Spike conduction properties of T-shaped C neurons in the rabbit nodose ganglion

The electrical activity of C-type neurons was recorded intracellularly in the rabbit nodose ganglion maintained in vitro. The initial segment of their axon is spirally wound close to the cell body and a primary branching point divides it into a central process (CP) projecting to the nucleus of solitary tract in the medulla oblongata and a peripheral process (PP) which conveys sensory inputs from the viscera. Stimulation of the CP induced either somatic ("S") spikes or low-amplitude axonal ("A") spikes ("A1" or "A2"). In some cases abrupt changes in the latency of "S" or "A" spikes (jumps) were observed by gradually increasing the stimulus intensity. They are discussed in relation to a secondary branching on the central axon located inside or near the ganglion. Collision experiments showed that antidromic "A" spikes are blocked at the primary bifurcation of the axon (T-shaped neuron). Stimulation of the PP induced either "S" spikes or high amplitude "A" spikes ("A3" or "A4"). Orthodromic spikes could be blocked either before or after the primary bifurcation. When blocking occurs after the bifurcation on the stem axon, the spike can invade the central axon without invading the soma. The study of the refractory periods of the two processes and the application of high frequency stimulation showed that the PP allows higher frequencies than the soma and the CP, and thus that branching and the CP act as low-pass filters. These data support the view that the primary branching point and the CP of these T-shaped cells represent a strategic area to modulate visceral afferent messages.