The mode of impulse conduction through the spinal ganglion.

It has been known that the spinal ganglion cell of vertebrates, which developed more highly than the shirk, has a monopolar configuration and that its axon ramifies into two branches, i.e., to the dorsal root fiber and to the peripheral nerve fiber, forming a Y-shaped bifurcation (Lenhossek, 1886). An afferent impulse from the peripheral nerve, therefore, conducts through this bifurcation to the dorsal root without passing the cell body (Erlanger, Bishop and Gasser, 1926; Svaetichin, 1951; Dun, 1955). On the other hand in the bipolartyped cell as that found in the spinal ganglion of selachian fishes ( Campbell, 1946) afferent impulses travel through the cell body. From the view point of comparative anatomy (Star, 1928) the latter has been believed less favorable to the conduction of afferent impulses than the monopolar-typed cell. As was reported in the preceding paper (Ito, 1957), the spinal ganglion cell of toads responds to nerve impulses conducted both from the peripheral nerve and from the dorsal root, producing an action potential composed of three component spikes. Two of them, the S and NM spikes, have been confirmed to be initiated from the cell body and from the non-myelinated segment, respectively (Ito, 1959). In this paper it will be shown that the M spike is generated from nodes of Ranvier of the myelinated axon including the one locating at the bifurcation, and the mode of impulse conduction through this bifurcation to the dorsal root fiber, to the peripheral nerve fiber and to the cell body will be compared.