An acoustic neuroma is a tumor of Schwann cells of the vestibulocochlear nerve (CNVIII), which is an important but relatively uncommon condition that can disrupt the function of cochlear nerve fibers. When the tumor grows, it may irritate the cochlear nerve fibers and cause a ringing sound in the affected ear (tinnitus). As the tumor enlarges, it can block the conduction in cochlear nerve fibers and lead to deafness in the ear. Early diagnosis is crucial as the tumor may be operable when it is still small. If the tumor is allowed to grow substantially, it can interrupt the entire vestibulocochlear nerve and result in both vestibular and auditory difficulties. Additionally, it can impinge on or distort neighboring cranial nerves (such as V, VII, IX, and X) and compress the cerebellar peduncles, producing cerebellar signs.

At low frequencies, individual auditory afferent fibers can respond to each cycle of the signal frequency. However, at higher frequencies, each afferent fiber generates an action potential only at certain cycles, limited by its maximum firing frequency. Nonetheless, the aggregate firing frequency of the overall population of afferent fibers can still signal the stimulus frequency.

Cochlear afferent fibers synapse on neurons in the dorsal and ventral cochlear nuclei, which have axons contributing to the central auditory pathways. Some axons from the cochlear nuclei cross to the opposite side and ascend in the lateral lemniscus, which is the primary ascending auditory tract. Others connect with various nuclei, such as the superior olivary nuclei, projecting through the ipsilateral and contralateral lateral lemnisci. Each lateral lemniscus terminates in the inferior colliculus. Neurons in the inferior colliculus project to the medial geniculate nucleus of the thalamus, which gives rise to the auditory radiation. The auditory radiation ends in the primary auditory cortex, located on the superior surface of the temporal lobe.

The representation of auditory space is complex, even at the brainstem level, as the input from each ear is bilaterally represented in the ascending auditory system pathway at the level of the lateral lemniscus and above. Consequently, isolated lesions of the cochlear nuclei or more peripheral structures can cause unilateral deafness, whereas central lesions do not cause unilateral deafness but may affect overall sensitivity to speech or sound localization.

The responses of neurons in various structures within the auditory system can be described by tuning curves, revealing tonotopic maps in which neurons are ordered based on their characteristic frequencies. Tonotopic maps have been identified in the cochlear nuclei, superior olivary complex, inferior colliculus, medial geniculate nucleus, and auditory cortex. Most auditory neurons at higher levels than the cochlear nuclei respond to stimulation from either ear and have binaural receptive fields. Binaural receptive fields are important for sound localization, as humans can distinguish sounds originating from sources separated by as little as 1 degree. The auditory system utilizes various cues, including differences in the time or phase of arrival of sound at the two ears and differences in sound intensity on each side of the head, to determine the origin of sounds.
Animportant,althoughrelativelyuncommon,conditionthatcaninterruptthefunctionofcochlearnervefibersisanacoustic neuroma, atumorofSchwanncellsofthevestibulocochlearnerve(CNVIII).Asthetumorgrows,irritationofcochlearnervefibersmaycausearingingsoundintheaffectedear(tinnitus). Eventually,conductionincochlearnervefibersisblocked,andtheearbecomesdeaf.Thetumormaybeoperablewhilestillsmall;therefore,earlydiagnosisisimportant.Ifthetumorisallowedtoenlargesubstantially,itcouldinterrupttheentirevestibulocochlearnerveandcausevestibularaswellasauditorydifficulties.Itcouldalsoimpingeonordistortneighboringcranialnerves(e.g.,V,VII,IX,andX),anditcouldproducecerebellarsignsbycompressingthecerebellarpeduncles.
•Fig. 8.22 A, Atlowfrequencies,individualauditoryafferentfiberscanrespondateachcycletothesignalfrequency.B, Athigherfrequencies,eachafferentfibergeneratesanactionpotentialonlyatcertaincycles,limitedbyitsmaximumfiringfrequency.However,theoverallpopulationofafferentfiberscanstillsignalstimulusfrequencybytheiraggregatefiringfrequency.
Cochlear afferent fibers synapse on neurons of the dorsal and ventral cochlear nuclei. The neurons in these nuclei have axons that contribute to the central auditory pathways. Some of the axons from the cochlear nuclei cross to the contralateral side and ascend in the lateral lemniscus, the main ascending auditory tract. Others connect with various ipsilateral or contralateral nuclei, such as the superior olivary nuclei, which project through the ipsilateral and contralateral lateral lemnisci. Each lateral lemniscus ends in an inferior colliculus. Neurons of the inferior colliculus project to the medial geniculate nucleus of the thalamus, which gives rise to the auditory radiation. The auditory radiation ends in the primary auditory cortex (Brodmann areas 41 and 42), located on the superior surface of the temporal lobe.
The input from each ear is bilaterally represented in the ascending auditory system pathway at the level of the lateral lemniscus and above. Thus the representation of auditory space is complex, even at the brainstem level. Consequently, unilateral deafness may occur with isolated lesions of the cochlear nuclei or more peripheral structures. Central lesions do not cause unilateral deafness, although they may interfere with overall sensitivity to speech or with sound localization.
Functional Organization of the Central Auditory System
The responses of neurons in several structures that belong to the auditory system can be described by tuning curves (see
Fig. 8.21B ). By plotting the distribution of the characteristic frequencies of neurons within a nucleus or in the auditory cortex, a tonotopic map may be revealed in which neurons are ordered according to their “best” frequencies. Tonotopic maps have been found in the cochlear nuclei, superior olivary complex, inferior colliculus, medial geniculate nucleus, and auditory cortex. A given auditory structure may, in fact, contain several tonotopic maps.
Most auditory neurons at levels above the cochlear nuclei respond to stimulation of either ear (i.e., they have binaural receptive fields). Binaural receptive fields contribute to sound localization. A human can distinguish sounds originating from sources separated by as little as 1 degree. The auditory system uses several clues to judge the origin of sounds, including differences in the time (or phase) of arrival of the sound at the two ears and differences in sound intensity on the two sides of the head.