Neurons in the vestibular nuclei project their axons to targets in the brain stem. The cerebellum is primarily responsible for initiating movements on the basis of equilibrium information. Some axons project from the vestibular ganglion directly to the cerebellum, with no intervening synapse in the vestibular nuclei. Most of the axons terminate in the vestibular nuclei of the medulla. An important function of the vestibular system is coordinating eye and head movements to maintain visual attention. The system contributes to controlling head and neck movements in response to vestibular signals. Central Processing of Vestibular Informationīalance is coordinated through the vestibular system, the nerves of which are composed of axons from the vestibular ganglion that carries information from the utricle, saccule, and semicircular canals. The movement of two canals within a plane results in information about the direction in which the head is moving, and activation of all six canals can give a very precise indication of head movement in three dimensions. As one of the canals moves in an arc with the head, the internal fluid moves in the opposite direction, causing the cupula and stereocilia to bend. Figure 15.4.2 – Rotational Coding by Semicircular Canals: Rotational movement of the head is encoded by the hair cells in the base of the semicircular canals. By comparing the relative movements of both the horizontal and vertical ampullae, the vestibular system can detect the direction of most head movements within three-dimensional (3-D) space. The semicircular canals contain several ampullae, with some oriented horizontally and others oriented vertically. As the head rotates in a plane parallel to the semicircular canal, the fluid lags, deflecting the cupula in the direction opposite to the head movement. The ampulla contains the hair cells that respond to rotational movement, such as turning the head while saying “no.” The stereocilia of these hair cells extend into the cupula, a membrane that attaches to the top of the ampulla. The base of each semicircular canal, where it meets with the vestibule, connects to an enlarged region known as the ampulla. The anterior and posterior vertical canals are oriented at approximately 45 degrees relative to the sagittal plane ( Figure 15.4.2). One is oriented in the horizontal plane, whereas the other two are oriented in the vertical plane. The semicircular canals are three ring-like extensions of the vestibule. The difference in inertia between the hair cell stereocilia and the otolithic membrane in which they are embedded leads to a shearing force that causes the stereocilia to bend in the direction of that linear acceleration. Figure 15.4.1 – Linear Acceleration Coding by Maculae: The maculae are specialized for sensing linear acceleration, such as when gravity acts on the tilting head, or if the head starts moving in a straight line. The exact position of the head is interpreted by the brain based on the pattern of hair-cell depolarization. The moving otolithic membrane, in turn, bends the sterocilia, causing some hair cells to depolarize as others hyperpolarize. Tilting the head causes the otolithic membrane to slide over the macula in the direction of gravity. The otolithic membrane moves separately from the macula in response to head movements. The otoliths essentially make the otolithic membrane top-heavy. On top of the otolithic membrane is a layer of calcium carbonate crystals, called otoliths. The stereocilia of the hair cells extend into a viscous gel called the otolithic membrane ( Figure 15.4.1). The macula is composed of hair cells surrounded by support cells. The utricle and saccule are both largely composed of macula tissue (plural = maculae). The neural signals generated in the vestibular ganglion are transmitted through the vestibulocochlear nerve to the brain stem and cerebellum. Head position is sensed by the utricle and saccule, whereas head movement is sensed by the semicircular canals. These cells are located within the vestibule of the inner ear. A similar mechanoreceptor-a hair cell with stereocilia-senses head position, head movement, and whether our bodies are in motion. Describe the means of mechanoreception for hearing.Īlong with audition, the inner ear is responsible for encoding information about equilibrium, the sense of balance.By the end of this section, you will be able to:
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