Constancy of central conduction delays during development in man: investigation of motor and somatosensory pathways.

1. A cross-sectional study has been performed on 457 normal subjects to determine changes in conduction delays with age in central and peripheral motor and somatosensory pathways to the upper limb. 2. Electromagnetic stimulation was used to investigate central and peripheral conduction in motor pathways from the cortex to biceps brachii and hypothenar muscles in 308 normal human subjects aged from 32 weeks gestation to 55 years. The responses were recorded in the surface electromyogram. 3. Somatosensory potentials evoked by electrical stimulation of the median nerve have been recorded at Erb's point and over the somatosensory cortex in 149 normal subjects aged from 34 weeks gestation to 52 years to determine central and peripheral somatosensory conduction delays. 4. The conduction delays in the central components of both motor and somatosensory pathways rapidly decrease over the first 2 years after birth and thereafter remain constant at adult values. 5. The conduction delays in the peripheral components of both motor and somatosensory pathways also decrease initially but then from the age of 5 years progressively increase in proportion to arm length. 6. The threshold stimulus intensity for evoking muscle responses following electromagnetic stimulation of the cortex is high initially and falls progressively until the age of 16 years. A linear relationship exists between the threshold intensity and height for the height range 70-180 cm. 7. The threshold stimulus intensities for exciting peripheral motor and somatosensory nerves decrease up to the age of 5 years and then reach a plateau. 8. The results support the conclusion, already reported in the literature that peripheral nerves attain maximum value for fibre diameter and conduction velocity at approximately 5 years of age. 9. In contrast, it is concluded that the maximum fibre diameters in both motor and somatosensory central pathways increase in proportion to height, leading to constant central conduction delays with growth.