The ear deserve to be separated into three parts: the outer ear, center ear and inner ear.

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The inner ear is the innermost part of the ear, and also houses the vestibulocochlear organs. It has actually two key functions:

To convert mechanical signal from the middle ear right into electrical signals, which can transfer info to the listening pathway in the brain.To maintain balance through detecting position and motion.

In this article, we shall look at the anatomy the the inner ear – that is position, structure, and also neurovascular supply.


by TeachMeSeries Ltd (2021)
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Fig 1 – synopsis of the ear


Anatomical Position and also Structure

The inside ear is situated within the petrous part of the temporal bone. It lies in between the center ear and also the interior acoustic meatus, which lie laterally and also medially respectively. The inner ear has actually two main materials – the bony labyrinth and membranous labyrinth.

Bony labyrinth – is composed of a collection of bony cavities within the petrous part of the temporal bone. It is composed of the cochlea, vestibule and also three semi-circular canals. Every these structures space lined internally v periosteum and save a fluid referred to as perilymph.Membranous labyrinth – lies in ~ the bony labyrinth. It consists of the cochlear duct, semi-circular ducts, utricle and the saccule. The membranous labyrinth is filled v fluid referred to as endolymph.

The inner ear has actually two openings right into the middle ear, both extended by membranes. The oval window lies in between the center ear and the vestibule, whilst the ring window off the middle ear indigenous the scala tympani (part the the cochlear duct).

Bony Labyrinth

The bony labyrinth is a series of bony cavities within the petrous component of the temporal bone. It is composed of three parts – the cochlea, vestibule and the three semi-circular canals.

Vestibule

The vestibule is the main part the the bony labyrinth. That is separated from the center ear through the oval window, and also communicates anteriorly v the cochlea and also posteriorly through the semi-circular canals.

Two components of the membranous labyrinth; the saccule and utricle, are situated within the vestibule.

Cochlea

The cochlea dwellings the cochlea duct that the membranous labyrinth – the auditory part of the inner ear. It twists ~ above itself roughly a central portion that bone called the modiolus, producing a cone form which point out in an anterolateral direction. Branches indigenous the cochlear part of the vestibulocochlear (VIII) nerve are uncovered at the base of the modiolus.

Extending outwards from the modiolus is a ledge that bone recognized as spiral lamina, i beg your pardon attaches come the cochlear duct, holding that in position. The visibility of the cochlear duct creates 2 perilymph-filled chambers over and below:

Scala vestibuli: located superiorly to the cochlear duct. As its name suggests, that is continuous with the vestibule.Scala tympani: situated inferiorly to the cochlear duct. The terminates in ~ the round window.

Semi-circular Canals

There room three semi-circular canals: anterior, lateral and also posterior. Lock contain the semi-circular ducts, which are responsible because that balance (along v the utricle and saccule).

The canals are located superoposterior come the vestibule, at right angles to every other. They have a swelling at one end, known as the ampulla.


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Fig 2 – The three parts of the bony labyrinth.


Membranous Labyrinth

The membranous labyrinth is a continuous system that ducts filled with endolymph. It lies in ~ the bony labyrinth, surrounded by perilymph. The is written of the cochlear duct, three semi-circular ducts, saccule and also the utricle.

The cochlear duct is positioned within the cochlea and is the body organ of hearing. The semi-circular ducts, saccule and also utricle room the offal of balance (also recognized as the vestibular apparatus).

Cochlear Duct

The cochlear duct is situated within the bony scaffolding that the cochlea. It is hosted in ar by the spiral lamina. The existence of the duct creates 2 canals above and listed below it – the scala vestibuli and scala tympani respectively. The cochlear duct deserve to be described as having actually a triangle shape:

Lateral wall – developed by thickened periosteum, known as the spiral ligament.Roof – formed by a membrane i m sorry separates the cochlear duct indigenous the scala vestibuli, well-known as the Reissner’s membrane.Floor – developed by a membrane i m sorry separates the cochlear duct from the scala tympani, recognized as the basilar membrane.

The basilar membrane dwellings the epithelial cells of hearing – the Organ of Corti. A an ext detailed summary of the body organ of Corti is past the scope of this article.


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Fig 3 – framework of the cochlea, and also borders of the cochlear duct.


Saccule and Utricle

The saccule and also utricle space two membranous sacs situated in the vestibule. They space organs of balance i beg your pardon detect movement or acceleration that the head in the vertical and also horizontal planes, respectively.

The utricle is the larger of the two, receiving the 3 semi-circular ducts. The saccule is globular in shape and also receives the cochlear duct.

Endolymph drains indigenous the saccule and also utricle right into the endolymphatic duct. The duct travels through the vestibular aqueduct to the posterior aspect of the petrous component of the temporal bone. Here, the duct broadens to a sac whereby endolymph can be secreted and also absorbed.

Semi-circular Ducts

The semi-circular ducts are situated within the semi-circular canals, and share their orientation. Upon motion of the head, the circulation of endolymph within the ducts alters speed and/or direction. Sensory receptor in the ampullae that the semi-circular canals detect this change, and send signals to the brain, permitting for the processing of balance.


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Fig 4 – The contents of the membranous labyrinth.


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Fig 5 – The labyrinthine artery emerging from the basilary artery


The bony labyrinth and also membranous labyrinth have various arterial supplies. The bony labyrinth obtain its blood it is provided from three arteries, which likewise supply the bordering temporal bone:

Anterior tympanic branch (from maxillary artery).Petrosal branch (from center meningeal artery).Stylomastoid branch (from posterior auricular artery).

The membranous labyrinth is provided by the labyrinthine artery, a branch that the worse cerebellar artery (or, occasionally, the basilar artery). That divides into three branches:

Cochlear branch – provides the cochlear duct.Vestibular branches (x2) – it is provided the vestibular apparatus.

Venous drainage the the inner ear is through the labyrinthine vein, which empties into the sigmoid sinus or inferior petrosal sinus.


Innervation

The within ear is innervated by the vestibulocochlear nerve (CN VIII). The enters the inner ear via the internal acoustic meatus, whereby it divides right into the vestibular nerve (responsible because that balance) and also the cochlear nerve (responsible for hearing):

Vestibular nerve – enlarges to form the vestibular ganglion, which climate splits into superior and also inferior components to supply the utricle, saccule and also three semi-circular ducts.Cochlear nerve – enters at the basic of the modiolus and its branches pass through the lamina to supply the receptor of the organ of Corti.

The face nerve, CN VII, likewise passes with the inside ear, but does not innervate any of the structures present.


Clinical Relevance: Meniere’s Disease

Meniere’s condition is a disorder of the within ear, qualified by episodes of vertigo, low-pitched tinnitus, and also hearing loss.

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The symptoms room thought come be resulted in by an excess buildup of endolymph in ~ the membranous labyrinth, causing progressive distension of the ducts. The resulting push fluctuations damages the slim membranes of the ear the detect balance and sound.