Coactivity of secondary spindle afferents and alpha 2, alpha 3, gamma 1 and gamma 2-motoneurons innervating anal and urinary bladder sphincters in humans.

1. Mixed efferent action potential (Ap) activity of human dorsal lower sacral nerve roots which contained only a few alpha 1 and gamma beta-motoneurons was split up into single alpha 2, alpha 3 (extrafusal) and gamma 1,2-motoneuron (intrafusal) activities by recording with 2 pairs of wire electrodes, constructing conduction velocity frequency distribution histograms, identifying different nerve fibre groups by their conduction velocity distribution peaks and counting the number of occurring conduction velocities under a peak within certain distribution limits. alpha-motoneurons in the oscillatory firing mode were identified by their conduction velocity and firing pattern. The only existing secondary spindle afferents were identified by their continuous firing and their conduction velocity. 2. Following natural stimuli monosynaptic responses were not observed. 3. alpha 3-motoneurons (S-type) responded slowly and unspecifically and alpha 2-motoneurons (FR-type) responded faster, transiently and specifically in the occasional spike mode and oscillatory firing mode. 4. In the oscillatory firing mode alpha 2-motoneurons responded within 1 oscillation cycle period (110-170 msec) or after 2 sec and once additionally after about 4 sec. In the occasional spike mode activity levels of alpha 2-motoneurons increased within 0.5 sec after stimulation. 5. Activity levels of alpha 2 and alpha 3-motoneurons in the occasional spike mode varied between 0 and 3 Ap's/sec per fibre. In the continuous oscillatory firing mode the activity level of single alpha 2-motoneurons was in the range of 18 Ap's/sec and the one of a alpha 3-motoneuron was in the range of 30 Ap's/sec. 6. In the occasional spike mode alpha 2 and gamma 1-motoneurons showed coactivity following touch and pain application and the pulling of anal and bladder catheters. This coactivity lasted for about 3 sec, was not time locked and was not always in phase. 7. Secondary spindle afferents were transiently activated mainly by the gamma 1-motoneurons with a time course similar to that of the gamma 1-motoneurons. A very strong bladder catheter pull activated gamma 2-motoneurons over several seconds which resulted in a strong increase in secondary spindle afferent activity for more than 5 minutes (cumulative reaction). 8. The delay between secondary spindle afferent activity and gamma-motoneuron activity, mainly from gamma 1-fibres, was about 400 msec. Taking the conduction times of gamma 1-motoneurons and secondary spindle afferents into account, it was found that the spindle afferent activity lagged about 350 msec behind the gamma-motoneuron activity at the muscle spindle site.(ABSTRACT TRUNCATED AT 400 WORDS)