The Vestibular System

The survival of vertebrates is dependent on maintaining their body equilibrium in the gravitational field and being capable of orienting themselves in their environment. Studies have demonstrated that well-defined neuronal networks in the central nervous system implement these complex sensorimotor transformations, known as the vestibuloocular, cervical and optokinetic reflexes. In this chapter, the main characteristics of the vestibular system are described, including the physiologic properties related to the anatomic features of the peripheral and central vestibular system. Electrophysiological aspects and the neurotransmitters related to each vestibular nuclei are discussed, focusing on the medial vestibular nucleus. An understanding of how vestibular inputs are integrated with multimodal signals to create the internal representation of head direction throughout the Papez circuit is presented. The pharmacology of the central vestibular system is reviewed, with emphasis on recent studies which pin-point the pharmacological interactions between identified central vestibular neurons and their inputs in defined circuits. A section on the influences of the brainstem regions involved in homeostatic regulation is included. In the final section, the anatomy of the main vestibular nuclei is discussed to the extent that they are useful for understanding some important morpho-functional correlations.