Discharge rates and discharge variability of muscle spindle afferents in human chronic spinal cord injury

The aim of this study was to test the hypothesis that the firing rates and discharge variability of human muscle spindles are not affected by spinal cord injury. Tungsten microelectrodes were inserted into muscle fascicles of the peroneal nerve in six individuals with complete paralysis of the lower limbs following spinal cord injury: 12 afferents were spontaneously active at rest and 7 were recruited during passive muscle stretch. For comparison, recordings were made from 17 spontaneously active and 9 stretch-recruited afferents in 12 intact subjects. Firing rates for the spontaneously active muscle spindles were not significantly different between the spinal (9.8 +/- 1.6 HZ) and intact (10.2 +/- 1.3 HZ) subjects; the same was true for the stretch-recruited afferents - static firing rates, measured over the final 1 s of a ramp-and-hold stretch, were not different between the spinal and intact groups (13.1 +/- 3.1% vs 10.0 +/- 2.5 HZ). There were also no differences in discharge variability between the spinal and intact subjects, either for the spontaneously active spindles (8.1 +/- 2.0% vs 5.7 +/- 0.9%) or for the stretch-activated spindles, calculated over the final 1 s of static stretch (19.7 +/- 5.6% vs 17.0 +/- 1.9%). In addition, the responses to stretch imposed manually by the experimenter provided no evidence for an increase in the dynamic response to stretch in the patients. The static stretch sensitivity of human muscle spindles is not affected by chronic spinal cord injury, suggesting that there is no difference in static (and possibly dynamic) fusimotor drive to paralyzed muscles in chronic spinal cord injury. This study provides no evidence for an increase in fusimotor drive as a mechanism for the spasticity associated with chronic spinal injury, though further studies using controlled stretch would be required before it can be concluded that dynamic fusimotor drive is “normal” in these patients.