Purpose of the Vestibular System
The human vestibular system estimates body position and motion. Motion inputs to the vestibular system include the inner ear signals (“vestibular” in Fig. 1.1), as well as position sensation, (“proprioception”) visual signals, and intended movement (“motor commands”). These redundant inputs are integrated by the central processor, the “vestibular nuclear complex,” which generates motor commands to drive the eyes and body. The system is normally very accurate. To maintain accuracy, the vestibular system is monitored and calibrated by the cerebellum.
Block diagram illustrating the organization of the vestibular system. (Copyright Timothy C. Hain, MD.)
The eye and body movement output of the central vestibular system is generally described in terms of three simple reflexes, the vestibulo-ocular reflex (VOR), the vestibulocollic reflex (VCR), and the vestibulospinal reflex (VSR). The VOR generates eye movements that enable clear vision while the head is in motion. The VCR acts on the neck musculature to stabilize the head. The VSR generates compensatory body movement to maintain head and postural stability and thereby prevent falls. Although the nomenclature of these reflexes, for example “vestibular ocular reflex,” might make one think that these circuits are only concerned with inner ear input, the vestibular nucleus that drives all these reflexes also processes input from the other sources listed above.
After an acute loss of peripheral vestibular function, as for example after surgical removal of a tumor of the vestibular nerve, postural and oculomotor deficits appear. With the head still, spontaneous jumping of the eyes (nystagmus) and tilting of the body away from upright appear. When the head is moved, vision and balance are further impaired. Associated with these deficits are a reduced propensity to move the head as well as behavioral changes aimed at minimizing the risk of disorientation by avoiding visual input and minimizing the risk of falling by adopting a more cautious and stable stance.
Recovery from vestibular lesions has been studied for over 100 years.1 Orientation in space and being able to walk upright are critical functions. It is understandable that the vestibular system is supported by multiple vestibular repair mechanisms. The capability for repair and adaptation is remarkable! Plasticity consists of neural adjustments that restore original function. This is supplemented by substitution of other sensory input or internal estimates. Finally, one may change one's behavior to “work around” problems presented by a vestibular lesion.
Given sufficient time, persons with up to approximately 50% loss of vestibular function adapt so well that a casual observer may find them indistinguishable from someone without a vestibular lesion. Nevertheless, such persons can rarely attain the same degree of performance as normal, and a sophisticated clinician can nearly always detect this situation.
In this chapter, we describe the anatomy and neurophysiology of the vestibular system, ...