Unmyelinated axons in a muscle nerve. Electron microscopic morphometry of the sternomastoid nerve in normal and sympathectomized rats.

In continuation of earlier studies on the innervation of the sternomastoid muscle of the rat, a detailed morphometric analysis was performed on the unmyelinated axons of the nerve, in normal rats and after extensive cervical sympathectomy. In 4 normal rats an average of 314 myelinated and 319 unmyelinated axons were present. 42 days after surgery, the 3 animals showed Horner's syndrome and a highly significant 40% loss of unmyelinated axons. We therefore suggest that 40% of the C-fibers in this nerve are postganglionic sympathetic efferents and that the remaining 60% are type IV fibers, i.e., unmyelinated afferents. Our counts also indicate that part of the Remak bundles of the Schwann cells contain only sympathetic axons, whereas others contain mixed groups of sympathetic and afferent axons. Myelinated nerve fibers were not lost due to sympathectomy. Unexpectedly, the 3 animals analyzed 7-13 days after surgery showed Horner's syndrome but only a 16% loss of unmyelinated axons, which was not even statistically significant. Morphological signs of degeneration and sprouting did not provide any clue, but a possible explanation would be that a transitory sprouting of the remaining afferent C-fibers or Schwann cells occurred.