Comparative terrestrial locomotion physiology

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The long-term objective of this research program is to determine the effect of body size on the neuromuscular control of locomotion in terrestrial mammals. My general hypothesis is that there is a compromise between the responsiveness and resolution of sensorimotor systems that becomes more acute with increases in body size thereby causing size-dependent changes in the physiological mechanisms that underlie movement. Responsiveness refers to the time required for the nervous system to recognize sensory stimuli and generate an appropriate response. Maximizing responsiveness is important for minimizing the time required to initiate a movement and for maintaining stability in feedback pathways. Resolution refers to the ability of the sensory system to distinguish between sensory stimuli and to the ability of the motor system to generate finely graded responses. Maximizing resolution is important for accurately sensing the state of the body and for generating appropriate muscle forces. The compromise between sensorimotor responsiveness and resolution manifests itself in peripheral nerve bundles as a trade-off between axon diameters, and thus reflex latency, and total number of axons. The proposed research tests the presence and implications of this theoretical comprise as it applies to peripheral nerves, spinal reflex pathways, and locomotor performance and does so across a wide size range--from shrews to elephants. My first aim uses anatomical techniques to determine the effect of body size on peripheral nerve bundle dimensions as well as the diameters and total number of axons within peripheral nerve bundles. Aim 2 uses electrophysiology to determine the effect of body size on peripheral nerve conduction velocity and reflex latency. Aim 3 uses mathematical modeling to gain insight into the effect of size-dependent changes in biomechanics and spinal reflexes on the stability of quadrupedal locomotion. The results will provide fundamental insight into the organization and function of neuromuscular control systems.