Ontogenesis of lumbar spinal somatosensory evoked potentials after posterior tibial nerve stimulation in the rat.

Although data are available regarding lumbar spinal somatosensory evoked potentials (SSEPs) after posterior tibial nerve stimulation in the mature rat, age-related changes spanning the period of early development have not been defined. We obtained lumbar spinal SEPs after posterior tibial nerve stimulation in 6 cohorts of animals (N = 36) ranging in age from weanling (15 days) to early adulthood (110 days) by recording from needle electrodes placed in the L1-2 and L5-6 interspinous ligaments. Absolute latencies of the major wave form components at the two recording sites declined rapidly until the mid-juvenile period (36 days) and more slowly thereafter. Mean peripheral conduction velocities (+/- S.D.) increased from 11.06 (+/- 0.02) to 33.22 (+/- 3.55) m/sec and mean central conduction velocities (+/- S.D.) increased from 6.27 (+/- 1.07) to 23.64 (+/- 3.84) m/sec from 15 to 110 days respectively. The linear relationships of central and peripheral conduction velocities to both age and weight as defined by standard regression were highly significant. No sex differences were noted for peripheral velocities at all ages studied. Central velocities revealed significant sex differences at 110 days but not earlier. This study demonstrates that lumbar spinal SEPs after posterior tibial nerve stimulation undergo a predictable evolution which can be represented by a simple cable model of a lengthening myelinated pathway.