Nerve compound action potentials analysed with the simultaneously measured single fibre action potentials in humans

1. Compound action potentials (APs) and single fibre APs were recorded with two pairs of wire electrodes from an S5 root from a paraplegic patient during surgery. 2. Frequency distribution histograms of single nerve fibre conduction velocities were constructed and nerve fibre group conduction velocities compared with the conduction velocities of the peaks of the compound APs. By increasing the strength of the stimulation, the peaks in the compound AP could be identified, and the threshold order of efferent nerve fibre groups determined. Using additional literature data, it is likely that the primary spindle afferents have the lowest threshold due to electrical nerve root stimulation followed by the alpha 1-motoneurons (FF), the secondary muscle spindle afferents, the alpha 2-motoneurons (FR), the alpha 3-motoneurons (S), the gamma beta, gamma 1 (dynamic), gamma 21 (static), gamma 22 (static), and the parasympathetic motoneurons. 3. In first approximations, the Ap duration increases in the same way as the AP amplitude decreases with decreasing conduction velocity, and the area between the average single fibre AP curve and the base-line is the same for all single fibre APs with an average same distance to the recording electrodes in the root cross-section. By comparing the mean area of a single fibre AP with the areas of the peaks of the compound APs, it was found that 230 single fibre APs contributed to the compound AP. In the secondary spindle afferent fibre and alpha 2-motoneurons groups 53 fibres were stimulated (23%). The alpha 3-motoneuron peak and the afferents in the same velocity range contained 101 fibres (44%), the gamma beta peak contained 9 fibres (4%), the gamma 1 32 (14%), the gamma 21 23 (10%) and the gamma 22 12 (5%) fibres. Additionally two primary spindle afferents and two alpha 1-motoneurons most likely contributed to the compound AP. Since the large peaks in the compound APs did not change their area with increasing stimulation, most likely all muscle spindle afferents and alpha-motoneurons were activated to contribute to the compound AP. 4. Transfer functions of nerves and the stimulation with natural impulse patterns of the adequate afferents to spinal oscillators with respect to continence in paraplegia are discussed.