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One of the most important tasks of the human postural control system is that of balancing the body over the small base of support provided by the feet. As a sensor of gravity and head acceleration, the vestibular system is one of the nervous system's most important tools in controlling posture. The vestibular system is both a sensory and a motor system. As a sensory system, the vestibular information is closely integrated with somatosensory and visual information so that the central nervous system (CNS) can estimate the position and movement of the entire body as well as the surrounding environment. In addition to providing sensory information, the vestibular system also contributes directly to motor control. Descending motor pathways such as the vestibulospinal tracts receive vestibular and other types of information to control eye, head, and trunk orientation and to coordinate postural movements.

Because the vestibular system is both a sensory and a motor system, it plays many different roles in postural control. In this chapter, we explore the four most important roles (Fig. 3.1). First, we discuss the role of the vestibular system in the perception of body position and self-motion. Second, we discuss its role in orienting the trunk to vertical using sensory orientation and weighting-appropriate sensory cues under different sensory environments. Third, we discuss the role of the vestibular system in controlling the position of the body's center of mass (COM), both for static positions and dynamic movements via postural responses. Fourth, we discuss its role in stabilizing the head during postural movements.

Figure 3.1

Four important roles of the vestibular system in postural control. As the skier leans into a curve, he (1) perceives his body orientation with respect to both gravity and the mountain, (2) orients his upper body to gravity (sensory orientation), (3) controls his COM (postural reactions), and (4) stabilizes his head in space.

Perceiving Position and Self-Motion

The vestibular system provides information about the movement of the head and its position with respect to gravity and other inertial forces (like those generated by moving vehicles). Therefore, this system contributes important information to the sensation and perception of the motion and position of the body. Graviceptive information from the vestibular system is combined with information from the somatosensory system to perceive gravitoinertial forces and use them to orient the body. For example, the skier in Figure 3.1 maintains equilibrium by integrating gravitational and centripetal forces from the vestibular and somatosensory systems together with orientation information from vision. The vestibular system provides information about the position and motion of the head. Because the largest head motions during quiet stance sway and while walking or running usually occur in the sagittal (forward-backward) and frontal (left-right) planes, the vertical canals and otoliths are more critical for postural control than are the horizontal ...

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