Observations on the morphology of axons and somata of slowly conducting dorsal root ganglion cells in the cat.

In anaesthetized cats, the somata of cervical dorsal root ganglion (DRG) cells were impaled with glass micropipettes containing horseradish peroxidase. Slowly conducting afferent units were first classified by the conduction velocity of the peripheral axon (group IV: less than 2.5 m/s, group III: 2.5-30 m/s) and then injected iontophoretically with horseradish peroxidase in order to visualize the perikaryon and the axons close to the ganglion. All units classified as group IV had non-myelinated peripheral and central axons. DRG cells classified as group III were morphologically heterogeneous. Units conducting in the lower group III range had non-myelinated axons on both sides of the bifurcation; in those having an intermediate conduction velocity only the peripheral axon was myelinated, and in the fastest conducting group III units both the peripheral and central axons were myelinated. One out of the 36 stained cells gave rise to 3 processes, namely 1 central axon and 2 peripheral branches. The latter ones left the ganglion at its distal pole. Within the ganglion a significant tapering of the peripheral axon of group IV afferent units was observed. In the frequency distribution histogram of cell sizes, the cross-sectional areas of group IV somata were not restricted to the lower extreme of the distribution, but showed a great overlap with the somata of group III and even group II and I units.