Evidence for Increased Activation of Persistent Inward Currents in Individuals With Chronic Hemiparetic Stroke

Abstract

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 (EMG) responses of the upper limbs. Elbow joint torque and the EMG activity of biceps, brachioradialis, and the long and lateral heads of triceps brachii were recorded during 8 s of 112-Hz biceps vibration (evoking the TVR) and for 5 s after cessation of stimulation. Repeated-measures ANOVA tests revealed significantly (P <or= 0.05) greater increases in elbow flexion torque and EMG activity in the paretic as compared with the nonparetic limbs, both during and up to 5 s 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 brain stem, or both.

Bilateral oscillatory hip movements induce windup of multijoint lower extremity spastic reflexes in chronic spinal cord injury

“…Much of the current evidence regarding alterations in intrinsic electrical properties and subsequent windup behavior following injury to the nervous system has focused on MNs (human: Gorassini et al 2004;McPherson et al 2008;Mottram et al 2009;animal: Bennett et al 1998aanimal: Bennett et al , 1998banimal: Bennett et al , 2001Lee and Heckman 2001;Li et al 2004). Indirect measurements from this study are consistent with a plateau-driven windup response for several reasons.…”

Origins of Abnormal Excitability in Biceps Brachii Motoneurons of Spastic-Paretic Stroke Survivors

“…As discussed in the following text, these novel findings together suggest that the lower-threshold for stretch reflexes observed in spastic-paretic stroke survivors, while at rest (Chardon et al 2008;Powers et al 1988), in addition to the enhanced tonic vibration reflex (TVR) (McPherson et al 2008) may not be attributable to increases in motoneuron PIC amplitude, yet rather the result of a low-level depolarizing synaptic drive to the resting motoneuron pool.…”

The effects of asymmetric tonic neck reflex during reaching movement following stroke: Preliminary results

“…Such abnormal co-activations have been shown between shoulder abductors with elbow flexors and shoulder adductors with elbow extensors [1,2,3,4,5]. One of the hypothesized neural mechanisms underlying these abnormal muscle co-contraction patterns is an increased reliance on brainstem pathways [6]. In animal studies, Magnus and de Kleijin [7] reported postural reflex patterns, i.e.…”