Efferent and afferent fibres in human sacral ventral nerve roots: basic research and clinical implications

Single extracellular action potentials have been recorded at 2 sites from human S4 ventral nerve roots, and their amplitude, duration and conduction time measured. Conduction velocity frequency distribution histograms have been constructed. Three classes of a-motoneurons could be identified, which had mean conduction velocity values of 61.5 (a1), 49 (a2) and 37.5 m/sec (a3) for a young adult at about 37 degrees C. The conduction velocity of single motor fibres has been correlated with its action potential amplitude and duration. The action potential amplitude increased and the duration decreased with the conduction velocity. Touch-stimulated and other afferences have been identified in these motor roots. The fastest afferents had about the same conduction velocity as the a1-motoneurons and the touch-stimulated afferents had conduction velocities of between 20 and 41 m/sec at about 34 degrees C. Also the amplitude of the afferent single unit potentials increased and the duration decreased with the conduction velocity. The electrophysiologically measured roots have been removed and morphologically analysed with the light and electron microscope. Nerve fibre diameter frequency distribution histograms have been constructed with respect to 4 myelin sheath thickness ranges. In the diameter histograms 3 a-motoneuron peaks with mean values of about 12.5 (a1), 10.3 (a2) and 8.3 microns (a3) and 1 peak of touch stimulated afferences with a mean value of 11.2 microns could be identified for myelin sheath thicknesses between 1.8 and 2.3 microns. A teased fibre dissection gave a factor of 100 between the internode length and the nerve fibre diameter. The electrophysiologic parameters have been correlated with the morphologic parameters. Approximate factors between the mean conduction velocities and the mean nerve fibre diameters of the a-motoneuron classes were 5.1 (a1), 4.85 (a2) and 4.4 m/sec/microns (a3) at about 37 degrees C. Comparable approximate conversion factors for group I and fastest touch-stimulated ventral root afferents were 4.5 (gr. I) and 3.5 m/sec/microns (touch). By comparing the number of nerve fibres of each class of motoneurons with the number of spontaneously occurring action potentials, it was found that the a3-motoneurons, most likely supplying the slow fatigue resistant muscle fibres, had the highest activation at rest. The existance of ventral root afferents has been discussed with respect to pain treatments by deafferentation and ventral root stimulation to improve the bladder function in paraplegia.(ABSTRACT TRUNCATED AT 400 WORDS)