Title: “Characteristics of spinal reflex circuitry during standing in health, ageing, and stroke”
Supervisor: Dr. J. Timothy Inglis (Kinesiology)
Committee Members: Dr. Mark Carpenter (Kinesiology), Dr. Jean-Sébastien Blouin (Kinesiology)
Abstract: There are many interconnected networks in the nervous system that, as a whole, are responsible for the control of human movement and balance. Understanding these networks in health, ageing, and disease has a wide range of implications in balance and mobility, neuroprosthetics, robotics, and functional rehabilitation after neurological injury. One important facet of the distributed sensorimotor network is the circuitry in the spinal cord. Circuits in the spinal cord integrate sensory signals from the periphery and signals from the brain to ultimately control muscle activation and movement. The goal of this thesis is to expand our understanding of some basic circuits in the spinal cord involved in the processing of muscle and cutaneous sensory feedback to control the muscles used to stand. Study 1 develops a novel methodology to characterize somatosensory reflexes in leg muscles during standing by examining the association between noisy Achilles tendon vibration and ongoing muscle (and cortical) electrical activity. Using intramuscular (single motor unit) recordings, the second study examines differences in tendon vibration reflex characteristics between two homologous extensor muscles – soleus and medial gastrocnemius. Study 3 investigates how two important somatosensory channels interact at the spinal level: cutaneous signals from the foot sole and muscle stretch signals from Achilles tendon vibration. Next, it is well known that many components of the sensorimotor system change with healthy ageing. Therefore, study 4 aims to examine the influence of age on reflex strength and frequency characteristics, as well as the spinal integration of cutaneous and muscle sensory feedback. Finally, stroke disrupts descending signals from the brain and brainstem to spinal cord circuitry. Therefore, the final study of this thesis aims to examine how stroke, and the plasticity that occurs with motor recovery, affect the reflexive activation of muscle groups (i.e., synergies), as well as the expression of reciprocal reflex connections between opposing muscles (ankle flexors and extensors) during standing.