Possible branching of myelinated primary afferent fibres in the dorsal root of the rat.

A combined physiological and morphological examination of rat dorsal root ganglion cells revealed branching of the central process of neurones with myelinated fibres (conduction velocity > 2 m/s; n = 24). Single shock electrical stimulation of spinal dorsal roots triggered double action potentials (early and late spike) in two dorsal root ganglion cells recorded by intracellular electrodes in the in vitro spinal cord-dorsal root ganglion preparation from 12-20 day-old rats. The action potentials had different stimulus thresholds (lower for the late spike). In one dorsal root ganglion cell the differences in delay between the two spikes inhibited the soma invasion of the second spike, when both action potentials were evoked at resting membrane potential. Depolarization of the soma membrane by DC current, injected through the electrode, recovered the blocked action potential. Membrane depolarization by GABA reduced the threshold for the early spike and changed the pattern of activation. After electrophysiological characterisation, intracellular biotin/avidin staining of the neurone revealed branching of the central axon in the dorsal root. None of the other cells, which responded with single action potentials after dorsal root stimulation showed secondary branching (n = 5). This rare observation shows that differences between the conduction velocities and activation thresholds in branches of individual dorsal root ganglion cell axons may produce block of spike invasion into the soma and perhaps the spinal terminal field of large primary afferents.