Title: Evidence for Increased Activation of Persistent Inward Currents in Individuals with Chronic Hemiparetic Stroke

Despite the prevalence of hyperactive stretch reflexes in the paretic limbs of individuals with chronic hemiparetic stroke, the fundamental pathophysiological mechanisms responsible for their expression remain poorly understood. This study tests whether the manifestation of hyperactive stretch reflexes following stroke is related to the development of persistent inward currents (PICs) leading to hyperexcitability of motoneurons innervating the paretic limbs. Because repetitive volleys of 1a afferent feedback can elicit PICs, this investigation assessed motoneuronal excitability by evoking the Tonic Vibration Reflex (TVR) of the biceps muscle in 10 awake individuals with chronic hemiparetic stroke and measuring the joint torque and electromyographic responses of the upper limbs. Elbow joint torque and the EMG activity of biceps, brachioradialis and the long and lateral heads of triceps were recorded during 8 seconds of 112Hz biceps vibration (evoking the TVR) and for 5 seconds after cessation of stimulation. Repeated measures ANOVA tests revealed significantly (p 0.05) greater increases in elbow flexion torque and EMG activity in the paretic as compared to the non-paretic limbs, both during and up to 5 seconds following biceps vibration. The finding of these augmentations exclusively in the paretic limb suggests that contralesional motoneurons may become hyperexcitable and readily invoke PICs following stroke. An enhanced tendency to evoke PICs may be due to an increased subthreshold depolarization of motoneurons, an increased monoaminergic input from the brainstem, or both.