Unmyelinated Sensory Fibers.

Abstract

Unmyelinated sensory fibers are present in large numbers in the spinal nerves. They take origin from the small cells of the spinal ganglia and divide like the myelinated fibers into central and peripheral branches. 1 The central branches enter the tract of Lissauer through the lateral division of the dorsal roots and probably end in the substantia gelatinosa Rolandi. 2 The peripheral branches run in the spinal nerves and are distributed chiefly in the cutaneous branches; very few are found in the motor nerves. 3 Ingvar has been able to confirm most of these observations on human material. 4

In order to exclude the possibility of confusion due to the presence of sympathetic unmyelinated fibers the question has been reinvestigated on sympathectomized cats. The right abdominal sympathetic chain was removed from the diaphragm to the pelvis and 5 weeks or more allowed for the degeneration of the sympathetic fibers in the femoral nerve. Pyridine-silver preparations were made of the saphenous nerve and of the nerve to the vastus medialis. Great numbers of unmyelinated fibers were found in the saphenous nerve; but very few in the nerve to the vastus medialis. Other pieces of the same nerves from the same sympathectomized cats were treated with osmic acid. Counts showed that as many myelinated fibers could be seen in the pyridine silver as in the osmic acid preparations. In one fascicle of a saphenous nerve 1294 myelinated fibers were counted in the osmic acid preparation and 1258 in the pyridine silver preparation. It is obvious that when the unmyelinated fibers are taken into consideration the number of axons present in the sympathectomized saphenous nerve far exceed the number of myelin sheaths.

The unmyelinated axons stain much darker than the myelinated axons in pyridine silver preparations.