Postnatal maturation of phrenic nerve in children

Abstract

The phrenic nerve at the pericardial level was examined postmortem in 17 children, ages 3 days to 8 years. Detailed macroscopic and histologic examination of the central nervous system in all patients disclosed no abnormalities. Quantitative developmental studies demonstrated that myelinated axons doubled in number from birth to age 1 year when a plateau was reached. The main period of growth in diameter of myelinated axons also corresponded to the first year when median diameters increased from 1.75 μm at 3 days of age to 3.0 μm by 8 months of age. Unmyelinated axons also grew significantly in the first 11 months when median diameters reached 1.4 μm. There was no significant increase in axonal diameter at later ages. The slope of the regression line for the number of myelin lamellae on axonal diameters increased with age until 6 months of age, whereas the dispersion around the regression lines decreased in the same period. This finding suggests a direct relationship between myelination and axonal growth. Significant maturation of the phrenic nerve occurs during the first year of life.