The fibre spectrum of the chorda nerve in the chicken (Gallus gallus var. domesticus).

Using transmission electron microscopy, the numbers and diameters of all nerve fibres were studied in the chorda tympani nerve of the domestic fowl. The mean diameter of the whole nerve was 64.6 micron and contained 203-407 (mean 302) myelinated and 205-345 (mean 265.6) unmyelinated fibres. Of the myelinated fibres (including sheath) 95% were in the range 0.5-3 micron in diameter and 94% of the unmyelinated fibres ranged from 0.2 to 0.9 micron in diameter. After section of the facial nerve within the facial canal, there was a marked reduction in the number of myelinated fibres present in the chorda tympani nerve (range 12-160, mean 103) but a less marked reduction in the unmyelinated fibres (range 94-362, mean 223). The loss of myelinated fibres was not confined to any one fibre size whereas the loss of unmyelinated fibres was mainly in the smaller fibres (range 0.1-0.5 micron). Since facial nerve section results in degeneration of efferent fibres it would seem that a large proportion of the afferent fibres are unmyelinated.