Tapering of human nerve fibres

To determine the tapering of human nerve fibres, rostral and caudal root pieces of cauda equina nerve roots were removed and nerve fibre diameter distributions were constructed for 4 myelin sheath thickness ranges for the two sites, and compared with each other. The reduction of the group diameter in the different alpha-motoneuron groups was 0.2 % per 13 cm. Accounting for systematic errors, there may be even less tapering. An identified single nerve fibre showed no tapering. Further, there is indication that gamma-motoneurons, preganglionic sympathetic and parasympathetic fibres and skin afferents also reduce their fibre diameter by 0.2 % per 13 cm or less. Consequently, a nerve fibre with a diameter of 10 microm would be reduced to approximately 9.8 microm at 1m from the cell soma. Preganglionic parasympathetic fibres were found to be represented in roots S1 to S5. At similar distances from the spinal cord, the mean diameter of ventral root alpha1-motoneuron (FF) axons increased from the thoracic towards the lumbo-sacral region before decreasing again in the lower sacral region. Usually no alpha1-motoneuron axons were found in S5 roots. The diameter distribution of unmyelinated nerve fibres of a ventral S5 root showed three peaks at 0.25, 0.95 and 1.2 microm. The unmyelinated fibres with diameters around 0.25 microm may represent parasympathetic fibres. In six selected areas of the ventral S5 root, 6.6 times more unmyelinated nerve fibres than myelinated fibres were found on the average.     

Pulse evolution on coupled nerve fibres

A numerical study of the coupled nerve fibre problem is given which verifies and extends the perturbation theory of Luzader. Pulses on adjacent fibres can couple together with two possible stable pulse separations.     

Selective fasciculation of nerve fibres in culture

Small aggregates of embryonic rat retina and perinatal rat sympathetic ganglia were put into culture and allowed to form axonal outgrowths. Neuritic outgrowths from adjacent sympathetic explants grew freely into one another and appeared to form common bundles; neurites from adjacent retinal explants showed a similar pattern of interaction. In contrast, when neurites from retinal and sympathetic explants confronted one another they showed a marked avoidance reaction. This response included the partial retraction of some axons, changes in the direction of their growth and, eventually, the formation of discrete bundles of a single kind of axon. In a second kind of experiment, single-cell preparations from retina and sympathetic ganglia were mixed and allowed to form aggregates. These were put into culture and the distribution of sympathetic fibres within the resulting outgrowth was detected by incubation with radioactive norepinephrine followed by radioautography. It was found that the sympathetic axons segregated from the retinal axons as they grew and formed separate bundles of predominantly one kind of fibre. It is concluded that selective fasciculation of nerve axons can occur in culture and we discuss some possible contributory mechanisms.     

Electromechanical coupling of waves in nerve fibres

The propagation of an action potential (AP) in a nerve fibre is accompanied by mechanical and thermal effects. In this paper, an attempt is made to build up a mathematical model which couples the AP with a possible pressure wave (PW) in the axoplasm and waves in the nerve fibre wall (longitudinal—LW and transverse—TW) made of a lipid bilayer (biomembrane). A system of differential equations includes the governing equations of single waves with coupling forces between them. The single equations are kept as simple as possible in order to carry out the proof of concept. An assumption based on earlier studies is made that the coupling forces depend on changes (the gradient, time derivative) of the voltage. In addition, it is assumed that the transverse displacement of the biomembrane can be calculated from the gradient of the LW in the biomembrane. The computational simulation is focused to determining the influence of possible coupling forces on the emergence of mechanical waves from the AP. As a result, an ensemble of waves (AP, PW, LW, TW) emerges. The further experiments should verify assumptions about coupling forces.     

Spider silk fibres in artificial nerve constructs promote peripheral nerve regeneration

Abstract.  Objective : In our study, we describe the use of spider silk fibres as a new material in nerve tissue engineering, in a 20-mm sciatic nerve defect in rats. Materials and methods : We compared isogenic nerve grafts to vein grafts with spider silk fibres, either alone or supplemented with Schwann cells, or Schwann cells and matrigel. Controls, consisting of veins and matrigel, were transplanted. After 6 months, regeneration was evaluated for clinical outcome, as well as for histological and morphometrical performance. Results : Nerve regeneration was achieved with isogenic nerve grafts as well as with all constructs, but not in the control group. Effective regeneration by isogenic nerve grafts and grafts containing spider silk was corroborated by diminished degeneration of the gastrocnemius muscle and by good histological evaluation results. Nerves stained for S-100 and neurofilament indicated existence of Schwann cells and axonal re-growth. Axons were aligned regularly and had a healthy appearance on ultrastructural examination. Interestingly, in contrast to recently published studies, we found that bridging an extensive gap by cell-free constructs based on vein and spider silk was highly effective in nerve regeneration. Conclusion : We conclude that spider silk is a viable guiding material for Schwann cell migration and proliferation as well as for axonal re-growth in a long-distance model for peripheral nerve regeneration.     

A comparative scanning electron-microscopical study of endoneurial collagen around normal mouse nerve fibres,nerve fibres following crush injury and nerve fibres of the dystonic mouse mutant (dt/dt)

Summary The endoneurial collagen sheath around teased nerve fibres following crush injury was studied by scanning electron microscopy and compared with uninjured sciatic nerve fibres and with fibres from the dystonic mutant mouse. Following crush injury the endoneurial collagen became more abundant than seen in untreated nerve fibres and formed large, separate and longitudinally oriented bundles. However, by four weeks post injury the sheath regained a normal external appearance. Mutant nerve fibres were also associated with more than the usual amount of collagen, but the sheaths were more disorganised, with a marked disorientation and irregular aggregation of collagen, and these abnormalities were not confined to obviously degenerating or demyelinated regions of the fibres. The dystonic abnormalities of the endoneurial sheath may be important in the mechanism of the neuropathy.Medical Research Council, Radiobiology Unit, Harwell, Didcot, Oxon, OX11 ORD     

Scanning electron microscopy of isolated peripheral nerve fibres

Summary The surface morphology of normal myelinated nerve fibres prepared in different ways for scanning electron microscopy has been studied and compared with the surface features of similar fibres undergoing retrograde changes. Nodes of Ranvier, paranodal specializations, artefactual fractures of the myelin, and the endoneurial collagen sheaths are described. A regular pattern of elevations, usually with a pitted or depressed surface seen on normal myelinated fibres after certain preparative procedures are thought to be artefacts produced during preparation and to be related to the neurokeratin network.Alterations in the surface structure of fibres central to long-standing nerve transections include irregular protuberances, serial surface corrugations and large swellings, all associated with demyelination. Fibres that have undergone retrograde degeneration consist of endoneurial tubes with focal swellings occupied by macrophages or myelin debris, together with fine unmyelinated and small myelinated regenerating axons. Strict centrifugal progression of myelination of regenerating axons was not observed.We thank Mr. R. A. Willis for his collaboration and for taking the SEM photographs of normal nerve fibres, and the Cambridge Scientific Instrument Co. Ltd. for permission to reproduce the SEM photographs of experimental nerve fibres. We also thank Dr. A. Boyde for access to his SEM and for helpful comments on interpretation of the scanning electron micrographs, Prof. J. Z. Young, Dr. P. K. Thomas, and Dr. R. H. M. King for discussion, and Messrs. P. Reynolds and D. Gunn for photography.A grant from the Muscular Dystrophy Group of Great Britain is gratefully acknowledged.     

Simultaneous demonstration of adrenergic and non-adrenergic nerve fibres

Summary A technique for simultaneous demonstration of adrenergic and non-adrenergic nerve fibres is described, using methylene blue staining and fluorescence microscopy after formaldehyde treatment. The procedure is applicable to whole mounts as well as to microtome sections.