SCALING FACTOR RELATING CONDUCTION-VELOCITY AND DIAMETER FOR MYELINATED AFFERENT NERVE-FIBERS IN THE CAT HIND-LIMB

Compound action potentials were recorded from certain muscle and cutaneous nerves in normal and chronically de-efferentated hind limbs of cats during stimulation of the appropriate dorsal spinal roots. The peaks for groups I, II and III in the compound action potential were correlated with the corresponding peaks in the fiber-diameter histograms of the same de-efferentated nerve after processing it for light microscopy. The scaling factor (ratio of conduction velocity in m/s to total diameter in .mu.m) was not constant for all sizes of fiber nor did it increase progressively with fiber size. Evidence is presented that a logarithmic relation between conduction velocity and fiber diameter is not appropriate. In muscle nerves the scaling factor for fibers fixed by glutaraldehyde perfusion and embedded in Epon was 5.7 for group I afferent fibers and 4.6 for myelinated fibers in both group II and group III. In cutaneous nerves the scaling factor was 5.6 for large fibers (group I or A.beta.) and 4.6 for small fibers (group III or A.delta.). The scaling factor for group I fibers is the same as was found previously for .alpha.-efferent fibers, and that for groups II and III is the same as for .gamma.-efferent fibers (Boyd and Davey, 1968). The possibility that there is a clear discontinuity in scaling factor between fibers in groups I and .alpha., and those in other functional groups, is discussed. There must be some structural feature of .alpha. and group I fibers which differs from that of smaller myelinated fibers. A difference in the relative thickness of the myelin sheath is probably involved and possibly also in the conductances responsible for generating the action potential.