Differential atrophy of sensory and motor fibers following section of cat peripheral nerves.

Differential effects of peripheral nerve section on myelinated sensory and motor fiber populations were investigated in 5 hindlimb nerves of cats. Upon electrical stimulation of each nerve, monophasic compound action potentials were recorded from the L6, L7 and S1 dorsal and ventral roots, and the impedance of each root was measured. The decline in the electrical charge computed from potentials 43 to 252 days after nerve section gave a measure of the effect of axotomy on the diameters of sensory and of motor fibers in each nerve. No significant difference in the rate of atrophy of sensory and motor fibers was observed after about 45 days following nerve section. After about 145 and 245 days, however, dorsal root charge contributions had decreased significantly more than ventral root values. Exponential decay curves were fitted separately to charge data for sensory and for motor fibers. The calculated value for the endpoint of the decay was about 35% of the control value for motor fibers, and not significantly different from zero to sensory fibers. These results suggest that in response to axotomy, myelinated motor fiber diameters decline at first but later stabilize, while myelinated sensory fibers continue to decline and may atrophy completely if regeneration is prevented. Possible roles of electrical activity and of 'trophic' interactions with the periphery in the maintenance of cell properties are discussed.