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Ear

The ear is a complex part of an even more complex sensory system. It is situated bilaterally on the human skull, at the same level as the nose. The main functions of the ear are, of course, hearing, as well as constantly maintaining balance. The ear is anatomically divided into three portions:

Key facts about the ear
Function Hearing and maintaining balance
External ear Parts: auricle, external acoustic meatus, tympanic membrane
Function: capture and conduction of sound
Middle ear Parts: tympanic cavity, auditory ossicles, muscles of the ossicles
Function: transforming a high-amplitude low-force sound wave into a low-amplitude high-force vibration and transmitting it to the internal ear
Internal ear Parts: bony labyrinth (vestibule, semicircular canals, cochlea) and membranous labyrinth (utricle, saccule, semicircular ducts, cochlear duct)
Function
- Bony labyrinth supports its membranous counterparts 
- Utricle and saccule provide information about the position of the head
- Semicircular ducts provide information about movements of the head
- Cochlear duct provides hearing information
Clinical relations Auricular hematoma, otitis (externa, media, interna), blockage of the pharyngotympanic (Eustachian) tube, high tone deafness

This mixture of bones, nerves, vessels, membranes, and muscles that make up the ear will be described in this article.

External ear

The external ear, like the middle ear, serves only to conduct sound to the inner ear. It consists of the auricle and external acoustic meatus (or ear canal).

Key facts about the external ear
Parts Auricle, external acoustic meatus
Blood supply

Posterior auricular, anterior auricular, minor branches of the occipital, deep auricular, stylomastoid, inferior tympanic arteries

Innervation

Auriculotemporal branch of the mandibular nerve, lesser occipital, great auricular nerve, auricular branch of the vagus nerve, branches of the facial nerve

Function Conducts sound to the inner ear

At the bottom of the ear canal is the tympanic membrane which establishes the border between the external and middle ear.

Auricle

The auricle, also known as pinna, is a wrinkly musculocutaneous tissue that is attached to the skull and it functions to capture sound. The auricle is mostly made up of cartilage that is covered with skin. There are two aspects of the auricle: and medial (inner) and lateral (outer). The medial aspect of the ear lobe is attached to the skull and has no major practical significance.

The lateral aspect is concave and presents numerous grooves and ridges. The outer rim of the auricle is called the helix, which then inferiorly ends as soft tissue known as the lobule of auricle (or ear lobe). The helix has three parts: crus, spine, and tail. The crus is the anterosuperior convex arch the helix, the spine the thick superior part of the helix, while the tail is continuous with the lobule. Parallel to the helix is another convex curvature referred to as the antihelix, which has two parts: the triangular fossa bound by the crus of the helix and the antihelix; and the crura of the antihelix which is the widening of the antihelix directed posteriorly toward the helix. 

Auricle (anterior view)

The center of the auricle has a fossa that is continuous with the external acoustic meatus called the concha of the auricle. This concha is covered with a triangular cartilaginous process called the tragus. The base of the tragus is attached to facial skin, whereas its apex partially covers the entrance to the external acoustic meatus. Opposite to the tragus, there is also a cartilaginous elevation above the lobule called antitragus

External acoustic meatus

This is a bony-cartilaginous canal that projects from the auricle to the middle ear, from which it is separated by the tympanic membrane (eardrum). The external acoustic meatus consists of the lateral (outer) cartilaginous part, referred to as the cartilaginous or membranous external acoustic meatus, and the medial (inner) bony part, called the bony external acoustic meatus. 

Observed antero-posteriorly, this canal looks entirely like a broken line, meaning that it doesn’t follow a straight course. The lateral part of the canal is directed posteriorly and superiorly, whereas the medial part is directed anteriorly and inferiorly. This is important to know while examining the ear, because the auricle must be pulled backwards, upwards and slightly laterally in order for the lateral and medial portions to align, and so that the tympanic membrane can then be seen and examined properly. 

The membranous external acoustic meatus makes up the lateral one-third of this canal. Its anterior and inferior walls are made of cartilage because it is a continuation of the auricle. On the other hand the posterior and superior walls are actually fibrous membranes. The bony external acoustic meatus makes up the the medial two-thirds of the canal. Its anterior and inferior walls are built by the tympanic part of the temporal bone, whereas its posterior and superior walls are built by the squamous part of the temporal bone. Note that the posterior wall of the bony canal is anatomically related to the mastoid cells of the mastoid process, while the anterior and inferior walls are related to the temporomandibular joint.

Tympanic membrane

The tympanic membrane, or simply the eardrum, is found at the bottom of the bony external acoustic meatus and it is the border between the external and middle ear. It is attached with a fibrocartilaginous ring to the tympanic part of the temporal bone.

Based on its structure and tension, the tympanic membrane is divided into the two following parts:

  • Pars flaccida (flaccid part) also called Shrapnell’s membrane
  • Pars tensa (tense part) 

It also has two sides: medial (inner) and lateral (outer). The medial side of the tympanic membrane is covered with mucosa and is entirely convexed towards the middle ear. On this side, around the border between the pars tensa and pars flaccida is the ridge of the chorda tympani, below which the chorda tympani nerve (branch of the facial nerve (CN VII)) passes. Also, the malleus bone is found on the medial side of the membrane and makes impressions on it.

The lateral aspect of the tympanic membrane is divided into four quadrants: anterosuperior, anteroinferior, posterosuperior, and posteroinferior. Behind the two superior quadrants on the medial side are the auditory ossicles (stapes, malleus, and incus) and the chorda tympani. The attachment of the lower end and the handle of the malleus on the medial side of the tympanic membrane create the concavity on the lateral side that is called the umbo of the tympanic membrane.

Superior to the umbo is a stripe called the malleolar stria which is the impression made by the rest of the handle of the malleus. The superior end of the malleolar stria presents with a ridge called the malleolar prominence. This impression is because of the lateral process of the malleus. The lateral side of the tympanic membrane is covered with skin and a thin layer of the cerumen. 

Muscles of the external ear

All muscles of the external ear are associated with the auricle and are innervated by posterior auricular branches of the facial nerve (CN VII). They are classified into two groups, intrinsic and extrinsic. 

Intrinsic muscles contribute to defining the shape of the auricle by passing between its cartilaginous parts. They are: helicis major, helicis minor, tragus, pyramidal muscle of auricle, antitragus muscle, transverse muscle of auricle, and oblique muscle of auricle. 

Extrinsic muscles play a role in positioning the auricle, originating from the skull and attaching within the auricle itself. They are: auricularis anterior, auricularis superior, and auricularis posterior.

Key facts about the extrinsic muscles of the external ear
Auricularis anterior

Origin: Temporal fascia/Epicranial aponeurosis

Insertion: Spine of helix

Innervation: Temporal branches of facial nerve

Function: Draws auricle anteriorly

Auricularis superior

Origin: Epicranial aponeurosis

Insertion: Superior surface of auricle

Innervation: Temporal branches of facial nerve

Function: Draws auricle superiorly

Auricularis posterior

Origin: Mastoid process, Tendon of sternocleidomastoid muscle

Insertion: Ponticulus of conchal eminence

Innervation: Posterior auricular nerve (branch of facial nerve)

Function: Draws auricle posteriorly

A fun fact about the extrinsic muscles is that they are remnants from earlier evolution, and now we cannot consciously move them. To imagine how would it look like if we could, think of of a rabbit moving its ears around towards a sound! 

Vasculature of the external ear

Three arteries participate in the supply of the auricle: the posterior auricular artery (branch of the external carotid artery), anterior auricular arteries (branches of the superficial temporal artery), and minor branches of the occipital artery. Veins that drain the auricle mirror the arteries, while the lymph is drained into the superficial parotid, mastoid, deep cervical, and superficial cervical lymph nodes.

The tympanic membrane is vascularized by the branches of the maxillary artery (deep auricular, and anterior tympanic arteries), the stylomastoid artery (branch of the posterior auricular artery), and inferior tympanic artery (branch of the ascending pharyngeal artery). The lymph drains to the periauricular lymph nodes.

Innervation of the external ear

The sensory nerves that supply the auricle are numerous: 

  • Anterior superior part is supplied by the auriculotemporal branch of the mandibular nerve (CN V3) and the cervical plexus.
  • Posterosuperior part is supplied by the lesser occipital nerve.
  • Anterior and posteroinferior parts are supplied by the great auricular nerve.
  • The deeper parts, closer to the external acoustic meatus, are supplied by the auricular branch of the vagus nerve (CN X) and branches of the facial (CN VII) nerve.

Most of the sensory innervation of the external acoustic meatus is by: 

  • The lateral branch of the auriculotemporal nerve (which is a branch of the mandibular nerve (CN V3)); and also by 
  • The auricular branches of the vagus and facial nerves. 

Because of vagus nerve innervation, any irritation of the external ear may cause a reflex cough. The lateral side of the tympanic membrane is innervated by the anterior auricular nerves (branches of the auriculotemporal nerve) and the auricular branch of the vagus nerve. On the other hand the medial side has its sensory innervation by the glossopharyngeal (IX) nerve.

Middle ear

The middle ear is a complex system of openings and canals placed mostly within the temporal bone. Its function is to transfer vibrations of the tympanic membrane into sound waves and pass them to the inner ear.

The middle ear is made up of the tympanic cavity and epitympanic recess. The tympanic cavity is directly medial to the tympanic membrane, whereas the epitympanic recess is the space superior to the membrane. 

Anterior to the tympanic cavity is the pharyngotympanic tube (Eustachian tube) which extends to the pharynx, while posterior to the cavity are the mastoid cells of the mastoid process of the temporal bone. Mastoid cells are covered with mucosa that has extended from the nasopharynx through the pharyngotympanic tube and then through the tympanic cavity.

Key facts about the middle ear
Parts Tympanic cavity, epitympanic recess
Blood supply Anterior tympanic, posterior tympanic, superior tympanic, inferior tympanic arteries, mastoid branch of the occipital artery
Innervation

Tympanic nerve, tensor tympani nerve, stapedius nerve, tympanic plexus

Function Transfers vibrations of the tympanic membrane into sound waves and pass them to the inner ear

The anatomical communication between the pharynx, middle ear, and mastoid cells is a possible path of spreading infections.

Tympanic cavity

This is a bony-membranous cavity. It is shaped like a biconcave lens, but from an anatomical aspect it is usually described as a six-sided prism. Along with these six walls, the auditory ossicles can also be described within the anatomy of the tympanic cavity.

Walls of the tympanic cavity
Tegmental wall (roof) Tegmen tympani - formed by both squamous and petrous parts of the temporal bone, and it separates the tympanic cavity from the cranial cavity.
Jugular wall (floor) Separates the cavity from the jugular fossa.
Carotid wall (anterior) Corresponds with internal carotid artery canal
Contains: petrotympanic fissure, orifice of the pharyngotympanic tube, orifice of the semicanal of the tensor tympani muscle
Membranous wall (lateral) Medial aspect of the tympanic membrane
Labyrinthine wall (medial)

Contains: oval window, round window, promontory, prominence of the facial canal

Mastoid wall (posterior)

Corresponds with the mastoid process and its pneumatic cavities

Contains: aditus to the mastoid antrum (entrance to the mastoid antrum), the pyramidal eminence (contains the stapedius muscle)

Let's discuss briefly the features of the labyrinthine wall.

  • The oval window is covered with a membrane and connects the middle ear to the vestibule of the internal ear. 
  • The round window also leads to the internal ear and is sealed with the secondary tympanic membrane. The vibrations of this membrane allow the fluid of the cochlea to move.
  • The promontory is the projection of the first coil of the cochlea. The groove of the promontorium is a groove on its surface through which the tympanic nerve passes.
  • The prominence of the facial canal has one horizontal and one vertical portion. The facial nerve (CN VII) passes through the horizontal part. This part runs superiorly to the promontory on the labyrinthine wall and then goes backwards toward the aditus of the mastoid antrum. Near this entrance, the canal deviates inferiorly and continues as its vertical portion that runs over the mastoid wall of the tympanic cavity.

The projection of the facial canal here is important, since the wall can sometimes contain openings and therefore infections of the middle ear can easily spread to the facial nerve and cause paralysis of its innervated structures. 

Auditory ossicles

These are the three smallest bones of the human body. Their main function is to transmit sounds to the internal ear, precisely to the labyrinth within the internal ear. They articulate with each other with synovial joints, and transmit vibrations by being moved by the muscles of the middle ear. In order from the tympanic membrane to the internal ear (lateral to medial), these bones are: 

  • Malleus (hammer)
  • Incus (anvil)
  • Stapes (stirrup) 

A fun fact about these bones is that they lack osteogenic periosteum.

Malleus

The malleus is laterally attached to the tympanic membrane and medially it articulates with the incus through the incudomalleolar joint. From the tympanic membrane it receives sound vibrations which it further transmits to the incus. It has several parts that include: head, neck, anterior and lateral processes, and the handle of the malleus. The head is placed within the epitympanic recess and it contains the articulation surface for the incus. It is connected to the tegmental wall of the tympanic cavity by the superior ligament of the malleus. Inferior to the head is the neck of the malleus, which contains two processes: 

  • The anterior process which is attached by the anterior ligament of the malleus to the anterior wall of the middle ear, and 
  • The lateral process which is attached to the medial surface of the tympanic membrane by the lateral ligament of the malleus. 

The extension that runs inferior to the neck and attaches to the central part of the tympanic membrane is the handle of the malleus. 

Incus

The incus is the anatomical connection of the malleus and stapes. It consists of three parts: the body and the long and short limbs. 

  • The body is placed in the epitympanic recess and articulates with the head of the malleus via the incudomalleolar joint. 
  • The long limb is placed parallel to the handle of the malleus and at the end it medially projects the lenticular process. This articulates with the stapes through the incudostapedial joint. 
  • The short limb extends posteriorly and attaches to the posterior wall of the tympanic cavity through the posterior ligament of the incus. 

Stapes

The stapes articulates with the incus laterally through the incudostapedial joint, while medially it is attached to the membrane of the oval window on the labyrinthine wall of the tympanic cavity. The vibrations carried from the malleus through incus and to the stapes then cause the membrane on the oval window to vibrate further transmitting sound to the vestibule of the internal ear. The parts of the stapes are: 

  • The head which articulates with the lenticular process of the long limb of the incus, 
  • Anterior and posterior limbs that attach to the oval base, and 
  • The base that fits into the oval window.

Muscles of the auditory ossicles

Even though the muscles related to the auditory ossicles are small, they are very important because their synergistic actions enable sound transmission. They also protect the internal ear from stimuli that are too strong by controlling the movements of the ossicles. These muscles are the tensor tympani muscle and the stapedius muscle.

Key facts about the muscles of the auditory ossicles
Tensor tympani muscle Origin: the cartilaginous part of the pharyngotympanic tube (Eustachian tube), the greater wing of the sphenoid bone, the semicanal for tensor tympani 
Insertion: the superior part of the handle of malleus
Innervation: the mandibular branch of the trigeminal nerve (CN V3)
Function: pulls the handle of malleus medially thus tenses tympanic membrane
Stapedius muscle Origin: inside of the pyramidal eminence
Insertion: neck of stapes
Innervation: facial nerve (CN VII)
Function: pulls stapes posteriorly thus prevents excessive oscillation 

The tensor tympani muscle is attached to the walls of the semicanal for tensor tympani on the medial side, and to the malleus on the lateral side. Its contraction pulls the malleus medially and in this way it tenses the tympanic membrane and pushes the stapes into the oval window.

The stapedius muscle is placed within the pyramidal eminence of the mastoid wall of the tympanic cavity. It extends a very thin tendon that attaches to the incus. The stapedius pulls the incus laterally and therefore it pulls the stapes out of the oval window.

Vasculature of the middle ear

The arteries that supply the tympanic cavity are the following:

  • Anterior tympanic artery (branch of the maxillary artery)
  • Posterior tympanic artery (branch of the stylomastoid artery)
  • Superior tympanic artery (branch of the middle meningeal artery)
  • Inferior tympanic artery (branch of the ascending pharyngeal artery)
  • Anterior, superior, and inferior tympanic arteries (branches of the internal carotid artery)
  • Mastoid branch of the occipital artery

The tympanic veins drain deoxygenated blood into the superior petrosal sinus and the pterygoid venous plexus. Lymph drains into the retroauricular cervical lymph nodes.

Innervation of the middle ear

When it comes to innervation, the mucosa that covers the walls of the middle ear is supplied with the tympanic nerve (branch of the glossopharyngeal nerve (CN IX)), whereas the muscles of the auditory ossicles are innervated by the tensor tympani nerve (tensor tympani muscle) and stapedius nerve (stapedius muscle).

The tympanic nerve and caroticotympanic nerves of the internal carotid plexus form the tympanic plexus in the mucous membrane that covers the promontory on the labyrinthine wall. This plexus supplies the mucosa of the middle ear, pharyngotympanic tube, and mastoid antrum. It also gives off the lesser petrosal nerve, which provides parasympathetic innervation for the otic ganglion.

Pharyngotympanic tube

The pharyngotympanic (auditory) tube, commonly known as the Eustachian tube, connects the middle ear and the nasopharynx. The air pressure in the external ear and the nasopharynx is equal to atmospheric (open air) pressure, so this communication between the middle ear and the nasopharynx serves primarily to equalize pressure on both sides of the tympanic membrane.

Equalization happens with the tube staying closed most of the time. The action of yawning or swallowing opens the tube, which allows air to flow and pressure to equalize. We’re sure that anytime you were driving up a mountain, you felt that change of pressure as if someone put plugs into your ears. If it wasn’t for the pharyngotympanic tube, the membrane would easily rupture very quickly under pressure. 

The opening of the pharyngotympanic tube is on the carotid (anterior) wall of the middle ear, from which the tube extends forward, medially, and downward on its way to nasopharynx. The one-third of the tube that is closer to the middle ear (posterolateral third) is the bony part, whereas the rest of the tube is the cartilaginous part.

The tube is mostly vascularized by the ascending pharyngeal artery, which is a branch of the external carotid artery. The middle meningeal artery and the artery of the pterygoid canal (branches of the maxillary artery) also participate in blood supply of the pharyngotympanic tube. Venous blood drains into the pterygoid venous plexus, while lymph drains into the deep cervical lymph nodes.

Recommended video: Outer ear and auditory tube
Overview of the structures of the outer ear and auditory tube.

The mucosa that covers the tube is innervated by the tympanic plexus which is actually an indirect way for the tympanic nerve (a branch of the glossopharyngeal nerve (CN IX)) to supply the tube.

Want to strengthen your information about the middle ear? Make sure to read both of our articles:

Internal ear

This is definitely the most complex part of the ear, so it’s not a coincidence that it is called the labyrinth. It is placed in the petrous part of the temporal bone and is made up of bony cavities in which specific membranous parts fit. For this reason, the internal ear is analyzed as the bony labyrinth and the membranous labyrinth which fits within the bony labyrinth.

Recommended video: Labyrinth
Overview of the bony and membranous structures of the labyrinth.

Note that the bony labyrinth is filled with the perilymph, whereas the membranous is filled with endolymph. This means that the membranous labyrinth is suspended in the perilymph of the bony labyrinth.

Bony labyrinth

The bony labyrinth is placed in the petrous part of the temporal bone, and is separated from the middle ear by the labyrinthine (medial) wall of the tympanic cavity. Grossly, it is located medially and slightly posteriorly to the tympanic cavity. It contains the vestibulocochlear organ, presented with the spiral organ of Corti (the origin of the cochlear nerve which serves for hearing), and the vestibular nerve which provides the balance information. These two nerves are branches of the vestibulocochlear nerve (CN VIII). 

From its medial to lateral portion, the bony labyrinth contains the following structures:

  • Vestibule
  • Cochlea
  • Semicircular canals

All three of those are actually cavities that communicate with each other. Their main function is to house corresponding parts of the membranous labyrinth. 

Vestibule

The vestibule is a central bony cavity. It contains two sacs: the utricle and saccule of the vestibular labyrinth (part of the membranous labyrinth). The vestibule communicates with the tympanic membrane through the oval window on its lateral wall. Anteriorly it communicates with the cochlea, and postero-superiorly with the semicircular canals.

The vestibule communicates with the posterior cranial fossa through the vestibular aqueduct. It is a membranous structure that leaves the vestibule, courses medially, passes through the temporal bone and opens on the posterior surface of the petrous part of the temporal bone. 

Semicircular canals

These three canals are positioned postero-superiorly to the vestibule. The canals are placed in three different plans, where each canal makes an angle of 90 degrees with the other. Within the semicircular canal are the corresponding semicircular ducts of the membranous labyrinth.

The anterior semicircular canal is positioned on the sagittal plane, the posterior is on the frontal plane, and the the lateral canal is placed horizontally on the transverse plane. Each canal ends in the form of a dilated ampulla, while the rest of the canal is narrow. Note that the anterior and posterior semicircular canals unite and form the common bony limb.

Cochlea

Cochlea is Greek for snail, and that’s exactly how this structure looks–a spiral and hollow bone chamber in which sound waves propagate from the base (near the oval window) to the apex.

After the base of the cochlea is a tube, called spiral canal of the cochlea, that twists around a central bony column (called the modiolus) two and a half times. Inside the spiral canal of the cochlea is the osseous spiral lamina that is attached to the outer wall of modiolus and extends into the cochlear canal. In this way it follows the wrapping of the spiral canal around the modiolus.

Since the spiral lamina is attached only to the modiolus, it incompletely divides the inner space of the spiral canal into the two canals:

  • The superior scala vestibuli
  • The inferior scala tympani

These portions communicate with each other at the apex of the cochlea through a narrow slit called the helicotrema. Note that the scala vestibuli is continuous with the vestibule, whereas the scala tympani is placed in front of the round window on the labyrinthine (medial) wall of the middle ear–so it is separated from the middle ear by the secondary tympanic membrane which seals the round window.

Similar to the vestibule, the cochlea communicates with the posterior cranial fossa through its own cochlear aqueduct. This canal emerges near the round window, passes through the temporal bone and opens on its posterior surface in the cranium.

Membranous labyrinth

The membranous labyrinth is a system of membranous cavities filled with endolymph which are suspended in the perilymph of the bony labyrinth. 

Labyrinth (diagram)

The membranous structures found within the bony labyrinth are:

  • Vestibular labyrinth – Comprises of two sacs, the utricle and saccule, and three membranous semicircular ducts. They all comprise the vestibular apparatus that is the sensory organ of balance. The utricle and saccule are within the vestibule of the bony labyrinth while the semicircular ducts are in the bony semicircular canal. 
  • Cochlear labyrinth – The bony cochlea contains the cochlear duct, that is the organ of hearing

Utricle

The utricle is a sac which fills the postero-superior portion of the vestibule. On its superior and posterior walls it has openings that correspond to the openings of the vestibule and which are the place of the emerging of the semicircular ducts. 

The inner surface of the utricle is covered with sensory tissue for balance called the macula of the utricle. This tissue comprises of cells that are rich with cilia and otholites. The macula responds to both centrifugal and linear acceleration, and the nerve that conducts this information begins right at this spot and is called the utricular nerve (a branch of the utriculo-ampullary nerve).

Saccule

The saccule is smaller than the utricle and it is placed in the antero-inferior part of the vestibule. Through the ductus reuniens, the cochlea is connected to the saccule, and in this way empties into the saccule.

On the inner surface of the saccule is the sensory tissue called macula of the saccule that responds to linear acceleration. The information registered here transmits further through the saccular nerve that begins in this macula. The saccule extends a communicating duct with the utricle, called the utriculosaccular duct. From this duct the endolymphatic duct extends–it enters the vestibular aqueduct, passes through the temporal bone and ends as the endolymphatic sac at the posterior surface of the petrous part of the temporal bone.

Semicircular ducts

The membranous semicircular ducts are suspended in the bony semicircular canals. The spatial relation here is important for the function, so let’s recall that the canals, as well as the ducts, are placed on three different planes, where each duct makes an angle of 90 degrees to the other.

The anterior semicircular duct is positioned on the sagittal plane, the posterior semicircular duct is on the frontal plane, whereas the lateral semicircular duct is placed transversely. Each duct then opens to the saccule via a dilated end called the ampulla. Because of this arrangement, each duct responds to movements directed within the plane with which they are aligned.

Cochlear labyrinth

The organ of hearing (spiral organ) is presented within the cochlear duct. This is a triangular shaped membranous duct that wraps around the modiolus two and a half times, following the quilling of both the spiral canal of the cochlea and the osseous spiral lamina.

The cochlear duct completely follows the anatomy of the bony labyrinth, meaning it is also divided into two canals–scala vestibuli and scala tympani (previously described in the Cochlea section). The cochlear duct has three walls:

  • The spiral ligament (outer wall) leans onto the bony labyrinth and consists of thick periosteum. Together with the basilar and vestibular membranes, it encloses the triangular shaped cochlear duct.
  • The vestibular membrane (roof) separates the perilymph in the scala vestibuli from the endolymph in the bony cochlea. It extends from the modiolus and attaches to the lateral wall of the cochlear duct.
  • The basilar membrane (floor) completely separates the perilymph of the scala tympani from the endolymph in the bony cochlea. It connects the free end of the spiral lamina with the spiral ligament of the lateral wall of the cochlear duct, thus completely separating the membranous scala vestibuli from the scala tympani.

On the basilar membrane inside this duct is the spiral organ, which is the organ of hearing. This structure contains mechanoreceptors called hair cells. The tips of the cells project out of the spiral organ and into the endolymph of the cochlear duct. The hair cells are covered with the tectorial membrane, which moves during oscillations of the endolymph that happen each time a sound wave is transmitted. The movements of the tectorial membrane stimulate the hair cells which then generate electrical impulses that transmit through the cochlear nerve to the brain.

Vasculature of the internal ear

The bony labyrinth is vascularized by the anterior tympanic artery (a branch of the maxillary artery), stylomastoid artery (a branch of the posterior auricular artery), and the petrosal artery (a branch of the middle meningeal artery). On the other hand, the membranous labyrinth is supplied by the labyrinthine artery (a branch of the basilar artery). 

Venous blood is drained by the vestibular and cochlear veins. They merge and form the labyrinthine vein, which empties into the sigmoid or the inferior petrosal sinus. Lymphatic drainage is by the parotid, mastoid, and superficial cervical lymph nodes.

Innervation of the internal ear

When it comes to innervation, the cochlear nerve carries sensory hearing information after arising from the spiral ganglion. It passes through the modiolus and the base of the cochlea.

The vestibular nerve arises from the sensory tissues of the semicircular ducts, utricle, and saccule carries sensory balance information. It forms the vestibular ganglion which provides superior and inferior branches, which again, split and provide the branches: utriculo-ampullary nerve, saccular nerve, and posterior ampullary nerve. Then, the utriculo-ampullary nerve splits again into the utricular, anterior ampullary, and lateral ampullary nerve. 

The cochlear and vestibular nerves unite and form the vestibulocochlear nerve (CN VIII), which exits through the internal acoustic meatus together with the facial nerve (CN VII) and travels to the lateral surface of the brainstem.

Read more about the anatomical details of the internal ear in our comprehensive articles and don’t forget to take our quiz to reinforce your knowledge.

Transmission of sound

At the external ear, sound waves captured by the auricle enter the external acoustic meatus, and travel through it to the tympanic membrane. Under the strike of the wave, the membrane moves medially, affecting the handle of the malleus attached to the medial surface of the tympanic membrane.

In the middle ear, the handle of the malleus moves medially, so that its head moves laterally. Since the head of the malleus articulates with the body of the incus, the incus is also moved laterally, which pushes the long process of incus medially. Since the long process articulates with the stapes, this will push the stapes medially, and cause its base to hit the oval window which then moves medially as well.

This will cause a large amplitude wave of the low force to be transformed into a small amplitude vibration of a high force in the internal ear. This then vibrates the fluid inside the scala vestibuli of the cochlea.

The receptors then send the information via the cochlear part of the vestibulocochlear nerve (CN VIII) to the brain, where they will be interpreted as a sound via the auditory pathway.

Transmission of balance

The membranous semicircular ducts, utricle and saccule are responsible for maintaining balance. Note that the semicircular ducts detect movement of the head, while the saccule and utricle provide head position information when it is not moving.

Course and branches of vestibulocochlear nerve (diagram)

The anterior semicircular duct is positioned on the sagittal plane, the posterior semicircular duct is on the frontal plane, whereas the lateral semicircular duct is placed transversely. Because of this arrangement, each duct responds to movements directed within the plane with which they are aligned. The saccule detects accelerations and head tilts in the vertical plane, and the utricle detects accelerations and head tilts in the horizontal plane.

The semicircular ducts, utricle, and saccule are filled with endolymph and contain motion receptors (hair cells). As the head rotates or tilts, the endolymph lags in the opposite direction due to inertia. This stimulates the stereocilia of the hair cells which generate an electric signal that is transmitted in the following way:

  • Hair cells from the anterior duct transmit signals via the anterior ampullary nerve
  • Hair cells from the posterior duct transmit signals via the posterior ampullary nerve
  • Hair cells from the lateral duct transmit signals via the lateral ampullary nerve
  • Hair cells from the utricle transmit signals via the utricular nerve
  • Hair cells from the saccule transmit signals via the saccular nerve

The utricular, anterior ampullary, and lateral ampullary nerves form utriculo-ampullary nerve. Then, the utriculo-ampullary nerve, saccular nerve and posterior ampullary nerve synapse within the vestibular ganglion. The vestibular nerve emerges from the vestibular ganglion, joins the cochlear nerve, and via the vestibulocochlear nerve carries sensory balance information to the central nervous system. 

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Clinical relations

Otoscopic examination

This procedure provides an examination of the external ear and the tympanic membrane. Because of the natural curvature of the external acoustic meatus, the physician needs to pull the helix of the auricle posterosuperiorly, in order to align the meatus in a straight direction. This enables proper placement of the otoscope. Normally, the tympanic membrane is shown as translucent and grey coloured. The handle of the malleus can be seen in the central part of the tympanic membrane. The inferior end of the handle reflects the cone of light directed anteroinferiorly.

Auricular hematoma

Auricular hematoma is a subcutaneous collection of blood that can appear after the bleeding within the auricle. The mass of the hematoma can compromise the vascularization of the auricle.

Impacted cerumen

Cerumen (ear wax) is normally produced in the external ear. But when the cerumen builds up (impacts) in the external acoustic meatus, it can lead to its blockade and temporary hearing loss. It is treated by softening the earwax and removing it mechanically.

Inflammations 

Inflammations of the external and middle ear, called otitis can cause the damage of the tympanic membrane which may result with the hearing loss. Swelling of the mucosa of the pharyngotympanic (Eustachian) tube may lead to its occlusion. This eventually leads to a lowering of pressure within the tympanic cavity, retraction of the tympanic membrane, and its inability to move resulting in hearing impairment.

Deafness 

Chronic exposure to loud sounds may result with degeneration of the spiral organ which leads to the high tone deafness. It is also called the sensorineural hearing loss. It is usually managed by cochlear implants which amplify sounds coming from the environment to enhance hearing ability.

Motion sickness

This state is caused by the sensory conflict between the vestibulocochlear and visual systems. In daily life, it happens while traveling by bus, plane, or boat. If you are sitting in the car, your vestibular system detects that the body is still and not moving. But at the same time, by looking through the window, you will see that you are moving. This conflict between the two systems systems stimulates the area postrema (the emetic center in the brain) and presents as the nausea or even vomiting.

Vertigo

Simply speaking, vertigo is dizziness. It is usually a symptom of internal ear damage, such as infections, head or neck injuries, or tumors. Since vertigo is a symptom, and not a disease, its treatment depends on discovering its cause and treating it.
 

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Show references

References:

  • Drake, R. L., Vogl, A. W., & Mitchell, A. W. M. (2015). Gray’s Anatomy for Students (3rd ed.). Philadelphia, PA: Churchill Livingstone.
  • Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2014). Clinically Oriented Anatomy (7th ed.). Philadelphia, PA: Lippincott Williams & Wilkins

Article, review and layout:

  • Jana Vaskovic
  • Alexandra Osika
© Unless stated otherwise, all content, including illustrations are exclusive property of Kenhub GmbH, and are protected by German and international copyright laws. All rights reserved.

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