Electrophysiological assessment of human spinal circuits

Abstract

Assessing the function of spinal cord circuits non-invasively in humans is crucial both to understand healthy function and pathology. This thesis focuses on the assessment of different human spinal circuits through techniques such as peripheral nerve stimulation, transcranial magnetic stimulation (TMS) and intermuscular coherence with the aim of understanding their function in health and disease. The thesis investigates human spinal circuits in five studies. 1. An experimental protocol is developed measuring of the flexor carpi radialis (FCR) H-reflex conditioned by electrical stimulation of the extensor carpi radialis (ECR) muscle. At the interval of 30ms between the two stimuli there was a clear facilitation of the H-reflex and at 70ms interval a clear suppression. Several human and monkey experiments revealed that the facilitation was generated by a spinal circuit with convergent input from FCR afferents and ECR Ib afferents to the wrist flexor. 2. In stroke survivors the facilitation at 30ms was similar to in healthy subjects while the later inhibition was weak or absent, which could represent either excessive facilitation or loss of inhibition. 3. Possible descending inputs to these spinal circuits were investigated using TMS to activate the corticospinal tract and click sounds to activate the reticulospinal tract. 4. Intermuscular coherence between lower limb muscles was measured during natural spasms in spinal cord injury patients. Results showed that the spinal cord is capable of producing intermuscular coherence at low frequencies. 5. The ability of peripheral nerve stimulation to reduce tremor by producing plastic changes at the spinal cord level were explored by optimizing phase cancellation of 10 Hz oscillations in motor cortex and spinal cord.