The lateral and medial vestibular nuclei also have important connections with the spinal cord, mainly via the uncrossed lateral vestibulospinal tract and the crossed and uncrossed medial vestibulospinal tracts (Fig. 14-4). Presumably, vestibular effects on posture are mediated via these pathways—the axial muscles being acted upon predominantly by the medial vestibulospinal tract and the limb muscles, by the lateral tract. The nuclei of the third, fourth, and sixth cranial nerves come under the influence of the vestibular nuclei through the projection pathways, mainly the medial longitudinal fasciculus described in Chap. 13. In addition, all the vestibular nuclei have afferent and efferent connections with the pontine reticular formation (see Fig. 14-4). The latter connections subserve vestibuloocular and vestibulospinal reflexes that are essential for clear vision and stable posture.
Finally, there are projections from the vestibular nuclei to the cerebral cortex, specifically to the regions of the intraparietal sulcus and superior sylvian gyrus. In the monkey, these projections are almost exclusively contralateral, terminating near the “face area” of the first somatosensory cortex (area 2 of Brodmann). Lesions in the posterior insula impair the sense of verticality, body orientation, and movement. Whether the vestibular nuclei project to the thalamus in humans is not entirely settled; most anatomists indicate that there are no such direct connections.
These brief remarks convey some notion of the complexity of the anatomic and functional organization of the vestibular system (for a full discussion, see the monographs of Brodal and of Baloh and Honrubia). In view of the proximity of cochlear and vestibular elements, it is understandable that acoustic and vestibular functions are often affected together in the course of disease although each may also be affected separately.
DEAFNESS, TINNITUS, AND OTHER DISORDERS OF AUDITORY PERCEPTION
Figures from a National Health Survey (National Institute on Deafness and Other Communication Disorders) indicated that approximately 28 million Americans of all ages had a significant degree of deafness and that 2 million were profoundly deaf. More than one-third of persons older than age 75 years were handicapped to some extent by hearing loss.
Deafness is of three general types: (1) Conductive deafness, caused by a defect in the mechanism by which sound is transformed (amplified) and conducted to the cochlea. These are disorders of the external or middle ear—obstruction of the external auditory canal by atresia or cerumen, thickening of the tympanic membrane from infection or trauma, chronic otitis media, otosclerosis (the main cause of deafness in early adult life), and obstruction of the eustachian tube. (2) Sensorineural deafness (also called, imprecisely, nerve deafness), which is caused by disease of the cochlea or of the cochlear division of the eighth cranial nerve. Although cochlear and eighth nerve causes of deafness have conventionally been combined in one (sensorineural) category, the neurologist recognizes that the symptoms and causes of the two are quite different and that it is more practical to think of them as cochlear (end organ) and retrocochlear (nerve) deafness. (3) Central deafness, caused by lesions of the cochlear nuclei and their connections with the primary auditory receptive areas in the temporal lobes. For example, complete tone deafness, which is probably inherited as an autosomal dominant trait, is a central disorder.