Differentiation Between Types of Fibers in Certain Components of Involuntary Nervous System.

Criteria discussed in a previous report 1 have been employed to distinguish between 3 different types of fibers which occur together in nerves of the involuntary nervous system. Comparative measurements of threshold 2 have also been extended to these fibers. The cervical sympathetic trunk of one variety of turtle serves as a typical nerve containing such axons, recognizable by 3 groups of waves in their conducted action potential record on the oscillograph. These groups will be designated as the A, B, and C groups.

A second type of turtle has only 2 groups of waves in the cervical sympathetic nerve trunk, the second and third. Sections of the first type of nerve∗ show scattered, larger, and for the most part thinly myelinated fibers, with islands well marked off by trabeculae containing unmyelinated and small myelinated axons. Sections of the second type show only such islands containing practically no large fibers. The second and third groups of waves in these nerves are therefore inferred to correspond to the thinly myelinated and unmyelinated axons respectively of these islands. The relative areas of potential of the third wave in various other nerves studied corresponds in general to the relative number of unmyelinated axons. In the second type turtle the third wave group is larger than the second in area.

The white and gray rami of the frog's 7th nerve consist almost exclusively of these typical islands of myelinated and unmyelinated fibers, and Gasser and Erlanger (personal communication) have observed B and C waves in a gray ramus. Certain dorsal roots of bullfrog and turtle also have similar thinly myelinated axons which are being studied as a possible source of sympathetic outflow.