Muscle receptors with fine afferent fibers which may evoke circulatory reflexes

The majority of afferent nerve fibers in mammalian skeletal muscle are thin myelinated (A delta or group III) and unmyelinated (C or group IV) afferents. Some 50% of these units appear to be responsible for the reception of noxious chemical, mechanical, and thermal stimuli, i.e., they are nociceptors. The other receptive units with fine afferent fibers presumably are activated by moderate innocuous stimuli such as light stretch, contractions, and light local touching. They possibly play a role in the circulatory and respiratory adjustments during exercise, i.e. they may be ergoreceptors. The data presently available suggest that the nociceptive as well as the ergoreceptive units are very heterogeneous groups with diverse receptive properties. The sensitivity of an individual unit may be restricted to the mechanical domain or to a single chemical substance, whereas other receptors respond to a great variety of chemical, mechanical, and probably thermal stimuli. Each receptor, however, seems to have a preferred susceptibility or "dominant sensitivity" to one or the other stimulus. This sensitivity may be modified by various factors, such as the local concentration of prostaglandins or serotonin. The concept of different types of fine afferent units is supported by preliminary ultrastructural findings showing a great structural diversity both in regard to the localization and the structural design of the receptive endings of these afferents.