The bony labyrinth is filled with perilymph and contains the membranous labyrinth within it via a combination of delicate filaments as well as a thick ligament known as the spiral ligament. The membranous labyrinth contains endolymph within it. The fluids within the inner ear structures stimulate the end organs resulting in the perception of sound and the maintenance of balance.

The bony labyrinth consists of the cochlea, the vestibule and the three semicircular canals, which are a series of three cavities surrounded by the otic capsule of the petrous temporal bone – the bony labyrinth itself is the fluid filled space contained by the dense surrounding otic capsule. It is lined by periosteum and is separated from the membranous labyrinth by perilymph. The endolymph within the membranous labyrinth does not communicate with the perilymph of the bony labyrinth.

The cochlea is this curved shell-like structure of the bony labyrinth which contains the cochlear duct, and is responsible for sound perception.

The cochlear is oriented anteriorly. The spiral canal of the cochlear begins at the vestibule and makes 2 and half turns around a central conical core of spongy bone known as the modiolus.

The cochlea base is sited posteromedially, with the apex pointing anterolaterally and slightly inferiorly. The base of the modiolus is angled towards the internal auditory meatus where it receives branches of the cochlear nerve which arise from the 8th cranial nerve – the vestibulocochlear nerve. The lamina of the modiolus or spiral lamina, is a thin lateral projection of bone from the central modiolus. The cochlea duct which is part of the membranous labyrinth, is attached to the spiral lamina as it circles around the modiolus. The cochlea duct attaches to the outer wall of the cochlea and creates two separate compartments – the scala vestibuli and the scala tympani which are continuous with each other within the cochlea apex via a narrow opening known as the helicotrema.  The scala vestibuli communicates with the vestibule, whereas the scala tympani extends to the round window.

The base of the cochlear, or the basal turn, forms the cochlear promontory which in turn forms the medial wall of the middle ear compartment (see video on boundaries of the middle ear for further details). The basal turn of the cochlear is connected with the subarachnoid space of the posterior cranial fossa with an opening just superior to the jugular foramen via the cochlear aqueduct. The round window is also formed within the basal turn of the cochlear, which is sealed by the secondary tympanic membrane or round window membrane. The round window vibrates with opposite phase to the vibrations entering the inner ear via the oval window, which thereby allows movement of the perilymph within the cochlea.

The vestibule is a small chamber which forms the central component of the bony labyrinth, measuring approximately 3 to 5mm. It is located immediately medial to the tympanic cavity, behind the cochlear and in front of the semicircular canals.

The vestibule contains the utricle and the saccule as well as the basal end of the cochlear duct (cochlear recess). Within its lateral wall, is the oval window which is closed off by the stapes footplate.

Posteriorly the vestibule communicates wite semicircular canals, and has an opening into the posterior cranial fossa via the vestibular aqueduct which opens posterolateral to the internal auditory meatus and  contains the endolymphatic duct. Anteriorly, the vestibule is continuous with the cochlea.

There are three semicircular canals – the superior, posterior and lateral semicircular canals which are set perpendicular to each other in 3 different planes. The semicircular canals lie behind and above the vestibule with which they communicate. They each form approximately 2/3rds of a circle and form a small dilatation at one end known as the bony ampulla. The superior and posterior semicircular canals form a common limb (crus) before connecting with the vestibule, whereas the lateral semicircular canal opens directly into the vestibule. The semicircular ducts are contained within the semicircular canals and are responsible for kinetic balance.

The membranous labyrinth is a system of sacs and ducts continuous with one another and contained within the bony labyrinth. These structures are  filled with a fluid known as endolymph and separated from the walls of the bony labyrinth by perilymph. It is comprised of two sacs centrally within the vestibule (the utricle and the saccule), and a series of four ducts – one for each of the three semicircular canals and a fourth duct contained within the cochlea – the cochlear duct.

The membranous labyrinth functions in producing the sense of hearing and in the maintenance of balance. The structures related to balance, the vestibular apparatus are comprised of the utricle, the saccule and the three semicircular ducts. The cochlear duct is the structure related to the perception of sound. The membranous labyrinth may be considered as compromised of two parts – the vestibular labyrinth and the cochlear labyrinth.

We are going to take a look at cross section of the cochlear to review the anatomy of the cochlea duct and relevant the internal components of the cochlea.

The cochlear duct is a triangular shaped duct which divides the cochlear canal of the into two canals; the scala vestibuli and the scala tympani, sitting centrally in between them. Because it is located in between these two canals, it may also be referred to as the scala media. The scala vestibuli and the scala tympani are continuous with one another at the apex of cochlea at the helicotrema – a small slit which enables communication between the two channels. The cochlear duct houses the organ of Corti, which is the organ of hearing.

The cochlear duct is attached centrally to the bony spiral lamina of the modiolus, which is this thin bit of bone which projects outwards from the central modiolus.

Peripherally the cochlea duct is attached to the outer wall of the cochlea by this thickened bit of periosteum, known as the spiral ligament.

The roof of the cochlea duct is this part of which separates the endolymph within it from the perilymph within the scala vestibuli, and is known as the vestibular membrane. This structure consists of a core of connective tissue lined by epithelium.

The floor, which separates the endolymph from the perilymph within the scala tympani is comprised of two components: a thin membrane known as the basilar membrane, and the free edge of the lamina of the modiolus. The basilar membrane attaches and blends within the spiral ligament on one end, and connects to the free edge of the lamina of the modiolus at the other end.

The spiral organ or organ of Corti is the organ of hearing which sits on the basilar membrane, projecting into the endolymph off the cochlear duct. It enables transduction of auditory signals through the vibration of the inner ear structures which results in displacement of the cochlear fluid, ultimately producing electrochemical signals through the movement of hair cells which are transmitted back to the brain via the vestibulocochlear nerve. The tectorial membrane lies over the organ of Corti.

The utricle and saccule are contained within the vestibule. The utricle is contained within the posterosuperior part of the vestibule and is larger than the saccule, with an oval and elongated shape. The three semicircular ducts drain into the utricle.

The three semicircular canal ducts mirror the shape of their bony container, with a dilatation at one end corresponding to the ampulla.

The saccule is the smaller sac within the vestibule, located anteroinferiorly within it and communicating with the cochlear duct via the ductus reuniens.

The utricle and saccule are connected via the utriculosaccular duct which means that all the components of the membranous labyrinth are in continuity with one another. The endolymphatic duct arises from the utriculosaccular duct, and opens into the posterior cranial fossa via the bony channel known as the vestibular aqueduct. At its opening within the posterior cranial fossa, it forms a small pouch known as the endolymphatic sac which is extradural – it lies just beneath the dura which overlies the posterior surface of the petrous temporal bone. This structure serves both as a site for reabsorption of endolymph into the epidural space as well as a reservoir for endolymphatic fluid. This is different to the perilymphatic duct which arises from the scala tympani of the cochlea and passes via the cochlear aqueduct, draining perilymph into the subarachnoid space of the posterior cranial fossa.

There are specialized areas of sensory epithelium within the vestibular apparatus , which are found within each of the components. In the ampulla of the semicircular ducts, this is known as the crista ampullaris, or ampullary crests , and within the components of the vestibule, they are known as the macula of the utricle and macula of the saccule. Within the utricle the sensory receptor is located within the floor, and within the saccule it is found within the medial wall in the vertical plane.

The crista ampullaris is covered in receptor cells called hair cells which is covered by a gelatinous structure called the cupula which extends from the crista to the ampullary roof. With rotational movement, the endolymph within the semicircular duct displaces the cupula against the hair cells in the opposite direction of head movement. The stimulation of the hair cells ultimately results in neuronal stimulation with the corresponding signals transmitted to the brain via the vestibulocochlear nerve.

The receptors within the semicircular canals can respond to movement in the plane of their respective ducts, whereas the utricle is involved with centrifugal and vertical acceleration, and the saccule is involved with linear acceleration.

Sound waves enter via the external auditory canal and cause vibrations of the tympanic membrane which are transmitted via the ossicles of the middle of the ear, ultimately to the stapes – the last ossicle in the chain, which attaches via its footplate to the oval window. This process transfers the vibrations of the tympanic membrane due to the sound waves, into a wave within the perilymph of the scala vestibuli within the cochlear.

This sound wave passes through the perilymph within the spiral structure of the cochlear within the scala vestibuli, to the helicotrema within the cochlea apex, where it is continuous with the scala tympani. The pressure wave is then transmitted back down through the perilymph of the scala tympani to the base of the cochlear where it the pressure wave is converted into vibrations of the secondary tympanic membrane which covers the round window. This process results in vibration of the basilar membrane, stimulating the receptor cells with the organ of Corti, and generating neural impulses which are transmitted to the brainstem via the vestibulocochlear nerve