Inner ear

Boundaries

The inner ear is embedded within the petrous part of the temporal bone, anterolateral to the posterior cranial fossa, with the medial wall of the middle ear, the promontory, serving as its lateral wall. The internal ear is comprised of a bony and a membranous component. The bony part, known as the bony (osseous) labyrinth, encases the membranous part, also known as the membranous labyrinth. Due to the size difference between the bony and membranous labyrinths, there is a space between the two structures that is filled with a fluid known as perilymph (similar in ionic composition to that of the extracellular fluid).

The lumen of the membranous labyrinth contains another fluid known as endolymph (possess a composition similar to cytosol). These two substances do not communicate with each other. It is noteworthy that in the literature, the lumen of the bony components is often referred to as “canals” (i.e. semi-circular canals), while that of the membranous components are referred to as “ducts” (i.e. semi-circular ducts).

Cochlear component

The bony casing of the cochlea is spirally coiled like that of a snail’s shell (from the Greek, cochlos, meaning snail). It is thickest at the base, and gradually becomes thinner with each coil until it terminates centrally at the cochlear cupula. The canal makes its spirals around a bony core known as the modiolus; the base of which rests at the bottom of the internal acoustic meatus, while the apex projects laterally (towards the middle ear) along the axis of the petrous bone. A thin, delicate bony ledge, called the spiral lamina, projects from the modiolus into the cochlear canal.

Like the vestibule and semi-circular canals (described below), the cochlear canal is covered by endosteum. The canal also communicates with the middle ear via the fenestra cochleae (round window), which, in life, is covered by the secondary tympanic membrane. The cochlear canal houses the membranous cochlear duct, which has attachments to both the spiral lamina and the canal’s outer wall and contains endolymph. The endolymph enters the cochlear duct from the saccule via the ductus reuniens.

The cochlear duct follows the spirals of the bony cochlea and terminates deep to the cochlear cupula. Because the duct is significantly thinner than the cochlear canal, it divides the canal into two parts: the scala tympani, which is found on the basal part and the scala vestibuli, located on the apical region. The scala tympani communicates with the scala vestibuli at the apex of the modiolus, a point known as the helicotrema. The distal end of the cochlear canal drains its perilymph via the cochlear aqueduct and the cochlear canaliculus into the subarachnoid space just inferior to the internal acoustic meatus in the anterior segment of the jugular foramen.

A cross section of the cochlea will reveal the microscopic apparatus that is responsible for conversion of vibrational energy into neuronal impulses. Within a particular cochlear coil, three spaces can be appreciated. The osseous spiral lamina projects horizontally into the cochlear duct. A membranous band, called Reissner’s (vestibular) membrane travels obliquely from the apical surface of the spiral lamina to the bony (osseous) cochlear. A second membrane, called the basilar membrane, extends horizontally from the apex of the spiral lamina to the osseous cochlea. Both the vestibular and basilar membranes are connected along the osseous cochlea by the spiral ligament.

Superomedial to Reissner’s membrane, and superior to the spiral lamina is the scala vestibuli; while inferior to the basal membrane and spiral lamina is the scala tympani. The resulting space between the three aforementioned membranes (roughly the shape of a sector) is the endolymph-filled cochlear duct. It is the home of the spiral organ of Corti, the sensory epithelium that underlies hearing. The organ of Corti consists of stereociliae suspended in supporting cells that are covered by the tectorial membrane.

Vestibular component

Semi-circular component

Three semi-circular bony structures project from the vestibule in three different directions:

1. Superior (anterior) semi-circular canal projects vertically and perpendicular relative to the petrous bone.
2. Posterior semi-circular canal also extends vertically, but along the vertical axis of the petrous bone.
3. Lateral semi-circular canal projects almost horizontally and laterally, with respect to the petrous bone.

At one end of each canal is an enlargement called the bony ampulla. The ampullae of all three canals open into the vestibule independently. However, the non-ampullated ends of the anterior and posterior canals fuse to form the common bony crus. The non-ampullated end of the lateral semi-circular canal is known as the simple bony crus.

Like the other membranous structures, the semi-circular ducts that occupy the semi-circular canals are smaller than their bony counterparts. The only exception to the rule is at the area of the membranous ampullae, which almost completely occupy the bony ampullae. The three ampullated ends and two non-ampullated ends of the membranous semi-circular duct open into the utricle. The medial aspect of each ampullated end of the semi-circular ducts has a transverse crest that houses a specialised sensory neuroepithelium that is sensitive to angular acceleration of the head.

Like the maculae of the saccule and utricle, the sensory neuroepithelium in the transverse crest is covered by a gelatinous structure called the cupula (plural cupulae). Due to its density, the cupula lags behind during head movement, allowing activation of the mechanosensitive hair cells in the neuroepithelium.

This quiz will challenge to you identify the structures of the internal ear.

Vasculature

Innervation

The vestibulocochlear nerve (CN VIII) provides special sensory innervation to the inner ear. Immediately after leaving the internal acoustic meatus, CN VIII divides into its cochlear division and its vestibular division. The cochlear nerve spirals towards the cochlea where it gives branches throughout the coils of the canal. Fibers then branch off and form spiral ganglia of Corti in the modiolus of the cochlea. Nerve fibers of these ganglia travel through the osseous spiral lamina and their dendrites communicate with the stereocilia of the spiral organs of Corti.

After branching from CN VIII, the vestibular division branches again to form a superior and inferior division – each with their own vestibular ganglion. The fibers of the superior vestibular ganglion innervate the ampullae of the anterior and lateral semi-circular ducts, and the utricle. The ampulla of the posterior semi-circular duct and the saccule are innervated by fibers of the inferior vestibular ganglion.

Of note, the facial nerve (CN VII) traverses the internal acoustic meatus along with the labyrinthine vessels and CN VIII. However, CN VII does not innervate any structure in the inner ear.