Afferent Auditory Pathways

The fibers of the ventral cochlear nucleus then project to the superior olive located in the medulla. The fibers then project by way of the lateral lemniscus nuclei and other relays to the inferior colliculus. The dorsal cochlear nucleus projects directly to the inferior colliculus via the lateral lemniscus. Fibers from both the ventral and dorsal cochlear nucleus project from the inferior colliculus to the medial geniculate nucleus of the thalamus. Within the colliculi signals from both ears interact on their way toward the cerebral cortex. From the medial geniculate nucleus, the auditory signals travel to the primary auditory cortex, which is located in the temporal lobe and is Brodmann’s area 41. Despite the extensive intermixing among afferent fibers, the bulk of the neural activity reaching the auditory cortex originates in the contralateral ear. Tonotopic ordering is preserved throughout the ascending pathway so that individual cortical regions are sensitive to specific frequencies. The width of the band of frequencies to which an individual neuron responds is approximately the same in area 41 as at the level of the cochlear nuclei.

In the analysis of acoustic information, relatively little is known about the function of the various stages along the auditory pathway. Neurons within the superior olivary complex are specifically adapted for analyzing the location of a sound in space. Olivary neurons receive excitatory input from the contralateral cochlear nuclei and inhibitory input from the ipsilateral cochlear nuclei. In the medial portion of the complex, where neurons are sensitive to sounds of low frequency, these opposing inputs result in individual neurons becoming attuned to a fixed time delay between the arrival of sound at each ear. In the lateral portion of the complex, where neurons are sensitive to higher frequencies, the opposing inputs result in neurons becoming sensitive to differences in the intensity of sound reaching each ear. The entire auditory pathway, including the auditory cortex, must be intact for sound localization to take place. Similarly, auditory structures as far as the level of the inferior colliculus are required for frequency discrimination, even though neurons at all levels of the auditory pathway are frequency selective. Intensity discriminations, on the other hand, can be made following the destruction of the inferior colliculus and higher centers. Such discrimination may involve the collateral pathways that relay auditory signals to the brainstem reticular formation. These pathways are probably also involved in the reflex reaction to a sudden sound.

Disorders. A common auditory pathway deficit is vestibular schwannoma. The patient suffers loss of sound localization, diminished speech discrimination, tinnitus, imbalance, and diminution of the stapedius reflex. Nerve-type deafness can be caused by toxins (e.g., arsenic, lead, quinine) and by antibiotics such as streptomycin, which can also damage the cochlea directly. Because of the multisynaptic and highly complex system of crossed pathways, damage to auditory brainstem tracts and nuclei by trauma, tumors, or vascular disorders results in only slight hearing impairment.

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