Disorders of the auditory system

The human auditory system possesses a remarkable ability to evaluate the acoustic environment and to provide the information necessary for normal function and survival. The process of evaluation begins at the periphery, in the ear. In the cochlea, auditory information is frequency analysed, amplified or attenuated, sharply tuned and transformed into electrical neural impulses, which are transmitted and processed in the auditory nerve. In the central auditory system, complex processing of acoustic signals, such as binaural fusion and sound localization, takes place, as well as various perceptual and cognitive processes. The efferent system plays a role in modulation of the auditory information, by balancing the processes of excitation and inhibition. Tonotopic organization, i.e. the anatomical arrangement according to sound frequencies, exists throughout the entire auditory system, and facilitates the maintenance and enhancement of frequency discrimination. The integral parts of the auditory system interact through complex, mainly feedback mechanisms, creating a highly dynamic system, in which abnormal functioning at one level may have functional consequences at other level(s). This functional plasticity in the auditory system is, for instance, reflected in the phenomenon of tonotopic reorganization in the cortex as a result of cochlear damage, and, in the opposite direction, an abnormality in the central auditory system may lead to disinhibition phenomena at the cochlear level. There is a myriad of functional disorders, resulting from pathology in the auditory system, with the loss of hearing sensitivity being the most important. From a neurological point of view, the understanding of auditory dysfunction has considerable importance, as neurological lesions may be associated with damage of auditory pathways. The identification of auditory dysfunction, its relationship to particular anatomical structure(s), and localization of the underlying pathological process, are subjects of continuing interest to both clinicians and scientists. With advances in science and technology, there has been significant progress in gaining better insight into this fascinating system. Functional anatomy of the auditory system Outer and middle ear The outer ear assists in localizing a sound source and serves to reinforce the resonance of the tympanic membrane. The middle ear (Fig. 45.1) contains the three interarticulated auditory ossicles, which form an elastic spring, the stiffness of which is controlled by the two middle ear muscles, the stapedius and the tensor tympani.