Investigating the Sophistication of Long-Latency Stretch Responses During Postural Control of the Upper Limb
Pruszynski, Jedrzej (Andrew)
Motor Control , Reflex , Feedback , Motor Cortex
A recent theory of motor control, based on optimal feedback control, posits that voluntary motor behaviour involves the sophisticated manipulation of sensory feedback. Although this theory can explain how people move in the world, it does not specifically describe how this control process is implemented by the nervous system. In this thesis, we propose and explore one physiological implication of this theory. Specifically, we hypothesize that rapid feedback responses should possess the key functional attributes of voluntary control because these two systems share a common neural pathway through motor areas of cerebral cortex. Our first four studies were designed to elaborate the functional attributes of the long-latency stretch reflex, a fast feedback response which occurs 50-100ms following the mechanical stretch of a muscle. Consistent with our hypothesis, we found that the long-latency response possesses many attributes commonly reserved for voluntary control: the long-latency response is continuously modulated by subject intent (Chapter 2), it compensates for the size-recruitment principle of the motoneuron pool (Chapter 3) and it accounts for the mechanical properties of the upper-limb (Chapter 5). Further investigation revealed that the long-latency response can be decomposed into two functionally-independent processes (Chapter 4), and that one of these components contributes all of the sophistication observed in Chapters 2 and 3. The goal of our fifth study was to investigate the neural basis of the long-latency response (Chapter 6). Our results provide strong evidence from both single-neuron recordings in non-human primates and transcranial magnetic stimulation in humans that primary motor cortex, which is known to be a critical node for voluntary control, also contributes to the sophistication of the long-latency response. Taken together, the studies presented in this thesis demonstrate that the long- latency response possesses several functional attributes typically reserved for voluntary control and that this sophistication likely arises via a transcortical pathway through primary motor cortex.