title: Changes in motoneuron afterhyperpolarization duration in stroke survivors.
abstract: Hemispheric brain injury resulting from a stroke is often accompanied by muscle weakness in limbs contralateral to the lesion. In the present study, we investigated whether weakness in contralesional hand muscle in stroke survivors is partially attributable to alterations in motor unit activation, including alterations in firing rate modulation range. The afterhyperpolarization (AHP) potential of a motoneuron is a primary determinant of motoneuron firing rate. We examined differences in AHP duration in motoneurons innervating paretic and less impaired (contralateral) limb muscles of hemiparetic stroke survivors as well as in control subjects. A novel surface EMG (sEMG) electrode was used to record motor units from the first dorsal interosseous muscle. The sEMG data were subsequently decomposed to derive single-motor unit events, which were then utilized to produce interval (ISI) histograms of the motoneuron discharges. A modified version of interval death rate (IDR) analysis was used to estimate AHP duration. Results from data analyses performed on both arms of 11 stroke subjects and in 7 age-matched control subjects suggest that AHP duration is significantly longer for motor units innervating paretic muscle compared with units in contralateral muscles and in units of intact subjects. These results were supported by a coefficient of variation (CV) analysis showing that paretic motor unit discharges have a lower CV than either contralateral or control units. This study suggests that after stroke biophysical changes occur at the motoneuron level, potentially contributing to lower firing rates and potentially leading to less efficient force production in paretic muscles.
authors.affiliation: Sensory Motor Performance Program, Rehabilitation Institute of Chicago, Chicago, Illinois; aneesha@uchicago.edu.
authors.forename: Aneesha K
authors.initials: AK
authors.lastname: Suresh
authors.affiliation: Sensory Motor Performance Program, Rehabilitation Institute of Chicago, Chicago, Illinois;
authors.forename: Xiaogang
authors.initials: X
authors.lastname: Hu
authors.affiliation: Department of Physiology and Biophysics, University of Washington, Seattle, Washington;
authors.forename: Randall K
authors.initials: RK
authors.lastname: Powers
authors.affiliation: Department of Physical Medicine and Rehabilitation, Feinberg School of Medicine, Northwestern University, Chicago, Illinois; Department of Physical Therapy and Human Movement Sciences, Northwestern University, Chicago, Illinois; Department of Physiology, Northwestern University, Chicago, Illinois; and.
authors.forename: C J
authors.initials: CJ
authors.lastname: Heckman
authors.affiliation: Sensory Motor Performance Program, Rehabilitation Institute of Chicago, Chicago, Illinois;
authors.forename: Nina L
authors.initials: NL
authors.lastname: Suresh
authors.affiliation: Department of Physical Medicine and Rehabilitation, Feinberg School of Medicine, Northwestern University, Chicago, Illinois; Department of Physical Therapy and Human Movement Sciences, Northwestern University, Chicago, Illinois; Department of Biomedical Engineering, Northwestern University, Chicago, Illinois.
authors.forename: William Zev
authors.initials: WZ
authors.lastname: Rymer
mesh_terms: D000200:Action Potentials; D000368:Aged; D016022:Case-Control Studies; D005260:Female; D006801:Humans; D008297:Male; D008564:Membrane Potentials; D008875:Middle Aged; D009046:Motor Neurons / Q000502:physiology*; D018482:Muscle, Skeletal / Q000294:innervation / Q000503:physiopathology*; D010291:Paresis / Q000503:physiopathology; D020521:Stroke / Q000150:complications / Q000503:physiopathology*
keywords: afterhyperpolarization; first dorsal interosseous; motor units; paresis; stroke