Electrophysiological aspects of peripheral nervous system development]

Different aspects of the maturation of the peripheral nervous system have been studied in infants, essentially during the first year of postnatal life. Absolute refractory periods which enable indirect estimation of excitability have been measured in both nerve and muscle fibers. It appears that already at birth, the absolute refractory period is characteristic for a given group of nerve fibres. In each group of nerve fibres, the absolute refractory period is not correlated to conduction velocity (ie fibre diameter) and remains rather constant during development. However, the absolute refractory period of the most excitable fibres is smaller than the absolute refractory period of motor fibres independent of the subject's age. On the other hand, the absolute refractory period of muscle fibres is slightly higher in premature in comparison with full-term neonates and adults. The diameter growth of different categories of nerve, fibers (motor--IA--non nociceptive cutaneous) is assessed by measurement of conduction velocities. At each age the IA fibre conduction velocities are the highest. The marked difference which exists between length development and postnatal increase of the fibre diameter explains the particular evolution of conduction times.