CHAPTER 9. THE AUDITORY SYSTEM

OBJECTIVE:
    BE ABLE TO IDENTIFY IN HISTOLOGICAL PREPARATIONS THE AUDITORY RECEPTORS AND IN GROSS SPECIMENS AND BRAINSTEM SECTIONS THE NUCLEI, TRACTS, AND CORTICAL AREAS ASSOCIATED WITH THE AUDITORY PATHS.

Internal Ear

In the human ear, the labyrinth comprises, from anterior to posterior, the cochlea, in which the cochlear duct is located, the vestibule, in which the saccule and utricle are located, and the three semicircular canals, in which the semicircular ducts are located. The relationships of these structures can be observed in Figure 9-1 as well as in a horizontal section of the temporal bone (Sl. 81).

Osseous Labyrinth

The osseous labyrinth consists of the otic capsule, dense bone in the petrous part of the temporal bone, and the perilymphatic space enclosed by the otic capsule. This space is lined by endosteum and contains perilymph which is similar in composition to cerebrospinal fluid except for a higher protein content. The perilymphatic space is arranged in a series of continuous cavities that include the cochlea, the vestibule, and the semicircular canals.

Vestibule   (Figs. 9-1, 9-2; Sl. 81)

The vestibule is in the middle of the osseous labyrinth just medial to the tympanic cavity. It contains the utricle and saccule, parts of the membranous labyrinth. At its lateral surface is the oval window (fenestra vestibuli) which is closed by the base of the stapes.

Semicircular Canals

Opening into the vestibule are the three semicircular canals, anterior, posterior, and lateral. The anterior, also called the superior, and the posterior are oriented vertically but their axes are at right angles to each other. The lateral canal is oriented horizontally (Fig. 9-1)

Cochlea    (Fig. 9-1, 9-2, 9-3; Sl. 82)

The cochlea is a spiral tube of about 2 1/2 turns. In the center of the cochlea is a bony core, the modiolus, which contains the spiral ganglion and the cochlear nerve. An osseous spiral lamina projects from the modiolus, like the rib of a screw, to the cochlear duct. The spiral lamina and the cochlear duct separate the anterior part of the cochlea, the scala vestibuli, from the posterior part, the scala tympani. At the apex of the cochlea, the modiolus and cochlear duct end blindly and the two scalae become continuous with one another as the helicotrema.

Membranous Labyrinth (Figs. 9-1, 9-2)

Within the bony labyrinth is a series of fluid-filled spaces, the membranous labyrinth, which includes chiefly the cochlear duct, the saccule and utricle, and the semicircular ducts. The walls of these consist of fibrous tissue lined by epithelium. The fluid within the membranous labyrinth is endolymph.

Cochlear Duct (Fig. 9-4; Sl. 83)

The receptors for sound are located in the cochlear duct within the cochlea. The cochlear duct extends from the osseous spiral lamina to the wall of the cochlea. Its roof is formed by the vestibular membrane which separates it from the scala vestibuli. Its floor is formed by the osseous spiral lamina and the basilar membrane which separate the cochlear duct from the scala tympani. The basilar membrane stretches from the free edge of the spiral lamina to the outer wall of the cochlea where the endosteum thickens to form the spiral ligament.  

Above the spiral ligament the endosteum also thickens and forms the spiral prominence. Beneath the spiral prominence is the external spiral sulcus. Between the spiral prominence and the attachment of the vestibular membrane the wall of the cochlear duct is lined by a stratified epithelium which contains a rich plexus of intraepithelial capillaries. This lining is called the stria vascularis and is thought to secrete the endolymph.

Auditory Receptors

Resting on the basilar membrane of the cochlear duct is the spiral organ of Corti (Fig. 9-5; Sl. 84), which contains the auditory receptor cells along with several types of supporting cells. The neuroepithelial receptor cells are classified as inner and outer hair cells. The inner hair cells are the receptors for hearing, while the outer hair cells are for modulation.

The inner hair cells are arranged in a single row along the entire length of the cochlear duct, that is, from its base to its apex. They are surrounded by supporting cells, the inner phalangeal cells. The outer hair cells are arranged in three rows in the basal coil of the cochlea and in four or five rows in the apical coil. They are supported by the outer phalangeal cells. Hence, both the inner and the outer hair cells rest on the supporting cells, not the basilar membrane. Extending laterally from the outer phalangeal cells are several rows of cells that become continuous with the epithelium lining the outer wall of the cochlear duct at the external spiral sulcus. Extending medially from the inner phalangeal cells are border cells which become continuous with the epithelium lining the inner wall of the cochlear duct at the internal spiral sulcus.

Overlying the spiral organ is the tectorial membrane, a gelatinous structure in which the tips of the stereocilia of the hair cells are embedded. The hair cells are stimulated by movement of the basilar membrane which produces a shearing effect between the hair cells and the tectorial membrane.

Auditory Path

First Order Auditory Neurons

The inner and outer hair cells are in synaptic contact at their basal poles with the terminal endings of the dendrites of the bipolar spiral ganglion cells. Observe these first order auditory neurons in the spiral canal in the modiolus (Sls. 81, 82, 83).  Their dendrites pass laterally in canals in the osseous spiral lamina to the spiral organ where approximately 90% terminate on the inner hair cells while the others terminate on the outer hair cells.

The central processes of the spiral ganglion cells form the cochlear division of the VIII nerve which passes toward the cranial cavity in the internal acoustic meatus and enters the brainstem in the cerebellar angle. At its entrance into the medulla, near the pontomedullary junction (Pl. 8), it lies lateral to the vestibular nerve (Sl. 19).

Second Order Auditory Neurons

The cochlear nerve terminates on second order neurons in the dorsal and ventral cochlear nuclei (Sls. 17, 18, 19, 20).  The dorsal cochlear nucleus is posterolateral to the inferior cerebellar peduncle and lies beneath the acoustic tubercle in the floor of the lateral recess of the fourth ventricle (Pl. 9).  The ventral cochlear nucleus is slightly more rostral and is located anterolateral to the inferior cerebellar peduncle

The axons of the secondary auditory neurons in the cochlear nuclei form acoustic striae which, for the most part, are inconspicuous as they course medially and slightly rostrally in the caudal part of the pons. Three groups exist: dorsal from the dorsal cochlear nucleus, and intermediate and ventral from the ventral cochlear nucleus. The dorsal and intermediate acoustic striae stream through the pontine tegmentum to the contralateral side. The ventral acoustic stria becomes incorporated in the trapezoid body (Sls. 21, 22) which consists of conspicuous bundles of transversely-oriented fibers that pierce the medial lemniscus as they pass toward the midline. Upon crossing to the contralateral side, the three groups of acoustic striae proceed to the region lateral to the medial lemniscus, where they begin to ascend in the lateral lemniscus (Sls. 21, 22).  Adjacent to the lateral lemniscus, from caudal to middle pontine levels, is the superior olivary nucleus.

Accessory Auditory Nuclei

The accessory auditory nuclei consist of several groups of neurons that receive impulses from the acoustic striae and lateral lemnisci, and then give rise to axons, some of which ascend on the ipsilateral side, while others cross and ascend on the contralateral side. The bilateral connections of the accessory auditory nuclei result in the bilateral representation of sound in the central ascending auditory paths on each side.

Each superior olivary nucleus (Sls. 21, 22) receives impulses bilaterally from the acoustic striae. The superior olivary nuclei then give rise to axons that either ascend in the ipsilateral lateral lemniscus or cross to the contralateral side, via the trapezoid body, and ascend in the contralateral lateral lemniscus. Neurons scattered among the fibers of the trapezoid body, the so-called nuclei of the trapezoid body, have similar connections. As a result of the bilateral connections of these nuclei, each lateral lemniscus carries auditory impulses from both ears. In addition to the crossings of auditory impulses via the superior olivary and trapezoid nuclei, more crossings occur via nuclei associated with more rostral parts of the lateral lemniscus - a ventral nucleus in the rostral pons (Sl. 26) and a dorsal nucleus in the isthmus (Sls. 27, 28).

Inferior Colliculus

Upon reaching the midbrain, the lateral lemniscus enters the inferior colliculus (Pls. 9, 13, 16, 30; Sls. 29, 30) and terminates.  Hence, the lateral lemniscus extends only from the caudal pons to the caudal midbrain.

Most of the fibers from the inferior colliculus emerge from its lateral aspect and ascend along the wall of the midbrain as the brachium of the inferior colliculus (Pls. 9, 29; Sls. 29, 30).

The nuclei of the inferior colliculi also aid in the bilaterality of the auditory paths by sending axons to the contralateral side via the commissure of the inferior colliculus.

Medial Geniculate Nucleus

The brachium of the inferior colliculus terminates in the medial geniculate nucleus (Pls. 9, 34; Sls. 31, 32, 33).  This thalamic auditory center then gives rise to the auditory radiation that passes laterally (Sl. 32) to join that part of the posterior limb of the internal capsule beneath the lentiform nucleus.  Hence, the auditory radiation lies in the sublenticular part of the posterior limb. From here it travels to the primary auditory area in the transverse temporal gyri (of Heschl). These are buried in the lateral fissure on the dorsomedial surface of the superior temporal gyrus (Pl. 14).