Conduction velocities of pyramidal tract fibres and lumbar motor nerve roots: normal values

Measurements of spinal cord and individual lumbar nerve root lengths were performed in 20 dissected cadavers. These data were correlated with the pyramidal tract and motor root conduction times obtained in 53 healthy subjects using motor evoked potentials. The distance between motor cortex and the level of the anterior horn cells ranged from 50.2 +/- 3.0 cm (mean +/- standard deviation) for the L1 segment to 54.4 +/- 3.6 cm for the L5 segment. The length of the motor roots from their exit from the myelon to their exit from the intervertebral foramen ranged from 10.3 +/- 1.7 cm in the L1 root to 17.5 +/- 1.9 cm in the L5 root. The central motor conduction velocity calculated for the distance motor cortex - anterior horn cells of the L5 segment was 50.1 +/- 4.5 m/s. The proximal peripheral conduction velocity of the motor nerve root between its exit from the spinal cord and its exit from the intervertebral foramen was 75.9 +/- 29.0 m/s. The overall conduction velocities between motor cortex and exit of the nerve roots from the intervertebral foramen were 57.4 +/- 6.3 m/s for the L4 fibers to the quadriceps femoris and 57.3 +/- 6.1 m/s for the L5 fibers to the anterior tibial muscle.