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Anatomy and Physiology of the Vestibular System

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The vestibular system consists of both peripheral and central components that function to sense and control motion. Head position and movement in space are first detected by the vestibular end-organs in the periphery, where vestibular hair cells transform mechanical stimuli into neuronal signals. These signals are carried along the vestibulocochlear nerve (cranial nerve eight) to the brain stem. This information is then integrated and distributed to complex pathways in the central nervous system (CNS), ultimately resulting in vestibular reflexes that control posture, balance, and eye movements. Specifically, the vestibulo-ocular reflex (VOR) functions to stabilize eye gaze, and it is readily evaluated clinically through optokinetics and nystagmus. In addition, peripheral vestibular signals interact with both cervical and lower spinal motor neurons to generate the vestibulocolic and vestibulospinal reflexes. These function to maintain and modulate posture, gait, and head position. The cerebellum also plays a critical role in coordination and the ability to adapt to vestibular injury. Finally, while their exact function remains a subject of ongoing investigation, both cortical and autonomic pathways are believed to play a role in various visceral responses to vertigo, such as nausea, as well as the conscious sense of motion.

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Figure 22–1. Anatomy and physiology of the vestibular system.A. The peripheral auditory and vestibular systems are composed of the external ear, including the auricle and external auditory canal (EAC); the middle ear, including the tympanic membrane (TM) and three ossicles, specifically the malleus (M), incus (I), and stapes (S); and the inner ear, composed of the cochlea and the three semicircular canals (SC) of the vestibular apparatus, specifically the lateral (Lat SC), superior (Sup SC), and posterior (Post SC).B. Focused view of the dilated, or ampullated, end of a semicircular canal showing the cristae ampullaris, neuroepithelium (including the hair cells), and the cupula. Fluid motion, generated by head rotation, generates forces across the cupula that bend the stereocilia of the hair cells, resulting in release of neurotransmitter into the vestibular synapse.C. Focused view of vestibular hair cells within the ampulla. Each hair cell has approximately 70 short stereocilia and one longer kinocilium that project into the gelatinous cupula. It is the laterally located kinocilium that is the primary determinant of the direction of polarization. Each hair cell is innervated by vestibular afferent neurons that allow transmission of positional information to the brain.

Figure 22–2. Vestibular ocular reflex.Connections among the vestibular, abducens, and oculomotor nuclei allow maintenance of vision during head movement. Rotational head movement yields both excitatory and inhibitory peripheral signals depending on the direction of motion. In this example, maintenance of an image on the retina during head rotation to the right requires conjugate leftward gaze. This is accomplished by stimulation of the right lateral semicircular canal and subsequent activation of the vestibular, abducens, and oculomotor nuclei. Ultimately, this neural circuitry culminates in activation of the left lateral and right medial rectus muscles and inhibition of left medial and right lateral recti. Integration of these signals takes place directly in the medial longitudinal fasciculus and indirectly in the pontine reticular formation (not shown).

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