Characterization of the postnatal development of superior laryngeal nerve fibers in the postnatal kitten.

A combined electron microscopic and electrophysiological study of the superior laryngeal nerve (SLN) was undertaken in postnatal kittens ranging in age from 1-63 days. The superior laryngeal nerve is predominantly a sensory nerve innervating the upper respiratory tract, and could play a potential role in the modulation of respiration, particularly in the infant animal. Distribution of fibers in the developing SLN indicates that within the first postnatal month, 75% of the fibers are unmyelinated, and by 42 days, the myelinated fibers increase in number to approximately 50%. Of the myelinated fibers present in the one day old kitten, 3-4% of those exceeded 4 mum in total diameter, which is the minimum diameter for normal conduction velocity of action potentials. The distribution of the diameter sizes of the myelinated fibers is bell-shaped within the first 45 days after which the curve becomes skewed to the right (43-61 days; mean 2.6 mum, range 0.5-8.0 mum) to resemble the adult distribution of myelinated fibers (mean 4.2 mum, range 1.6-13.0 mum). Two variable plots of myelin width to axon diameter suggest a steeper slope for developing fibers as compared to that of the adult fibers. Electrical stimulation of the sectioned SLN indicates that evoked potentials could be recorded from the recurrent laryngeal nerve innervating the laryngeal intrinsic muscles and from the hypoglossal nerve to the tongue musculature in the youngest kittens tested (i.e., age 9 days). Stimulation at selected frequencies of 3 and 30/sec readily evoked apnea in the youngest kitten studied (i.e., age 5 days), while swallowing was more readily evoked at 28-30 days when using electrical stimulation.