THE FIBER CONSTITUTION OF THE DEPRESSOR NERVE OF THE RABBIT

Abstract

A combined histologic and physiologic study of the depressor branch of the vagus nerve of the rabbit indicates that this nerve is not purely sensory, but contains vagus efferent fibers, both myelinated and non-myelinated, and that of the afferent fibers, not all have a depressor effect. The size range of depressor fibers lies between 5.5 and ca. 2[mu]. Fibers larger than this, up to 9 or 10[mu] occasionally present, are sometimes afferent pressor fibers, sometimes of other undetermined function. Autonomic myelinated fibers and the smaller afferent depressor fibers are indistinguishable anatomically by present methods, but differ in their physiologic characteristics. Pressor and presumably other afferent fibers may be present in the size range of typically depressor fibers, as well as in the larger size group. The max. depression obtainable from stimulation of one depressor nerve varies widely from rabbit to rabbit; the no. of myelinated fibers in the nerve is 150-600; there is some correspondence between the size of the nerve and the max. depression obtainable. The vagus has many depressor fibers larger than those in its depressor branch while most of its pressor fibers lie in the size range of depressor fibers in the depressor nerve. Evidence is presented that depressor fibers (presumably from the heart region) are variably distributed between vagus and depressor nerves, accounting in part at least for the variability in nos. of fibers in the depressor.