Ipsilateral versus contralateral cortical motor projections to a shoulder adductor in chronic hemiparetic stroke: implications for the expression of arm synergies

Reticulospinal neurons in the pontomedullary reticular formation of the monkey (Macaca fascicularis)

Recent neurophysiological studies indicate a role for reticulospinal neurons of the pontomedullary reticular formation (PMRF) in motor preparation and goal-directed reaching in the monkey. Although the macaque monkey is an important model for such investigations, little is known regarding the organization of the PMRF in the monkey. In the present study, we investigated the distribution of reticulospinal neurons in the macaque. Bilateral injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) were made into the cervical spinal cord. A wide band of retrogradely labeled cells was found in the gigantocellular reticular nucleus (Gi) and labeled cells continued rostrally into the caudal pontine reticular nucleus (PnC) and into the oral pontine reticular nucleus (PnO). Additional retrograde tracing studies following unilateral cervical spinal cord injections of cholera toxin subunit B revealed that there were more ipsilateral (60%) than contralateral (40%) projecting cells in Gi, while an approximately 50:50 ratio contralateral to ipsilateral split was found in PnC and more contralateral projections arose from PnO. Reticulospinal neurons in PMRF ranged widely in size from over 50 microm to under 25 microm across the major somatic axis. Labeled giant cells (soma diameters greater than 50 microm) comprised a small percentage of the neurons and were found in Gi, PnC and PnO. The present results define the origins of the reticulospinal system in the monkey and provide an important foundation for future investigations of the anatomy and physiology of this system in primates.

Modulation of stretch reflexes of the finger flexors by sensory feedback from the proximal upper limb poststroke

Neural coupling of proximal and distal upper limb segments may have functional implications in the recovery of hemiparesis after stroke. The goal of the present study was to investigate whether the stretch reflex response magnitude of spastic finger flexor muscles poststroke is influenced by sensory input from the shoulder and the elbow and whether reflex coupling of muscles throughout the upper limb is altered in spastic stroke survivors. Through imposed extension of the metacarpophalangeal (MCP) joints, stretch of the relaxed finger flexors of the four fingers was imposed in 10 relaxed stroke subjects under different conditions of proximal sensory input, namely static arm posture (3 different shoulder/elbow postures) and electrical stimulation (surface stimulation of biceps brachii or triceps brachii, or none). Fast (300 degrees/s) imposed stretch elicited stretch reflex flexion torque at the MCP joints and reflex electromyographic (EMG) activity in flexor digitorum superficialis. Both measures were greatest in an arm posture of 90 degrees of elbow flexion and neutral shoulder position. Biceps stimulation resulted in greater MCP stretch reflex flexion torque. Fast imposed stretch also elicited reflex EMG activity in nonstretched heteronymous upper limb muscles, both proximal and distal. These results suggest that in the spastic hemiparetic upper limb poststroke, sensorimotor coupling of proximal and distal upper limb segments is involved in both the increased stretch reflex response of the finger flexors and an increased reflex coupling of heteronymous muscles. Both phenomena may be mediated through changes poststroke in the spinal reflex circuits and/or in the descending influence of supraspinal pathways.

Multijoint reflex responses to constant-velocity volitional movements of the stroke elbow

Multijoint reflex coupling could impact the voluntary control of functional arm movements in people post stroke. The multijoint responses to active-assist, constant-velocity movements of the elbow joint were measured in 14 individuals post stroke and 9 neurologically intact controls. Resulting responses in the stroke group illustrated a change in the reflex coupling of the elbow and shoulder muscles compared with passive perturbations of the spastic elbow. Voluntary effort during constant-velocity elbow extension resulted in reflex shoulder abduction, differing from the reflex coupling observed between the elbow flexors and shoulder adductors observed during passive elbow extension. These results suggest that post stroke, voluntary drive alters reflex coupling of the elbow and shoulder. Flexion of the elbow during active-assist also resulted in reflex coupling. Shoulder abduction torque decreased with constant-velocity flexion of the elbow; however, no net adduction was observed at the end of the perturbation. Shoulder flexion/extension and internal/external rotation torque responses demonstrated similar modulations to imposed active-assist perturbations of the elbow in subjects post stroke. Responses were absent during passive perturbations of the control elbow; however, shoulder torque modulations were observed during constant-velocity, active-assist tasks. The active-assist response patterns in controls were similar to stroke subjects during the extension task but opposite during flexion of the elbow. This study provides evidence of a neural coupling between elbow and shoulder muscles and a modulation of this coupling during voluntary drive of the spastic arm.

The use of visual feedback, in particular mirror visual feedback, in restoring brain function

This article reviews the potential use of visual feedback, focusing on mirror visual feedback, introduced over 15 years ago, for the treatment of many chronic neurological disorders that have long been regarded as intractable such as phantom pain, hemiparesis from stroke and complex regional pain syndrome. Apart from its clinical importance, mirror visual feedback paves the way for a paradigm shift in the way we approach neurological disorders. Instead of resulting entirely from irreversible damage to specialized brain modules, some of them may arise from short-term functional shifts that are potentially reversible. If so, relatively simple therapies can be devised--of which mirror visual feedback is an example--to restore function.