BME PhD Proposal Seminar: Eric Brown

Adaptive Properties of Reflexive Head Movements Evoked During Linear Acceleration

Co-advised by Professor's Greg Gdowski and Gary Paige

Abstract

More than one in three elderly Americans experience a fall each year, often while walking. Vestibular loss is a key cause of postural instability; often leading to an altered sense of motion and the inability of stabilizing reflexes to produce sufficient muscle torque to compensate for the body’s inertia during a fall threat. In healthy individuals, as the body moves a variety of proprioceptive, vestibular, and other sensory signals arise. The central nervous system (CNS) then utilizes these signals to control reflexes to adapt to varying conditions. Diagnostic and rehabilitative tools that reduce the incidence of falls would benefit from incorporating strategies utilized by the CNS for controlling and adapting sensory driven postural reflexes.

The proposed studies are targeted at determining how vestibular and proprioceptive sensory signals are utilized by the CNS in controlling reflexes, collectively referred to as collic reflexes, which stabilize the head and upper trunk with respect to the body during whole body movement. Inertia of the head creates force on the neck during motion that depends upon the direction of acceleration and biomechanical properties of the head/neck plant. To control head movements in a variety of contexts the CNS must be capable of adapting stabilizing reflex activity to the different biomechanical challenges imposed by motion in a three-dimensional world subject to gravity, inertial properties, and active elements of the head/neck plant which are not constant. We hypothesize that the CNS accomplishes this task utilizing convergent vestibular sensory signals generated during linear and angular motion to evoke collic reflexes that stabilize head movement. To test this general hypothesis, the aims of this proposal biomechanically challenge collic reflexes by:

1. Varying the direction of linear acceleration throughout the horizontal plane
2. Altering head/neck plant biomechanics through applied changes in gravitational and inertial loading

Changes in the direction of motion alter the direction of compensatory torque necessary to stabilize head movement. We hypothesize that directionally tuned vestibular neurons in the CNS will evoke collic reflexes with directionally tuned response properties. Gravity is a persistent acceleration which imposes forces on the neck dependent upon the body’s orientation; tilt of the body creates a constant load on the head/neck which must be compensated to maintain head orientation on the body, as well as to produce active reflex performance. The load on the neck also changes during motion when inertia is added to the head, or the head’s center of mass changes, such when wearing a helmet. While the vestibular signals of motion remain the same, the stabilizing torque necessary to control head movement during motion changes. We hypothesize that collic reflexes will adapt to compensate for the biomechanical challenges of gravitational or inertial loading during linear translation with greater muscle activation to produce similar head kinematics. Neither the directional tuning nor adaptive properties of collic reflexes during whole body linear acceleration have previously been studied.