Degradation of neuronal function following a spinal cord injury: mechanisms and countermeasures

Abstract

The aim of this study was to evaluate the course of spinal neuronal activity following spinal cord injury (SCI). In patients with a complete SCI, the leg muscle EMG activity early and up to 33 years after an SCI was analysed during locomotor movements induced and assisted by a driven gait orthosis (DGO). Only in chronic SCI patients did a premature exhaustion of neuronal activity occur. This was reflected in a reduced density and fading of leg muscle EMG activity. The early exhaustion of EMG activity was more pronounced in the leg flexor (e.g. biceps femoris) than extensor (e.g. gastrocnemius) muscles. The timing of the leg muscle pattern remained unchanged in the chronic patients. A preserved amplitude of motor action potentials following repetitive peripheral nerve stimulation and during spasms indicated an interneuronal site of impairment. In patients who participated in a locomotor training programme lasting up to 13 weeks, no positive effect on the slope of exhaustion was seen. It is concluded that a degradation of spinal neuronal activity takes place following an SCI. If in the future regeneration of spinal tract fibres becomes feasible in patients with complete SCI, such an approach can only become functionally successful if neuronal activity below the level of the lesion is maintained. This might be achieved by a continuous training approach starting early after injury.

Neuronal dysfunction in chronic spinal cord injury

“…However, the exhaustion phenomenon could not yet be reversed by a locomotor training in the chronic stage of a motor complete SCI. 22 Preliminary observations indicate that the neuronal dysfunction is partially reversible in non-ambulatory AIS C subjects by locomotor training combined with functional electrical stimulation (unpublished observation). In fact, an improvement of ambulatory function through intensive locomotor training can be achieved in chronic incomplete SCI subjects.…”

“…36 Two observations support this assumption: first, the muscle action potentials and H-reflexes do not change in amplitude during repetitive nerve stimulation 22,37 and second, EMG activity of all leg muscles became enhanced during spasms induced by stumbling, despite of an exhaustion of locomotor activity at the end of a training session. 22 Corresponding to the EMG exhaustion phenomenon during assisted walking, SR behavior undergoes changes too insofar that a second late SR component can be evoked by tibial nerve stimulation, while the amplitude of the early component decreases. 21 There obviously exists a temporal relationship between the decrease of the early SR component, the increase of the late SR component and the degree of exhaustion of locomotor activity (see Figure 2).…”

“…22 This phenomenon called 'EMG exhaustion' cannot be reversed by locomotor training and is more pronounced in the leg flexor than in the extensor muscles. So far, no condition in animal models is known that corresponds to the phenomenon of EMG exhaustion in human SCI.…”

“…5 Studies with chronic complete spinal cord injury (cSCI) subjects during the past few years provided some evidence that the function of spinal neuronal circuits below the level of the lesion is impaired. 21,22 The purpose of this review is to depict the behavior of spinal neuronal circuits deprived from appropriate supraspinal and afferent input and to discuss potential countermeasures to prevent neuronal dysfunction below the level of lesion in the chronic stage of SCI.…”

The effect of timing electrical stimulation to robotic-assisted stepping on neuromuscular activity and associated kinematics

“…Thus, we used EMG measures as one indicator of plasticity, in particular at the level of the motor pool. EMG patterns recorded from hind-limb muscles have been shown to reflect training-enhanced plasticity [57], and EMG behavior has been accepted as a representation of damage to or recovery of the neuromotor control that underlies muscle output [58][59][60][61]. The EMG activation profiles of the FES+RTT group showed more consistent timing of muscle activation to the swing phase of the gait cycle than those of the RS group.…”