Axoplasmic flow in myelinated and unmyelinated nerves

[14C]Leucine was injected into the floor of the IVth ventricle of rabbits in order to label the axoplasmic flow of proteins in the efferent fibres of the vagus nerve. At intervals from 6 h to 180 days after injection, the rabbits were sacrificed and the cervical and abdominal vagus and recurrent laryngeal nerves removed. The radioactivity in the protein, lipid and low molecular weight fractions of the nerve segments was estimated by liquid scintillation counting. Two rates of axoplasmic flow were thus demonstrated in the abdominal vagus nerve; a fast rate of about 450 mm/day and a slow rate of 30 mm/day. This agrees very well with previous work. Peaks of activity in the recurrent laryngeal nerve corresponded to rates of 9 and 270 mm/day, but their level was low, being not significantly above sciatic nerve background levels. The number of efferent fibres in the two nerves and the corresponding axonal areas were calculated in order to estimate the amount of labelled material in the flow. The abdominal vagus was shown to contain 4-5 times as much label per unit area of axoplasm as the recurrent laryngeal. A difference was therefore demonstrated in the same anatomical nerve between myelinated fibres in the recurrent laryngeal and unmyelinated fibres in the abdominal vagus nerve, both in the rates of flow and in the amounts of material contained in the flow.