Changes in spinal motoneuron "fastness" in post-stroke spastic patients

Motor unit (MU) action potentials trains were recorded from brachial biceps of 11 post-stroke patients (Brunstrom stage 2-4) and 8 healthy volunteers. The variability analysis of interspike intervals (ISIs), aimed at the comparison of the duration of afterhyperpolarization (AHP) in motoneuron (MN), was performed on MUs of 39 controls and 64 patients. Our results concerning MU discharge characteristics conformed in general to the previous reports, showing positive serial correlation coefficients and lower MU firing rates in patients as compared with control subjects. We have found, however, that the positive correlation coefficients result from trends in MU firing rates related to less efficient control of the muscle force. ISI variability determined in short-interval range was the same in patients and control subjects. The transition intervals of variability-mean ISI characteristics, which were previously shown to correlate with AHP duration, were significantly longer in patients, but the prolongation decreased with patient's age and disease duration. Our results indicate that the spinal MNs respond to the cerebral stroke with prolongation of AHP duration, which tends to recover after the accident. These changes are less pronounced in older patients, presumably due to decreased MN plasticity. We conclude that the match between MN and muscle properties is preserved after stroke.