afferent units possessing receptive properties enabling them to serve as muscular nociceptors.