The sympathetic innervation of voluntary muscles.

Abstract

The experimental anatomical studies of Boeke (1913), 1 Boeke and Dusser de Barenne (1919), 2 and Agduhr (1919) 3 indicate clearly that, in addition to somatic afferent and efferent components of the cerebrospinal nerves, nerve-fibers of sympathetic origin also terminate on voluntary muscle-fibers. Not a few physiological studies tend to corroborate this anatomical finding, others yield only negative results. Consequently, the sympathetic innervation of voluntary muscles is not yet universally accepted even as an anatomical fact.

The material on which the present study is based was obtained from three dogs in which the somatic nerve-fibers supplying certain muscles had undergone degeneration following section of the corresponding nerve roots. 4 In two of the dogs the roots of the seventh, eighth and ninth thoracic nerves were exposed by laminectomy and cut distal to the spinal ganglia and proximal to the communicating rami. These animals were killed four weeks later. To avoid confusion which might arise by reason of overlapping of the areas of distribution of the intercostal nerves or the plurisegmental innervation of muscle-fibers, muscle was taken for study only from the eighth intercostal space. Control material was taken from intercostal spaces in which the nerves were left intact. In the third dog the mandibular nerve was cut intracranially. When this dog was killed twenty-three days later, portions of the masseter and pterygoid muscles on the side of the operation and control material from the corresponding muscles on the opposite side were taken for study. In all cases muscle tissue in which the somatic nerve fibers had undergone degeneration and control material was prepared both by the gold chloride and the pyridine-silver method.

In the control material prepared by both methods the terminal branches of the nerves comprising both myelinated and unmyelinated fibers as well as the terminations of fibers of both types on muscle-fibers may be observed. Our observations on the endplates of the ordinary myelinated somatic efferent fibers corroborate those of Boeke (1921) 5 who described these end-plates as hypolemmal in position and resting on nucleated 'sole-plates'composed of granular sarcoplasm which lies superficial to the myofibrillae and in which is imbedded a delicate reticular structure. As the myelinated somatic efferent fibers approach the end-plate some of them are accompanied by a slender unmyelinated fiber which terminates in a small end-net or end-loop within the area occupied by the 'sole-plate'on which the motor end-plate rests. Doubtless, these are the accessory fibers described by Boeke.