Intro | Anvil | Ear Canal | Semicircular Canals | Cochlea | Eardrum | Hammer | Auditory Nerve | Stirrup
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The Auditory Nerve conveys the neuronal activity generated by the hair cells to a specialized nucleus of the thalamus. This thalamic nucleus in turn relays this information to the auditory cortex, which is found within the temporal lobes of the brain.
The auditory nerve or eighth cranial nerve is composed of two branches, the cochlear nerve that transmits auditory information away from the cochlea, and the vestibular nerve that carries vestibular information away from the semicircular canals. Each cochlear nerve contains approximately 50,000 afferent axons. Approximately 95 percent of these axons carry information away from the inner hair cells. The other 5 percent of these axons synapse with the more plentiful outer hair cells. Although the inner hair cells are few in numbers, their extensive projections indicate their primary importance to the hearing process. Indeed, research indicates that inner hair cells are essential for normal hearing. Many investigators in this field believe the outer hair cells selectively tune the hearing process by altering the mechanical characteristics of the basilar membrane. The process by which this tuning may occur is not yet understood.
The auditory system can be broken down into primary and secondary subsystems similar to those of the visual system. In the primary auditory system, impulses from the sensory receptors are transmitted to one of two nuclei (dorsal or ventral cochlear nuclei) in the medulla. The axons of cells in these nuclei form the lateral lemniscus, which terminates in the inferior colliulus. Two distinct pathways emerge from the colliculus, heading for the ventral and dorsal medial geniculate nucleus of the thalamus. The ventral region of the medial geniculate nucleus projects to primary auditory cortex, whereas the dorsal region projects to secondary auditory cortex. Both primary and secondary auditory cortex, are located deep within the lateral fissure of the temporal lobe, with some extension into the parietal lobe. The primary system is essential for sound identification or perceptual processing, but is not necessary for the detection or localization of sounds. The tonotopic organization of each auditory cortical area remains unclear, but in general it appears that in each subfield that has been mapped low tones are represented caudally (toward the back of the head) and high tones more rostrally (toward the base of the head). Bilateral damage to auditory cortex is associated with auditory agnosia; characterized by word deafness or the inability to perceptually identify the meaning of both speech and nonverbal sounds.
The secondary auditory pathway is essential for sound localization. This pathway consists of projections from the dorsal and ventral nuclei of the medulla to the inferior colliculus of the midbrain. The midbrain in turn transmits relevant information to secondary auditory cortex. Sound localization occurs via two mechanisms. The medial system, arising from the medial superior olivary complex, responds to slight differences in the timing of sound arrival at each ear (intensity differences). The lateral system, arising from the lateral superior olivary complex, responds to slight differences in sound amplitude arriving at each ear (phase differences).