Two pairs of wire electrodes were used to record afferent and efferent single fibre extracellular action potentials (APs) from human nerve root filaments. The nerve fibres were identified according to the group to which they belong by comparing the afferent and efferent conduction velocity distribution histograms and identifying peaks and ranges of nerve fibre groups. Secondary muscle spindle afferents and a2-motoneurones (FR) were identified by having the same peak group conduction velocity (calibration relation), which is 50 m/s at 36 °C. On the basis of AP wave form comparisons, the natural impulse patterns of five secondary muscle spindle afferents, two fusimotor motoneurones and two oscillatory firing a2-motoneurones could be identified in the dorsal S4 root. The patterns of single endings of secondary spindle afferent fibres could be identified. The shortest interspike intervals of single endings of all secondary muscle spindle afferents had the same duration as the shortest interspike intervals of the two fusimotor fibres (80 ms) and equalled a half of the oscillation period of one repetitively firing a2-motoneurone (6 Hz) probably innervating the external anal sphincter (three AP impulse train firing). In another more rostral dorsal root filament (probably S3 or S2) of the same human, the interspike intervals of six secondary spindle afferents were more variable. The values of peaks in the interspike interval distributions ranged from 60 to 102 ms. In the coccygeal root, the interspike interval duration ranged from 160 to 185 ms, directly contributing to the drive of the oscillatory firing a2-motoneurone. The different agreement between the oscillation period and the interspike intervals of the spindle afferents in different segments indicate that the oscillatory firing CMS circuitry was localized within S3 to S5 segments of the conus medullaris for the drive of the anal sphincter. An a2-motoneurone firing repeatedly with 1 to 2 AP impulse trains, innervating most likely the external urethral sphincter, fired at a frequency of 9.1 to 6.7 Hz, a similar frequency of the oscillation as the interspike intervals from two activated stretch receptors of the urinary bladder wall. The measurements of this brain-dead human indicates that in this case the neuroneal circuitry driving the external anal sphincter was mainly confined to the sacral micturition and defecation centre, mainly located in the S3 to S5 segments.