What happens after vibrations have been transmitted across the tympanic membrane and through the auditory ossicles? What are the structures involved in the next segment of the auditory pathway?
The structures of the inner ear are designed to convert the mechanical energy transmitted in the form of waves generated by surrounding objects into neuronal impulses (transduction) that can be interpreted as sound. Likewise, the inner ear also plays pivotal roles in maintaining postural balance and visual focus on a single object (gaze fixation). As a result, the inner ear (which consists of a series of interlinked cavities termed labyrinths) can be divided into three general parts:
- the cochlear component that is concerned with hearing
- the vestibular component (comprised of the utricle and saccule) that deals with balance while stationary
- the semi-circular component that regulates balance while in motion.
The former is located anterior to the latter. The gross anatomical structures, their spatial relations, innervation and blood supply will be discussed in this article.
Boundaries of the Inner Ear
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).
The 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.
The vestibule is a hollow cavity located between the cochlea and the semi-circular canals and is located medial to the tympanic cavity, posterior to the cochlea and anterior to the semicircular canals. It contains three recesses:
- an elliptical recess closer to the ampullae of the anterior and lateral semi-circular canals
- a cochlear recess adjacent to the cochlea
- and a spherical recess adjacent to the opening of the scala vestibuli
Six orifices open into the vestibule: five belonging to the semi-circular canals and one resulting from the scala vestibuli of the cochlea. There are smaller openings on the side adjacent to the internal acoustic meatus that serve as conduits for the vestibular part of CN VIII (vestibulocochlear). The vestibule communicates with the middle ear via the fenestra vestibuli (oval window). In life, it is covered by the footplate of the stapes along with its annular ligament.
The membranous contents of the vestibule are the saccule and utricle. The saccule is a small, fibrous pouch that communicates inferiorly with the cochlear duct via the ductus reuniens. The utricle is also a fibrous sac and communicates with the semi-circular ducts. The saccule resides in the anteroinferior compartment of the vestibule, while the utricle occupies the posterosuperior portion. Close to the point at which the walls of both sacs abut, each gives off a small duct that fuses to form a Y shaped structure. The long stem of the ‘Y’ is the endolymphatic duct that traverses the vestibular aqueduct and terminates as the endolymphatic sac, deep to the dura mater in the posterior cranium.
It is important to note that the membranous labyrinth is a closed system; therefore endolymph is removed from the ducts by reabsorption via the epithelia of the endolymphatic sac. Lining the medial walls of each saccule and the floor of each utricle is a specialized area of neurosensory epithelium called macula (plural maculae) which is used to detect linear acceleration of the head. The orientation of the macula in the utricle makes it particularly sensitive to linear acceleration in the horizontal plane, meanwhile the macula in the saccule is more sensitive to acceleration in the vertical plane (such as the gravitational force). The epithelial surface of each macula is characterised by hair cells with numerous stereocilia and one prominent kinocilum. These cells are covered by a gelatinous structure known as the otolithic or statoconial membrane and embedded in it are several small crystals known as otoconia (also known as otoliths, from the greek oto-, ear + lithos, a stone). Linear acceleration of the head will cause the otolithic membrane to lag behind as a result of the inertia produced by its increased density and will lead to activation or inhibition of the mechanosensitive hair cells.
Three semi-circular bony structures project from the vestibule in three different directions:
- superior (anterior) semi-circular canal projects vertically and perpendicular relative to the petrous bone,
- posterior semi-circular canal also extends vertically, but along the vertical axis of the petrous bone, and
- 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), which due to its density, lags behind during head movement allowing activation of the mechanosensitive hair cells in the neuroepithelium.
The basilar artery, formed by the fusion of the left and right vertebral arteries, resides in the basilar groove of the ventral pons of the midbrain. This artery often gives rise to a labyrinthine (internal acoustic) artery just superior to the anatomical origin of CN VI (at the pontomedullary junction). The artery may also arise from first branch of the basilar artery, the anterior inferior cerebellar artery (AICA). The artery enters the inner ear via the internal acoustic meatus, where it divides to give branches to the cochlear and vestibular divisions of the inner ear. The stylomastoid artery (branch of either the posterior auricular or occipital branches of the external carotid artery) provides supplementary blood supply to the inner ear.
Venous drainage is achieved via the labyrinthine veins, which travel through the internal acoustic meatus to drain their contents in the superior petrosal sinus or the transverse sinus. Inconsistent veins also pierce the petrous bone to drain their contents to the superior petrosal.
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 posterior semi-circular ducts, and the utricle. The ampulla of the lateral 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.