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

Abstract

SUMMARY1. 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 (0): axon cross-sectional area area of a circle with same perimeter'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 a and y 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 y efferent fibres in one nerve were studied in a series of sections cut at 25 ,tm 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 0 varied widely from 1-0 (circular) to 0-5 or less from fibre to fibre within every functional group. However, the mean value of qS was less for y axons (0.68) than for a 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 qS between group I and a fibres. If values of qS < 0 5 were rejected, the difference between the between a and y fibres was still very highly significant. 5. The external perimeter (S) of a non-circular fibre differs from 2i times the diameter of a circle just enclosing the fibre (D). It is shown that S = 0 95 7TD for group I and II fibres, S = 0 90 nrD for a and group III fibres, and S = 0-85 iD for y fibres.6. The myelin period, or interperiod repeat distance, varied from 14 1 to 15-6 nm in different cats, implying radial shrinkage of the myelin sheath from 15 to 23% The myelin period in a particular cat was the same for several nerves, and the same for fibres in different functional groups.7. The possibility that repetitive firing of axons during fixation contributed to the varying degree of non-circularity is considered but rejected as unlikely.8. It is deduced that about 10 % radial shrinkage of the myelin sheath, but little or no osmotic shrinkage of the axon, occurred during fixation and rinsing. Further radial shrinkage of about 8 % in all components of the fibre probably occurred as a result of subsequen...

Ultrastructural dimensions of myelinated peripheral nerve fibres in the cat and their relation to conduction velocity

“…2. Values for all the fibres in each nerve are included in this and later figures save for those sectioned through a Schmidt-Lanterman incisure, or which deviated widely from circularity because of the proximity of the Schwann cell nucleus (see Arbuthnott et al 1980). Fibres cut through the region of paranodal folding are indicated by stars.…”

“…Afferent groups I, II and III tend to overlap considerably, however, even in nerves showing three distinct peaks in their afferent fibre-diameter histogram. Since fibres are rarely circular and vary in cross-sectional area along the internode (Berthold, 1968;Arbuthnott et al 1980) fibre diameter is not a very reliable criterion for the 127 1E. R. ARBUTHNOTT, I.…”

“…Details of the preparation of material for electron microscopical study of every fibre in each nerve, using the one-hole mount technique of Arbuthnott (1974), were given by Arbuthnott, Ballard, Boyd & Kalu (1980). Individual fibres were photographed at a nominal electron-microscopic magnification of 3000 or 5000 times for large fibres and 7500 times for small fibres.…”

“…Thus locs(-7lng)I. Arbuthnott et al (1980) showed that small axons are, in fact, more non-circular, on average, than large ones. This applied particularly to motor axons where the mean value of 0 was 0*79 for a axons and 0-69 for y axons, the difference being statistically highly significant.…”

“…Small fibres are significantly more non-circular than large fibres. From the quantitative data of Arbuthnott et al (1980) it is concluded that non-circularity may contribute to the fact that 0 cc s rather than 6 oc S, but cannot wholly account for it. Other possibilities considered are that axoplasmic resistivity or specific nodal conductance may differ for large and small fibres.…”