The Reversal of Neuromuscular Adaptation in Human with Spinal Cord Injury (II)

After activating Ia afferent by mechanical stretch or electrical stimulation, the monosynaptic reflex is suppressed up to ten seconds. This phenomenon is called post activation depression. Following injury to the spinal cord, the spinal circuit undergoes a series of adaptations. Diminishing amounts of post activation depression of monosynaptic reflex has been attributed to the spasticity syndrome. In parallel with the spinal circuit adaptation, the muscular properties also adapt due to immobilization after SCI. In human and animal studies, histochemical and physiological evidences showed that the paralyzed muscle transferred from slow, fatigue-resistant to fast, fatigable after injury. At present, the mechanism underlying post activation depression and its restoration after adaptations is a problem that awaits resolution. The actual site at which post activation depression in humans is yet known. Previous studies suggested that it might occur pre-synaptically, however, it differs from classic presynaptic inhibition, and short term inhibition may be related to the post-synaptic action of the Ranshaw cells. As invasive means of assessment is not possible in humans, non-invasive methods of assessment need to be developed. The neuromuscular adaptations after SCI are possibly due to the absolute immobilization below the level of injury. Passive motions and electrical stimulations are two methods to re-mobilize the paralyzed limbs. Past studies found that functional electrical stimulation could diminish the atrophy and reverse the fatigability, but failed to reverse the post activation depression. Conventional static stretch of muscle has instant effect on reduction of spasticity but not on the post activation functions. Recent animal studies showed that machine driven repeated stretching of muscle, which provided more Ia afferent input, could restore post activation depression. The purpose of this project is to investigate the site responsible for post activation depression in human and to investigate the effect of remobilization on the reversing the immobilization adaptation of human paralyzed muscle and spinal circuitries. In the previous year, we have established sensitive and reliable non-invasive neuromuscular adaptation indicators to study the excitability change of post-synaptic at segmental level in humans. The purposes of the next two years are as follows, In the first year, the post activation depression mechanism will be studied in individuals with spinal cord injury to avoid the influence of the suprasegmental level. The strength of depression will also be evaluated in individuals with and without SCI. In the second year, a rehabilitation program of machine driven passive stretch will be applied to individuals with spastic spinal cord injury and its effect on restoration of post activation depression, normalization of muscle tone, and influence of muscle property will be evaluated. The result of this project will advance the understanding of spinal circuitry adaptation and parallel muscular re-adaptation in humans after spinal cord injury. The result of this study can also be applied in clinical treatment.

Project ID:PC10301-0558
External Project ID:NSC102-2314-B182-021-MY2