Decreased conduction velocity in the proximal projections of myelinated dorsal root ganglion cells in the cat

Abstract

tThe myelinated primary afferent fibers arising from cells of the spinal dorsal root ganglia (DRG) have been thought to have generally constant fiber diameter and, hence, constant conduction velocity over their entire course6 except for branching terminally within the spinal cord17 and peripherally near the receptors. A similar assessment for motor neuron axons has recently been challenged by physiologic data showing a 25 % decrease in conduction velocity for the segment between brachial plexus and spinal cord in the baboon1. The same authors further claimed to have evidence for similar proximal slowing in myelinated afferents from evoked potential recordings in dorsal roots, although they presented no data on the subject. In this study, conduction velocities were measured in the distal and proximal processes of individual DRG cells. Eleven adult male and female cats were surgically prepared by L4-S1 laminectomy under deep pentobarbital anesthesia while positioned in a stereotaxic frame. During the experiment, the animal was paralyzed with Flaxedil and artificially ventilated; the temperature of the body and the mineral oilfilled spinal pool were maintained automatically at 36-38 C. Bipolar stimulating electrodes (pairs of platinum hooks) were placed on the distal stump of the cut right L7 dorsal root and on the sciatic nerve which was dissected free but left intact in the posterior groove between the hamstrings. Monophasic electrical pulses (0.1 or 0.3 msec duration) from isolated bipolar constant current stimulators were always delivered using the hoolc closest to the ganglion as cathode. Conduction distances were I measured in situ at the end of each experiment by laying a piece of string from the cathode to the center of the ganglion. Dorsal root distances varied from 15 to 22 mm and sciatic nerve varied from 91 to 133 mm. The methods are described in greater I detail elsewherel3. Extracellular single unit records were obtained with glass insulated platinumiridiumz1 or Parylene insulated tungsten14 microelectrodes which were inserted in pairs into the intact L7 DRG. Filtering and differential recording were used to cancel the mass potential evoked by synchronous electrical activation of the DRG cells, and single ganglion cell soma potentials were clearly discernible at threshold as all-or