Title: “The effects of periodic vs aperiodic muscle spindle afferent input on stretch reflex circuitry: the role of intent to responds to a perturbation”
Thesis Supervisor: Dr. J. Timothy Inglis
Committee Members: Dr. Romeo Chua, Dr. Mark Carpenter
Chair: Dr. Jean-Sebastien Blouin
Abstract: Muscle spindles lie in parallel with skeletal muscle and encode changes in both muscle length and velocity. The muscle spindles are innervated by large group Ia afferents, which project into the spinal cord to synapse with motoneurons, interneurons, and project to subcortical and cortical areas in the central nervous system. When a muscle is rapidly stretched, distinct muscle responses can be seen in the electromyography (EMG). In the flexor carpi radialis, the first response, known as the short latency response occurs between ~25-50ms, which is followed by a longer latency response between 50-100ms. The short latency response is mediated via monosynaptic and short polysynaptic spinal circuits, while the long latency response involves more complex circuits, with involvement from both long spinal circuits and transcortical loops. It is well known that if a participant is instructed to respond to a perturbation as fast as possible, there is a significant increase in the amplitude of the long latency response, but not the short latency response.
Muscle tendon vibration, evokes strong Ia afferent responses, and can been used to activate the Ia afferent pathway. Prolonged muscle vibration, and subsequent afferent volleys, produces three robust effects: proprioceptive illusions, tonic vibration reflexes, and a suppression of the stretch reflex circuit. This thesis will focus on the suppression of the stretch reflex.
The purpose of the current thesis is to investigate how periodic vs aperiodic afferent volleys influence the stretch reflex circuit, and how the role of intent to respond may influence this effect. Specifically, we will be using periodic and aperiodic stimuli to evoke Ia afferent responses in the flexor carpi radialis muscle, during rapid wrist extension perturbations. We hypothesize that the aperiodic stimulus will evoked less suppression of the stretch reflex circuit compared to the periodic, as periodic afferent volleys may increase the probability of non-linear phase locking of the motoneuron output, while aperiodic afferent volleys would not. Additionally, we hypothesize that a participants intent to respond to the perturbation will reduce the amount of suppression seen.
The results from this thesis will provide deeper insight in the spinal cord processing of muscle spindle afferent input, and how the central nervous system can modulate the muscle spindle afferent pathways.