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

ObjectivesTo test the hypothesis that the firing rates and discharge variability of human muscle spindles are not affected by spinal cord injury.MethodsTungsten 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.ResultsFiring 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.ConclusionsThe 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.SignificanceThis 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.

► For the first time, microneurographic recordings of muscle spindle afferents have been obtained from the paralyzed leg muscles of patients with spinal cord injury. ► Compared to intact individuals, there were no differences in either the ongoing firing rates or discharge variability of spontaneously active muscle spindle endings, nor were there overt differences in the responses to manually-applied passive stretch. ► These results argue against any increase in static (and possibly dynamic) stretch sensitivity of muscle spindle endings as a contributing factor to spasticity.