Chapter 29 - Auditory System

Abstract

Sequencing of the rat genome has allowed a better understanding of the rat physiology and biology, in turn promising to translate into a better understanding of human biology and disease processes, including neurobiology of hearing in humans. Sensory cells found in the organ of Corti are known as inner and outer hair cells; they have different functions, and these are discussed in this chapter. The cochlear nuclear complex is the first relay center in the ascending auditory pathway, and the site of termination of all auditory nerve fibers. In the cochlear nuclei, the auditory nerve fibers branch widely and are transmitted to a number of different types of second-order neurons that give rise to a number of parallel ascending tracts, each with a specific pattern of projection into the superior olivary complex, nuclei of the lateral lemniscus and inferior colliculus. The ascending auditory tracts converge towards the auditory midbrain, the inferior colliculus, which is an obligatory relay for many inputs in the route to the auditory cortex. Inferior colliculus neurons project to the medial geniculate body in the thalamus. From the inferior colliculus and upwards, the auditory pathway can be divided into a “lemniscal” projection in which tonotopic organization is very sharp and a “non-lemniscal” projection in which it is less sharp. In contrast to the minimum of two relay stations between the periphery and cerebral cortex in the visual and somatosensory cortices, there is a minimum of three relays in the auditory system, with several stages of convergence and divergence, and at least seven levels in the brainstem where axons cross the midline, making the organization of the auditory system highly complex. Parallel to the ascending pathway, there is a stepwise descending projection from the auditory cortex to the organ of Corti that serve to modulate, filter and/or enhance sound processing in a top-down fashion that clearly plays a critical role in maintaining the normal operations of the auditory system as a whole. A fascinating fact of the auditory system is that it has the extraordinary ability to extract the spectral, temporal and binaural properties of the world around us. From the pattern of sounds reaching our ears, it is possible to identify and distinguish a vast selection of objects. Moreover, our auditory system also provides us with the ability to recognize and communicate through speech, and even enjoy music. Auditory disorders are not fatal per se, but there is little doubt that they can have a devastating effect on our quality of life. This chapter highlights the functional organization of the rat auditory brain, and focuses on the rat as a useful animal model for hearing research. These issues, and more, are discussed in this chapter.