Absolute refractory period of the ulnar nerve motor fibers in the human newborn.

The absolute refractory period (ARP) of the most excitable motor fibers of the human ulnar nerve was measured in 12 full-term newborns (1-8 days old) and 15 adults. The mean calculated diameter (Hursh's equation) of the adult motor fibers was twice as large as that of the newborn fibers. The inverse relationship demonstrated in cat between the fibers' conduction velocity and the ARP is not confirmed in the present results. ARP, independently of the myelination process could be a specific membrane property of any given fiber with respect to the expected adult diameter.