Quantitative study of the non-circularity of myelinated peripheral nerve fibres in the cat

• Published in: The Journal of physiology 1980-11, Vol.308 (1), p.99-123
• Main Authors: Arbuthnott, E.R, Ballard, K.J, Boyd, I.A, Kalu, K.U
• Format: Article
• Language: English
• Subjects: animal physiology; Animals; Axons - ultrastructure; Cats; Hindlimb - innervation; Histological Techniques; Microscopy, Electron; Muscles - innervation; Nerve Fibers, Myelinated - physiology; Nerve Fibers, Myelinated - ultrastructure; Neural Conduction; Neurons, Afferent - ultrastructure; Neurons, Efferent - ultrastructure; Schwann Cells - ultrastructure
• ISSN: 0022-3751; 1469-7793
• DOI: 10.1113/jphysiol.1980.sp013464

1. One hind limb of each of four cats was either chronically de-efferentated, or chronically de-afferentated, and perfused with buffered glutaraldehyde fixative. Up to three different muscle nerves were dissected from each limb, post-fixed in osmium tetroxide and embedded in Epon. Ultrathin transverse sections were mounted on Formvar-coated single-hole specimen grids so that all the fibres in each nerve could be examined individually by electron microscopy. 2. Non-circularity was expressed as the ratio (ø): [Formula: see text] The degree of non-circularity of all the afferent axons, or all the efferent axons, in each muscle nerve was determined. The proportion of fibres cut through the paranodal region, or through the Schwann cell nucleus, was as expected for group I afferent and for α and γ efferent fibres, but hardly any typical paranodal sections of group II or III afferent fibres were encountered which suggests that their paranodal arrangement differs from that of other groups. In a quantitative comparison of noncircularity in different functional groups, fibres cut through paranodes, Schwann cell nuclei or Schmidt-Lanterman clefts were rejected. 3. All the γ efferent fibres in one nerve were studied in a series of sections cut at 25 μm intervals. The degree of non-circularity was found to be relatively constant along the internode of most fibres when the values at paranodes, Schwann cell nuclei or Schmidt-Lanterman clefts were ignored. 4. The value of ø varied widely from 1·0 (circular) to 0·5 or less from fibre to fibre within every functional group. However, the mean value of ø was less for γ axons (0·68) than for α axons (0·78), and less for group III axons (0·79) than for axons in groups I and II (both 0·84). When the results for all the nerves were aggregated, these differences were statistically very highly significant, as was the difference in ø between group I and α fibres. If values of ø < 0·5 were rejected, the difference between the mean ø for group III and group II was then of doubtful significance whereas that between α and γ fibres was still very highly significant. 5. The external perimeter ( S ) of a non-circular fibre differs from π times the diameter of a circle just enclosing the fibre ( D ). It is shown that S = 0·95 π D for group I and II fibres, S = 0·90 π D for α and group III fibres, and S = 0·85 π D for γ fibres. 6. The myelin period, or interperiod repeat distance, varied from 14·1 to 15·6 nm