INTERSPIKE INTERVALS OF SECONDARY MUSCLE-SPINDLE AND URINARY-BLADDER AFFERENTS IN RELATION TO THE OSCILLATION PERIODS OF SACRAL SPINAL OSCILLATORS FOR CONTINENCE IN MAN

Two pairs of wire electrodes were used to record afferent and efferent single fibre extracellular action potentials (APs) from human nerve r oot filaments. The nerve fibres were identified according to the group to which they belong by comparing the afferent and efferent conductio n velocity distribution histograms and identifying peaks and ranges of nerve fibre groups. Secondary muscle spindle afferents and alpha(2)-m otoneurones (FR) were identified by having the same peak group conduct ion velocity (calibration relation), which is 50 m/s at 36 degrees C. On the basis of AP wave form comparisons, the natural impulse patterns of five secondary muscle spindle afferents, two fusimotor motoneurone s and two oscillatory firing alpha(2)-motoneurones could be identified in the dorsal S4 root, The patterns of single endings of secondary sp indle afferent fibres could be identified. The shortest interspike int ervals of single endings of all secondary muscle spindle afferents had the same duration as the shortest interspike intervals of the two fus imotor fibres (80 ms) and equalled a half of the oscillation period of one repetitively firing alpha(2)-motoneurone (6 Hz) probably innervat ing the external anal sphincter (three AP impulse train firing). In an other more rostral dorsal root filament (probably S3 or S2) of the sam e human, the interspike intervals of six secondary spindle afferents w ere more variable. The values of peaks in the interspike interval dist ributions ranged from 60 to 102 ms. In the coccygeal root, the intersp ike interval duration ranged from 160 to 185 ms, directly contributing to the drive of the oscillatory firing alpha(2)-motoneurone. The diff erent agreement between the oscillation period and the interspike inte rvals of the spindle afferents in different segments indicate that the oscillatory firing CNS circuitry was localized within S3 to S5 segmen ts of the conus medullaris for the drive of the anal sphincter. An alp ha(2)-motoneurone firing repeatedly with 1 to 2 AP impulse trains, inn ervating most likely the external urethral sphincter, fired at a frequ ency of 9.1 to 6.7 Hz, a similar frequency of the oscillation as the i nterspike intervals from two activated stretch receptors of the urinar y bladder wall. The measurements of this brain-dead human indicates th at in this case the neuroneal circuitry driving the external anal sphi ncter was mainly confined to the sacral micturition and defecation cen tre, mainly located in the S3 to S5 segments.