The Inner Ear

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The ear is split into three sections: the outer ear, the middle ear, and the inner ear.

The vestibulocochlear organs are housed in the inner ear, the innermost region of the ear. It serves two primary purposes:

  • To transform mechanical impulses from the middle ear into electrical signals that can be sent to the brain’s auditory system.
  • To keep equilibrium by detecting position and motion.

This article will examine the inner ear’s anatomy, including its location, structure, and neurovascular supply.

Anatomical Position and Structure

The inner ear is positioned in the petrous region of the temporal bone. It is located between the middle ear and the internal acoustic meatus, which are located laterally and medially. The inner ear comprises the bone labyrinth and the membranous labyrinth.

  • Bony labyrinth – a series of bony cavities located within the petrous portion of the temporal bone. The cochlea, vestibule, and three semicircular canals make up the cochlea. Internally, these structures are lined by periosteum and contain a perilymph fluid.
  • Membranous labyrinth The bony labyrinth contains the membraneous labyrinth. It is made up of the cochlear duct, semicircular ducts, utricle, and saccule. Endolymph is the fluid that fills the membranous labyrinth.

The inner ear has two membrane-covered apertures in the middle ear. The oval window connects the middle ear to the vestibule. In contrast, the round window connects the middle ear to the scala tympani (part of the cochlear duct).

Anatomical Position and Structure

The inner ear is positioned in the petrous region of the temporal bone. It is located between the middle ear and the internal acoustic meatus, which are located laterally and medially. The inner ear comprises the bone labyrinth and the membranous labyrinth.

  • Bony labyrinth – a series of bony cavities located within the petrous portion of the temporal bone. The cochlea, vestibule, and three semicircular canals make up the cochlea. Internally, these structures are lined by periosteum and contain a perilymph fluid.
  • Membranous labyrinth The bony labyrinth contains the membraneous labyrinth. It is made up of the cochlear duct, semicircular ducts, utricle, and saccule. Endolymph is the fluid that fills the membranous labyrinth.

The inner ear has two membrane-covered apertures in the middle ear. The oval window connects the middle ear to the vestibule. In contrast, the round window connects the middle ear to the scala tympani (part of the cochlear duct).

Bony Labyrinth

The bony labyrinth is a collection of bony cavities within the temporal bone’s petrous section. It is divided into three sections: the cochlea, the vestibule, and the three semicircular canals.

Vestibule

The vestibule is located in the center of the bony labyrinth. The oval window separates it from the middle ear and communicates anteriorly with the cochlea and posteriorly with the semicircular canals.

The vestibule contains two components of the membrane labyrinth: the saccule and the utricle.

Cochlea

The cochlea duct of the membranous labyrinth – the auditory component of the inner ear – is housed in the cochlea. It coils around a core section of bone known as the modiolus, forming a cone shape that points anterolaterally. Branches from the vestibulocochlear (VIII) nerve’s cochlear component can be detected at the base of the modiolus.

A ledge of bone known as the spiral lamina extends outwardly from the modiolus and joins to the cochlear duct, keeping it in place. The existence of the cochlear duct results in the formation of two perilymph-filled chambers above and below:

  • Scala vestibuli: Located superiorly to the cochlear duct. As its name suggests, it is continuous with the vestibule.
  • Scala tympani: Located inferiorly to the cochlear duct. It terminates at the round window.

Semicircular Canals

The semicircular channels are anterior, lateral, and posterior. They contain the semicircular ducts in charge of balancing (along with the utricle and saccule).

The canals are super posterior to the vestibule and at right angles to one another. They have a swelling at one end called an ampulla.

Fig 2 – The three parts of the bony labyrinth.

Membranous Labyrinth

The membranous labyrinth is a continuous network of endolymph-filled channels. It is located within the bone labyrinth and is surrounded by perilymph. The cochlear duct comprises three semicircular ducts, the saccule and the utricle.

The cochlear duct is the organ of hearing located within the cochlea. The organs of balance are the semicircular ducts, saccule, and utricle (also known as the vestibular apparatus).

Cochlear Duct

The cochlea duct of the membranous labyrinth – the auditory component of the inner ear – is housed in the cochlea. It coils around a core section of bone known as the modiolus, forming a cone shape that points anterolaterally. Branches from the vestibulocochlear (VIII) nerve’s cochlear component can be detected at the base of the modiolus.

A ledge of bone known as the spiral lamina extends outwardly from the modiolus and joins to the cochlear duct, keeping it in place. The existence of the cochlear duct results in the formation of two perilymph-filled chambers above and below:

  • Scala vestibuli: Located superiorly to the cochlear duct. As its name suggests, it is continuous with the vestibule.
  • Scala tympani: Located inferiorly to the cochlear duct. It terminates at the round window.
Fig 3 – Structure of the cochlea, and borders of the cochlear duct.

Semicircular Canals

The semicircular channels are anterior, lateral, and posterior. They contain the semicircular ducts in charge of balancing (along with the utricle and saccule).

The canals are super posterior to the vestibule and at right angles to one another. They have a swelling at one end called an ampulla.

Cochlear Duct

The cochlear duct runs through the bone framework of the cochlea. The spiral lamina holds it in place. The existence of the duct results in the formation of two canals above and below it, known as the scala vestibuli and scala tympani, respectively. A triangle form may be defined for the cochlear duct:

  • Lateral wall – Formed by thickened periosteum, known as the spiral ligament.
  • Roof – Formed by a membrane that separates the cochlear duct from the scala vestibule, known as the Reissner’s membrane.
  • Floor – Formed by a membrane separating the cochlear duct from the scala tympani, the basilar membrane.

The basilar membrane is home to the Organ of Corti’s hearing epithelial cells. The scope of this page does not allow for a more extensive discussion of the Organ of Corti.

The cochlear duct is the organ of hearing located within the cochlea. The organs of balance are the semicircular ducts, saccule, and utricle (also known as the vestibular apparatus).

Saccule and Utricle

The vestibule has two membrane sacs called the saccule and utricle. They are balancing organs that sense movement or acceleration of the head in the vertical and horizontal planes.

The bigger of the two, the utricle, receives the three semicircular ducts. The cochlear duct enters the saccule, which is globular in form.

Endolymph feeds into the endolymphatic duct from the saccule and utricle. The duct connects to the posterior portion of the petrous section of the temporal bone through the vestibular aqueduct. The duct extends to form a bag where endolymph can be secreted and absorbed.

Semicircular Ducts

The semicircular ducts share the same orientation as the semicircular canals. Endolymph flow within the ducts changes speed and direction when the head moves. Sensory receptors in the semicircular canals’ ampullae detect this shift and provide signals to the brain, allowing balance to be processed.

Vasculature

The vascular supply to the bone and membrane labyrinths differ. The bony labyrinth is supplied with blood by three arteries that simultaneously serve the surrounding temporal bone:

  • Anterior tympanic branch (from a maxillary artery).
  • Petrosal branch (from a middle meningeal artery).
  • Stylomastoid branch (from a posterior auricular artery).

The labyrinthine artery, a branch of the inferior cerebellar artery, supplies the membranous labyrinth (or, occasionally, the basilar artery). It is divided into three sections:

  • Cochlear branch – supplies the cochlear duct.
  • Vestibular branches (x2) – supply the vestibular apparatus.
  • The labyrinthine vein drains the inner ear and exits into the sigmoid sinus or inferior petrosal sinus.
  • Innervation – The vestibulocochlear nerve innervates the inner ear (CN VIII). It reaches the inner ear through the internal acoustic meatus, where it separates into the vestibular nerve (which controls balance) and the cochlear nerve (which controls hearing):
  • Vestibular nerve – expands to create the vestibular ganglion, which divides into superior and inferior sections to feed the utricle, saccule, and three semicircular ducts.
  • Cochlear nerve – enters at the base of the modiolus and branches via the lamina to provide the Organ of Corti receptors.

CN VII, the facial nerve, similarly runs through the inner ear but does not innervate any of the tissues there.

Fig 4 - The labyrinthine artery arising from the basilary artey