The auditory system differs significantly from the visual and somatosensory pathways in that there is no large direct pathway from peripheral receptors to the cortex. Rather, information ultimately reaching the auditory cortex undergoes significant reorganization as it passes through the brainstem (Moore, 1994). A general conclusion reached from work on the anatomic and chemical composition of the auditory pathway is that inhibition plays an extremely important role at all levels of the system in shaping the exquisitely precise responses of central neurons. One implication of these complexities of central organization relates to the placement of CNS stimulating devices and the therapy of auditory disorders (see section on Cochlear Implantation later). Detailed description of peripheral and central auditory pathways is found in standard texts (Baloh and Honrubia, 1990; Jackler and Brackmann, 1994).

Sounds that reach the ear set the tympanic membrane in motion and this motion is conducted to the fluid of the cochlea by the three ossicles of the middle ear. The middle ear function as an impedance transformer. It improves the transmission of sound to the cochlear fluid. This improvement in transmission is mainly the result of the large ratio between the area of the tympanic membrane and that of the stapes footplate. Two small muscles are attached to the ossicles. One, the tensor tempani, is innervated by the trigeminal nerve and pulls the tympanic membrane inward when it contracts. The tympanic membrane is stretched and thereby attenuates sound transmission for low-frequency sounds. The other muscle, the stapedius muscle, is attached to the stapes and pulls the stapes in the direction perpendicular to its normal motion in response to sound. The stapedius muscle is innervated by the facial nerve and its contraction also decreases the middle ear=s ability to conduct low-frequency sounds. In humans, the stapedius muscle contracts in response to a strong sound, the acoustic reflex. Motion of the fluid in the cochlea sets the basilar membrane into motion. The sensory cells (hair cells) that are innervated by the fibers of the auditory nerve are located along the basilar membrane and convert the motion of the basilar membrane into a neural code. As a result of the hydromechanical properties of the cochlea, a sound gives rise to a traveling wave on the basilar membrane. The distance the wave travels before it reaches its peak amplitude is a direct function of the frequency of the sound. Because tones of different frequency give rise to maximal vibration amplitudes at different locations along the basilar membrane, the spectral components of a complex sound are separated along the basilar membrane according to frequency. Thus, the cochlea is a frequency analyzer. Since the cochlear hair cells located along the basilar membrane are innervated by fibers of the auditory nerve, the discharge pattern of individual auditory nerve fibers can be expected to reflect the vibratory pattern of the basilar membrane and therefore process frequency. A certain frequency exists at which the threshold of an auditory nerve fiber is lowest. The range of sound frequencies important to humans is well above threshold; in addition, the sounds that are significant to human communication are not pure-tones, but complex sounds that contain many spectral components. The discharges of single nerve fibers are phase-locked to the waveform of sounds within their response areas which is believed to be the basis for the temporal hypothesis for frequency discrimination in the auditory system originally known as the volley theory. Recent evidence has accumulated indicating that temporal coding plays an important role in the coding of frequency or spectral composition of sounds. In theory, temporal coding would be just as efficient in carrying information about a sound=s frequency or spectral composition to the brain as coding by the place principal which assumes that specific nerve cells are activated by tones of specific frequency or specific spectral components of a complex sound. However, because of the temporal jitter that occurs in synaptic transmission, the temporal code is converted to a place code at a peripheral location along the ascending auditory pathway. The anatomical location of such a conversion has not yet been identified but may be in the cochlear nucleus or other nuclei of the ascending auditory pathway.

The human auditory system is complex, yet highly ordered. It should be noted that the central auditory system, at least histologically, arises at the neuroglial-neurilemma junction of cranial nerve VIII within the internal auditory canal. The first-order neurons of the auditory system are cells of the spiral ganglion situated within the modiolus or central core of the cochlea. In humans, there are approximately 32,000 myelinated cochlear nerve fibers. The cochlear nerve occupies the anterior-inferior portion of the internal auditory canal and the vestibular nerve occupies the posterior half. The facial nerve or cranial nerve VII is located in the anterior-superior quadrant of the internal auditory canal. A tonotopic relationship is preserved throughout the entire auditory nervous system. Figure 1 represents a summary schematic diagram of the central pathways.

Axons leaving the cochlear nucleus have at least one synapse in cell groups between the pontomedullary junction and the midbrain. Auditory fibers from the dorsal and ventral cochlear nuclei form three pathways referred to as striae. The fibers from the anterior ventral portion of the ventral cochlear nucleus send an ipsilateral pathway to reach the superior olivary complex as well as to join the ipsilateral lemniscus (Benjamin and Troost, 1988). The superior olive is a cellular complex of about 4mm long that extends from level of the facial nucleus in the pons to about the level of the motor nucleus of the trigeminal nerve. Ventrally it is in contact with the lateral portion of the trapezoid body. The cytoarchitecture of this complex defines three nuclei: the medial superior olive, the lateral superior olive, and the nucleus of the trapezoid body. It should be emphasized that the trapezoid body itself is not a nucleus; it is a bundle of transverse fibers in the ventral part of the pontine tegmentum. The entire superior olive complex is surrounded by small cellular groups known as the preolivary or periolivary nuclei. The olivary nuclear complex is probably the first level in the auditory system where binaural integration of auditory signals occur. As such, unilateral hearing loss does not result from lesions at the level of the superior olive complex or more rostral auditory nuclear groups. The medial superior olivary nucleus give rise to fibers that ascend in the ipsilateral lemniscus and to a bundle of fibers known as the peduncle of the superior olive which passes dorsomedially toward the abducens (VI nerve) nucleus.

The lateral lemniscus is the principal ascending auditory pathway in the brainstem. The lateral lemniscus originates laterally to the superior olivary complex, but at the level of the inferior colliculus it lies at a more dorsal portion in the brainstem (see Figure 1). There are diffuse cellular groups within this bundle known to constitute the ventral and dorsal nuclei of the lateral lemniscus. Projections from these cells proceed to the midbrain and terminate in the inferior colliculus. The inferior colliculus, located in the midbrain tectum, serves as a relay center for all of the ascending and descending auditory fibers. Ascending fibers and some fibers in the lateral lemniscus constitute the afferent bundle known as the brachium of the inferior colliculus. These fibers synapse in the medial geniculate body of the thalamus. Interaural time intensity comparisons occur at the inferior colliculus so that it also plays a role in auditory localization. In addition to receiving ascending afferent fibers from the lateral lemniscus and the contralateral inferior colliculus, each colliculus receives descending projections from the ipsilateral medial geniculate body and the auditory cortex. The auditory cortex projects fibers bilaterally to the colliculi. A small portion of fibers pass from the inferior to the superior colliculus which might provide an anatomic connection enabling reflex circuits between auditory and visual systems.

The medial geniculate body is situated on the caudal aspect of the thalamus and is the last major relay station for ascending auditory fibers before they reach the cortex. There is a tonotopic arrangement in the medial geniculate body in which low frequencies are represented laterally and high frequencies are located medially in the principal division. The main projection of the medial geniculate body is to the superior temporal convolution or transverse gyrus of Heschl via the geniculotemporal (auditory) radiations. At the subcortical level, the auditory radiations can be seen in the sublenticular portion of the internal capsule. The medial geniculate body also sends fibers to other thalamic nuclei and may play a part in a regulatory feedback system, with descending projections to the inferior colliculus, the nucleus of the lateral lemniscus, the trapezoid body, and the superior olivary nucleus.

The primary auditory cortex corresponding to Brodmann areas 41 and 42 lies on the transverse gyrus of Heschl on the dorsal surface of the superior temporal convolution. Brodmann=s area 41 is a primary auditory reception area and receives its projections from the pars princapalis of the medial geniculate body. Areas 42, 52, and 22 lie immediately adjacent to the primary auditory cortex and are auditory-association areas (Figure 2). These association areas receive signals from the primary auditory cortex and send projections to the occipital, parietal, and insular cortex. Tonotopic organization of the auditory cortex is particularly impressive. In the simplest analysis, high frequencies are represented anteriorly and low frequencies posteriorly in the auditory cortex. Each auditory cortical area is reciprocally connected to a homotypic area in the contralateral hemisphere via projections in the corpus callosum. In addition, auditory-association areas connect with other sensory-association areas concerned with somatesthesia and vision. They also send projections that converge in the parietotemporal language area. It appears that the higher level of integration in the association areas is responsible for more complex interpretation of sounds. These properties of the auditory cortex may explain why patients with hemispheric lesions have little difficulty with hearing as measured by pure tone audiometry. However, such patients may have impaired ability to discriminate the distorted or interrupted speech patterns and have difficulty focusing on an isolated speech sample when a competing message is introduced.

There is a descending efferent auditory pathway that parallels the afferent pathway and is influenced by ascending fibers via multiple feedback loops. The specific function of this system in audition is not well understood, but clearly modulates central processing and regulates the input from peripheral receptors in a fashion similar to the role played by the efferent vestibular system.

It should be apparent from this brief overview of the peripheral and central auditory systems that the pathway is a complex multisynaptic system throughout which tonotopic organization is preserved. Maintenance of this tonotopic organization allows the cochlea to be represented at each synaptic locus and at various areas on the auditory cortex. A cardinal feature of the auditory nervous system is the extensive binaural representation of acoustic information at various levels resulting from the interaction of neural input from both ipsilateral and contralateral pathways.