Topography and Ultrastructure of Group III and IV Nerve Terminals of the Cat’s Gastrocnemius-Soleus Muscle

Abstract

Topography, structure and values of muscle spindles and Golgi tendon organs within the gastrocnemius-soleus muscle of the cat are well documented (Swett et al., 1960; Chin et al., 1962; Boyd and Davey, 1968; Ariano et al., 1973). More than two thirds of thick myelinated nerve fibers in a muscle nerve innervate these proprioreceptors (Boyd and Davey, 1968; Boyd and Smith, 1984). There is much evidence that the receptor rich portion of the striated muscle is rich in oxidative muscle fibers (Gonyea and Ericson, 1978; Botterman et al., 1978; Richmond and Stuart, 1985). In physiological studies of slowly conducting fine afferent units (group III and group IV afferents) in the gastrocnemius-soleus muscle different receptor types are classified: 1. low threshold mechanoreceptors (LTM), 2. receptors that are stimulated by contraction only (ergoreceptors), 3. thermoreceptors and 4. receptors stimulated by ischemic contraction (nociceptors) (Mense and Stahnke, 1983; Kaufman et al., 1984; Mense and Meyer, 1985). Other investigators describe the thin-fiber receptors in the skeletal muscle as polymodal receptors responding to mechanical, chemical and thermal stimulation (Kumazawa and Mizumura, 1977; Kumazawa and Tadaki, 1983; Kumazawa et al., 1983). The functional significance of fine afferent fibers of skeletal muscle receptors for the control of cardiovascular reflexes has been reviewed by Mitchell and Schmidt (1983). Little is known about the morphology of these fine afferents.