Site-dependent differences in density of sympathetic nerve fibers in muscle-innervating nerves of the human head and neck

The autonomic nerve supply of skeletal muscle has become a focus of interest because it is closely related to the adaptation of energy metabolism with aging. We have performed an immunohistochemistry study on tyrosine hydroxylase (TH) and neuronal nitric oxide synthase (nNOS) using specimens obtained from ten selected elderly cadavers (mean age 83.3 years) in which we examined muscle-innervating nerves (abbreviated "muscle-nerves" hereafter) of ten striated muscles (soleus, infraspinatus, extra-ocular inferior rectus, lateral rectus, superior obliquus, temporalis, orbicularis oculi, posterior cricoarytenoideus, trapezius and genioglossus) and, as a positive control, the submandibular ganglion. We found that the extra-ocular muscles received no or very few TH-positive nerve fibers. Muscle-nerves to the other head and neck muscles contained a few or several TH-positive fibers per section, but their density (proportional area of TH-positive fibers per nerve cross-section) was one-half to one-third of that in nerves to the soleus or infraspinatus. We did not find nNOS-positive fibers in any of these muscle-nerves. In the head and neck muscles, with the exception of those of the tongue, there appeared to be very few TH-positive nerve fibers along the feeding artery. Consequently, the head and neck muscles seemed to receive much fewer sympathetic nerves than limb muscles. There was no evidence that nNOS-positive nerves contributed to vasodilation of feeding arteries in striated muscles. This site-dependent difference in sympathetic innervation would reflect its commitment to muscle activity. However, we did not find any rules determining the density of nerves according to muscle fiber type and the mode of muscle activity.