Neuroanatomy of the Auditory System

Osteology

The inner ear, within the petrous portion of the temporal bone, is encased in a bony structure called the osseous labyrinth or bony labyrinth. The bone that forms the cochlea and vestibular labyrinth is the hardest bone in the human body and is akin to ivory in its density. The labyrinth consists of the three following contiguous sections:

• The cochlea

• The vestibule

• The semicircular canals

The initial point for the communication of sound energy between the middle and inner ears occurs at the oval window of the vestibule, where the stapes footplate abuts the oval window membrane. The cochlea is a snail-shaped structure that has a wide diameter at the base and narrows over turns until it reaches its apex. Fig. 127.1 shows a cross section of the cochlea through the basal, middle, and apical turns.

The central core of the cochlea is the modiolus, a highly porous bone that allows the passage of auditory nerve fibers from the internal auditory meatus to the hair cell synapses. Extending from the modiolus is the osseous spiral lamina , which coils around the center of the cochlea and provides partial division of the upper and lower cochlear chambers into the scala vestibuli and scala tympani, respectively. At the apex of the cochlea is the helicotrema, where these fluid-filled scalae are in communication. The spiral lamina is also the point of attachment for the basilar membrane, which is the lower border of the scala media.

Along the length of the cochlea, the widths of the spiral lamina and basilar membrane are inversely related. Specifically, they appear as follows:

• Osseous spiral lamina

  • Wider at the base and narrower toward the apex

• Basilar membrane

  • Narrower at the base and wider at the apex

  • Thicker at the base and thinner at the apex

This anatomy is a key factor that confers frequency specificity to the basilar membrane. Namely, the cochlea is responsive to high frequencies at the base and low frequencies at the apex.

Membranous Labyrinth and Inner Ear Fluids

The membranous labyrinth of the cochlea follows the shape of the osseous cochlea and encloses a third cochlear chamber, the scala media. It is distinct from the scala vestibuli and scala tympani, which—as already mentioned—are connected at the helicotrema. The scala media is bordered

• Superiorly by the Reissner membrane

• Inferiorly by the basilar membrane

• Laterally by a portion of the outer cochlear wall

This sets up a two-fluid system within the cochlea, which creates an environment crucial to

• The mechanical displacement of the basilar membrane from the traveling wave

• The cellular depolarization and subsequent synaptic activity in the hair cells

Key aspects of the two-fluid system are as follows:

• Endolymph

  • Scala media

  • High potassium; low sodium

  • Akin to intracellular fluid

  • Maintained by the stria vascularis

  • Communicates with the endolymphatic duct and sac

  • Disorders include endolymphatic hydrops (Ménière disease) or wide vestibular aqueduct

• Perilymph

  • Found in scala vestibuli, scala tympani, and internal spaces of the organ of Corti

  • Low potassium; high sodium

  • Akin to extracellular fluid

  • Communicates with cerebrospinal fluid via cochlear aqueduct

  • Likely conduit for bacterial meningitis into the inner ear

The organ of Corti is situated on the basilar membrane ( Fig. 127.2 ). It runs longitudinally along the length of the basilar membrane and comprises multiple types of epithelial cells and structures. Medially, seated atop the osseous spiral lamina, is the spiral limbus, a thickened band of periosteum that serves as the point of medial attachment for the Reissner membrane and gives rise to the tectorial membrane, which lies over the inner and outer hair cells. The tectorial membrane is a compliant gelatinous structure composed primarily of collagen II fibers; it serves as a mass load that moves similarly to a rubber band. Lateral to the spiral limbus is the inner spiral sulcus, which is lined with the border cells of Held. In addition, one row of inner hair cells is present, the cell bodies of which are surrounded by supporting cells called phalangeal cells.

Although the scala media is filled with endolymph, the spaces within the organ of Corti are filled with perilymph. The perilymph-filled spaces and their boundaries within the organ of Corti are shown in Fig. 127.2 and include

• The tunnel of Corti

  • Between the inner and outer pillar cells

• Space of Nuel

  • Between the outer pillar cells and first outer hair cells

• Outer tunnel

  • Between the third outer hair cells and Hensen cells

• Intercellular spaces

  • Surrounding hair cells themselves

The phalangeal cells, phalangeal processes of the Deiters cells, and superior surfaces of the hair cells form the reticular lamina, a tightly interwoven matrix that supports the apices of the hair cells. The reticular lamina forms a barrier from endolymph, the fluid in the scala media, which, owing to its ionic composition, is toxic to hair cells.

Hair Cells

The inner and outer hair cells function as receptor cells that transduce mechanical energy into an electrochemical signal to stimulate the auditory nerve. Fig. 127.3 presents schematic examples of inner and outer hair cells. The apical portion of all hair cells includes a thickened region called the cuticular plate, which, in conjunction with the supporting cells, forms the reticular lamina. Rooted in the cuticular plate of each hair cell and projecting through the reticular lamina (into endolymph) are bundles of actin filaments known as stereocilia, stiff hair-like structures that deflect with mechanical disturbances. Adjacent to this is a noncuticular region that contains a rudimentary kinocilium. Cellular distinction between the hair cells include

• Inner hair cells

  • Flask shaped: wide at bottom, narrow at the top

  • High concentration of metabolic organelles

  • Golgi bodies, mitochondria

  • Passive transducers of mechanical (acoustic) motion to electrical activity

  • Many afferent fibers contact the base

  • Indirect contact by efferent fibers

• Outer hair cells

  • Cylindrical: narrow at bottom and top

  • High concentration of motile structures

• Microfilaments and microtubules

  • Active participants in tuning the cochlea to specific frequencies

  • Fewer afferent fibers contact the base

  • Direct contact by efferent fibers

The inner and outer hair cells are differently arranged within the organ of Corti but have some similarities ( Fig. 127.4 ). Key points include the following:

• Inner hair cells

  • Approximately 3500

  • Arranged in one row

  • Modiolar side of tunnel of Corti

  • Stereocilia are shallow and U shaped

  • Open toward the modiolus

  • Two or more rows of stereocilia per cell

  • Stereocilia have graduated lengths

  • Longest on the strial side and shortest on the modiolar side

• Outer hair cells

  • Approximately 12,000

  • Arranged in three rows

  • Strial side of the organ of Corti

  • Stereocilia are V or W shaped

  • Open toward the modiolus

  • Three or more rows of stereocilia per cell

  • Stereocilia have graduated lengths

  • Longest on the strial side and shortest on the modiolar side

The longest stereocilia on the outer hair cells contact the tectorial membrane, which results in deflection of the stereocilia with basilar membrane movement. The stereocilia are connected to each other by filamentous links laterally, by cross links, and from the tips of shorter stereocilia to the sides of the taller ones by tip links. These multiple links ensure that the connected stereocilia move as a unit when the longer stereocilia are deflected.