Recruitment of motoneurons in the occasional firing mode in paraplegics.

1. Single-fibre action potentials were recorded from lower sacral nerve roots of two humans with thoracical spinal cord lesions. Conduction velocities were calculated, distribution histograms of representative conduction velocity frequencies constructed, and recruitment of motoneurons in the different groups was measured. 2. Following anal and bladder catheter pulling, slowly conducting gamma 1, alpha 3, alpha 2 and alpha 1-motoneurons were recruited before the faster conducting motoneurons in each group separately. Motoneurons were recruited repeatedly at approx. every 2.5 sec. 3. The slowly conducting alpha 2-motoneurons (FR) were recruited approx. 1 sec following stimulation, the slowly conducting alpha 3-motoneurons (S) were mostly recruited following stimulation. 4. Upon no additional stimulation, slowly and fast conducting motoneurons were recruited repeatedly within the groups of alpha 2 and alpha 3-motoneurons every 2.5 sec. 5. The faster recruitment of alpha 3-motoneurons in comparison to alpha 2-motoneurons differed from that observed during measurements in brain-dead humans and in rats and dogs, and is discussed with respect to the loss of interneurons in the spinal cord.