Postural reflexes have evolved to stabilize the head and to protect the neck from injury. During whole body translations, the inertia of the head exerts forces on the neck. These forces balanced by forces that are generated by reflexes evoked by vestibular sensory information (linear vestibulocollic reflex, LVCR) and proprioceptive sensory information (cervicocollic reflex, CCR). The dynamics of the LVCR and CCR are poorly understood, in part because they have been most often been studied in decerebrate cats. A non-human primate model of reflexive head stabilization is essential in order to translate results to humans. To this end, this study proposes a bottom-up approach to studying both the LVCR and CCR in Squirrel monkeys. This study will characterize both the LVCR and CCR individually in terms of 1) magnitude and direction total force produced reflexively by the neck musculature 2) muscle activation patterns and directional specificity of muscle activation 3) the relationship of signals carried by vestibulospinal neurons (VS) in the vestibular nucleus (VN) to muscle activity and the integration of vestibular and proprioceptive sensory information in the VN. A system that consists of a vestibular and proprioceptive stimulator as well as a 3-D force transducer which is fixed to the animal's head will be used to quantify the total reflexively generated forces during LVCR and CCR stimulation. Additionally, animals will be implanted chronically with electromyographic (EMG) electrodes and recording chambers so that the activity of neck muscles and the activity of single units in the VN can be recorded simultaneously during vestibular and proprioceptive stimulation. The combined results of these studies will pave the way for functional modeling of reflexive head stabilization during whole body translations in humans. One million US lives are affected by cervical injury annually, in auto and sporting accidents. Without a better understanding of how postural reflexes which stabilize the head are produced and controlled, better safety technologies which protect the head are unlikely to emerge. ? ? ?
