Neck rotation modulates flexion synergy torques, indicating an ipsilateral reticulospinal source for impairment in stroke

Abstract

Ellis MD, Drogos J, Carmona C, Keller T, Dewald JP. Neck rotation modulates flexion synergy torques, indicating an ipsilateral reticulospinal source for impairment in stroke. J Neurophysiol 108: 3096 -3104, 2012. First published September 5, 2012 doi:10.1152/jn.01030.2011.-The effect of reticular formation excitability on maximum voluntary torque (MVT) generation and associated muscle activation at the shoulder and elbow was investigated through natural elicitation (active head rotation) of the asymmetric tonic neck reflex (ATNR) in 26 individuals with stroke and 9 agerange-matched controls. Isometric MVT generation at the shoulder and elbow was quantified with the head rotated (face pointing) contralateral and ipsilateral to the paretic (stroke) and dominant (control) arm. Given the dominance of abnormal torque coupling of elbow flexion with shoulder abduction (flexion synergy) in stroke and welldeveloped animal models demonstrating a linkage between reticular formation and ipsilateral elbow flexors and shoulder abductors, we hypothesized that constituent torques of flexion synergy, specifically elbow flexion and shoulder abduction, would increase with contralateral head rotation. The findings of this investigation support this hypothesis. Increases in MVT for three of four flexion synergy constituents (elbow flexion, shoulder abduction, and shoulder external rotation) were observed during contralateral head rotation only in individuals with stroke. Electromyographic data of the associated muscle coactivations were nonsignificant but are presented for consideration in light of a likely underpowered statistical design for this specific variable. This study not only provides evidence for the reemergence of ATNR following stroke but also indicates a common neuroanatomical link, namely, an increased reliance on ipsilateral reticulospinal pathways, as the likely mechanism underlying the expression of both ATNR and flexion synergy that results in the loss of independent joint control. flexion synergy; strength; tonic neck reflex; asymmetric tonic neck reflex REACHING FUNCTION FOLLOWING STROKE can be profoundly impaired and has been historically described as being constrained, at least in part, to stereotypic multijoint movement patterns or synergies (Brunnstrom 1970;Foerster 1936;Twitchell 1951). Initial quantitative investigations thoroughly described the phenomena under isometric conditions highlighting the abnormal coactivation of brachialis, biceps brachii, and brachioradialis with deltoid (Dewald et al. 1995). The coactivation manifests as a "flexion synergy" and is significant for abnormal joint torque coupling of elbow flexion during shoulder abduc-

Deficits in startle-evoked arm movements increase with impairment following stroke

“…This phenomenon has been observed in other subcortical reflexes e.g. hypermetric stretch reflexes or spasticity (Krakauer, 2005, Levin, 1996, Zackowski et al, 2004) and a resurgence of the typically dormant asymmetric tonic neck reflexes following stroke (Ellis et al, 2012, Lee et al, 2009, Yamshon et al, 1949). Further, the task-inappropriate flexor activity shares several common features with the classic startle reflex.…”

Asymmetric tonic neck reflex and symptoms of attention deficit and hyperactivity disorder in children

“…An important postnatal developmental deficit that likely may have various etiological backgrounds is persisting of the so-called primitive reflexes that are in early developmental stages in postnatal periods replaced by higher motor and cognitive functions [4][5][6]. One of the particularly important primitive reflexes that may pathologically persist in later stages of development is asymmetric tonic neck reflex (ATNR), which typically occurs when turning of the head to one side causes an increase in the ipsilateral and a decrease in the contralateral extensor muscle tone and may be used as a diagnostic instrument to assess spasticity and cognitive functions related to certain specific movements [7,8].…”

Adaptive neuron-to-EMG decoder training for FES neuroprostheses

“…There is reason to believe that the flexor muscles receive disproportionate input from other sources, including the reticular formation [52]. This asymmetry is thought to underlie the characteristic flexor tone following stroke [53, 54]. The implications of this apparent disparity for cortical BMIs have not been directly investigated.…”