Examination of afterhyperpolarization duration changes in motoneurons innervating paretic muscles in stroke survivors

The after hyperpolarization (AHP) of a motoneuron is a primary determinant of motoneuron firing rate. Any increase in its duration or amplitude could alter normal motor unit (MU) firing rate properties in stroke, and potentially impact muscle force generation. The objective of this preliminary study was to examine potential differences in afterhyperpolarization (AHP) duration of motoneurons innervating paretic and contralateral limb muscles of hemiparetic stroke survivors. A novel surface EMG (sEMG) electrode was used to record from the first dorsal interosseous muscle (FDI) of three hemiparetic stroke survivors. sEMG data was decomposed to derive single motor unit (SMU) events, which were subsequently utilized to produce interval (ISI) histograms of the motor unit discharge. Interval Death Rate (IDR) analysis was then used to transform ISI histograms into death rate plots. [1] The prescribed IDR analysis method [1] involves a final transformation of death rate plots into an estimated AHP time course. The present study uses a modified method of interpreting death rate plots in order to determine AHP duration. AHP durations from this analysis are similar to durations obtained from ISI variability analysis. [2] Results from three subjects indicate that on average, motor units on the paretic side have a longer AHP duration than the contralateral side, potentially contributing to lower firing rates, and to less efficient force production in paretic muscles.