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Middle ear

Recommended video: Introduction to the ear [21:34]
Overview of the structures of the outer ear and auditory tube.

The ear is structurally divided into three parts: the outer (external), middle and inner ear. The middle ear is an air-filled pressurized space within the petrous portion of the temporal bone, extending from the tympanic membrane (eardrum) to the lateral wall of the inner ear. It is lined by mucous membrane and communicates with the nasopharynx anteriorly via the pharyngotympanic (Eustachian) tube and the mastoid antrum and air cells posteriorly. Structurally, the middle ear is made up of two parts: 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.

The middle ear houses three small bones called the auditory ossicles, which are responsible for transmitting sound vibrations from the tympanic membrane to the inner ear. Additionally, the middle ear contains two small muscles, the stapedius and tensor tympani which are associated with the ossicles, the chorda tympani nerve (a branch of the facial nerve, CN VII) and the tympanic plexus of nerves from the glossopharyngeal nerve (CN IX).

This article will discuss the anatomy and function of the middle ear and relevant neighboring structures including the neurovascular supply of this part of the ear and its components, as well as some clinically relevant points.

Key facts about the middle ear
Components Tympanic cavity (tympanic cavity proper and superior epitympanic recess), auditory ossicles, muscles of the ossicles (stapedius, tensor tympani), nerves (tympanic plexus, chorda tympani)
Boundaries Roof: Tegmen tympani
Anterior wall:
Internal carotid artery canal, tympanic opening of auditory tube
Posterior wall:
Aditus to mastoid antrum, pyramidal eminence
Lateral wall:
Tympanic membrane
Medial wall:
Promontory of tympanic cavity
Floor:
Tympanic canaliculus
Function Transformation of sound waves into vibrations → Transmission of vibrations to the internal ear
Contents
  1. Walls and boundaries
  2. Tympanic membrane
  3. Auditory ossicles
    1. Malleus
    2. Incus
    3. Stapes
  4. Middle ear muscles
    1. Stapedius
    2. Tensor tympani
  5. Relating structures
  6. Vasculature
    1. Arterial supply
    2. Venous drainage
  7. Innervation
  8. Conclusion
  9. Clinical points
    1. Otitis media
    2. Hyperacusis
  10. Sources
+ Show all

Walls and boundaries

The middle ear cavity, also known as the tympanic cavity is bounded by six walls which can be better appreciated when viewed from a coronal section that transects the external acoustic meatus.

The tympanic cavity is bounded by the following:

  • Tegmental wall (roof): The tegmen tympani, a thin plate of squamous and petrous parts of the temporal bone, forms the upper boundary (roof) of the tympanic cavity. It separates the tympanic cavity from the cranial cavity, specifically the middle cranial fossa.
  • Jugular wall (floor): A thin plate of bone forms the floor of the tympanic cavity and separates it from the superior bulb of the internal jugular vein. The floor also has a microscopic canal, known as the tympanic canaliculus, which permits the passage of the tympanic part of the glossopharyngeal nerve (CN IX).
  • Carotid wall (anterior wall): The carotid wall consists of two parts: its inferior part is formed by the wall of the internal carotid artery canal which separates the cavity from the internal carotid artery, while its superior part is occupied by the opening of the pharyngotympanic (auditory or Eustachian) tube as well as a smaller opening of the canal for the tensor tympani.
  • Mastoid wall (posterior wall): This corresponds with the mastoid process. The superior part of the mastoid wall is the aditus (entrance) to the mastoid antrum. The lateral semicircular canal and the facial nerve canal lie medial to the aditus. The inferior part of the mastoid wall consists of the bony pyramidal eminence, which contains the stapedius muscle.
  • Membranous wall (lateral wall): Laterally, the tympanic cavity is bounded mainly by the tympanic membrane. This membrane has two sides: medial (inner) and lateral (outer). The medial side is entirely convexed towards the middle ear. On this side, the ridge of the chorda tympani, below which the chorda tympani nerve passes, can be found between the pars tensa (tense part) and pars flaccida (flaccid part) of the membrane. The tympanic membrane, however, does not extend into the epitympanic recess. Therefore, the superior part of the membranous wall is formed by the bony wall of the epitympanic recess.
  • Labyrinthine wall (medial wall): The promontory of tympanic cavity which is formed by the basal coil (initial part) of the cochlea. The tympanic plexus which is derived primarily from the tympanic branch of the glossopharyngeal nerve (CN IX) and the caroticotympanic branches of the internal carotid plexus covers the surface of the promontory. The labyrinthine wall also bears the round window which lies posterosuperior to the promontory, and the oval window which is situated posteroinferior to the promontory.

Tympanic membrane

The tympanic membrane is an oval structure that is situated at an angle relative to the external acoustic meatus. Not only does this membrane serve as a physical boundary for the middle ear, but it is an important part in the auditory conduction pathway. It is among the first to begin vibrating once it is struck by the sound waves collected by the external auditory meatus. This thin, fibrous structure has an external surface that is exposed to the external environment and lined by stratified squamous epithelium, and an inner surface (facing the tympanic cavity) that is lined by low columnar epithelium.

When viewed laterally with an otoscope, the important landmarks can be described with reference to the face of a clock. At the center of the clock (where the hands would be attached) is a depression in the tympanic membrane known as the umbo. This concavity is created by the attachment of the malleus to the center of the tympanic membrane. The handle of the malleus then extends from the umbo towards the one o’clock position, where it becomes the lateral process of the malleus. The light from the otoscope is reflected from the umbo in the five o’clock direction and is described as the cone of light.

The anterior and posterior mallear folds are fibrous extensions from the lateral process of the malleus that project in the one and eleven o’clock directions, respectively. The superior part of the tympanic membrane, between the mallear folds, is known as Shrapnell’s membrane or pars flaccida and is relatively loose. The remainder of the membrane is held tight by the tensor tympani muscle, and is therefore known as the pars tensa. The perimeter of the tympanic membrane is thicker than the central portion; this allows it to rest adequately in the groove of the tympanic plate.

Auditory ossicles

The auditory ossicles are a chain of three small bones located in the middle ear. From lateral to medial, these are called the malleus (hammer), incus (anvil), and the stapes (stirrup). Although they are the three smallest bones in the human body, they are extremely important components of the auditory pathway. Through synovial articulations, these bones are able to transmit mechanical energy from the tympanic membrane to the oval window of the internal ear.

Malleus

The malleus is the largest, and most lateral of the bony chain. It is a club-shaped structure whose head rests in the epitympanic recess and articulates with the incus. The constricted region below the head forms the neck, inferior to which are the anterior and lateral processes of the malleus. The malleus extends below these processes as the handle of the malleus which is connected to the medial surface of the tympanic membrane.

Three ligaments secure the malleus to the walls of the tympanic cavity. These are the anterior, superior and lateral ligaments of the malleus. The anterior ligament of the malleus connects the neck of the malleus, just above the anterior process, to the anterior wall of the tympanic cavity. The superior ligament attaches on one end to the head of the malleus and on the other end to the roof of the epitympanic recess. The head of the malleus is equally connected to the tympanic membrane via the lateral ligament.

Incus

The incus is the anvil-shaped central ossicle which connects the malleus to the stapes. It has two limbs and a cuboidal body. The body of the incus sits in the epitympanic recess, where it articulates with the head of the malleus. Its long limb extends into the tympanic cavity alongside the handle of the malleus and articulates with the stapes. The short limb of the incus extends posteriorly and is connected to the upper posterior wall of the tympanic cavity by a ligament.

Stapes

The stapes is the smallest and most medial in the chain of ossicles. The head of stapes articulates with the long process of the incus laterally. The head is supported by a constricted narrow neck, from which two processes or crura, namely the anterior crus and posterior crus diverge. The crura are connected to an oval plate which forms the base or footplate of the stapes and sits over the oval window (fenestra vestibuli). The articulation between the footplate of the stapes and the oval window plays an essential role in the transmission of sound vibrations in the middle ear to the fluid in the vestibule of the inner ear.

Middle ear muscles

Although the auditory ossicles and the tympanic membrane are the primary structures responsible for the transmission of vibration, their activities are modified by two muscles: the stapedius and the tensor tympani which are extremely important modulators of the hearing pathway.

Stapedius

The stapedius muscle originates from the internal concavity of a small projection on the mastoid wall called the pyramidal eminence (pyramid). Its tendon exits from the opening at the apex of the eminence and inserts onto the posterior aspect of the neck of the stapes. When the incoming sound is loud, the muscle acts by pulling back the neck of the stapes, which indirectly pulls back the footplate from the oval window reducing further vibrations passed to cochlea. The stapedius muscle is innervated by the nerve to the stapedius, which is a branch of the facial nerve (CN VII).

Tensor tympani

The tensor tympani muscle originates from the cartilaginous portion of the auditory tube, the greater wing of the sphenoid bone, and the petrous part of the temporal bone. It lies in a canal superior to the bony part of the auditory tube and inserts into the handle of the malleus to pull it medially. The action of the muscle increases the concavity and rigidity of the tympanic membrane.The increased tension reduces the amplitude of the tympanic membrane oscillations and thereby reduces sound transmission to the vestibular window. This lowers the perceived amplitude of sounds. The tensor tympani is innervated by a branch of the mandibular nerve (CNV3).

Relating structures

The outer ear is situated lateral to the middle ear, while the inner ear has a medial relation. Superior to the tympanic cavity, above the tegmen tympani, is the temporal lobe of the cerebrum. In addition to serving as the medial wall for the tympanic cavity, the promontory also serves as a landmark for the oval window (fenestra vestibuli) and the round window (fenestra cochleae) . The former is located posterosuperiorly, while the latter is located posteroinferiorly. Also, above the promontory is the canal for the facial nerve (CN VII), which is inferior to the prominence created by the lateral semicircular canal.

The auditory tube (pharyngotympanic tube or Eustachian tube) is an anteromedial continuation of the middle ear cavity. It extends from the anterior wall of the middle ear cavity to the lateral wall of the nasopharynx , allowing for communication between these two regions. The mastoid antrum is related posteriorly to the epitympanic recess of the tympanic cavity. Communication between the two spaces is permitted by the aditus.

Test your knowledge on the middle ear with this quiz.

Vasculature

Arterial supply

The blood supply to the middle ear is primarily derived from the maxillary division of the external carotid artery, which is complemented by branches of the internal carotid artery. The deep auricular artery (from the maxillary artery) contributes to the tympanic arterial network in the floor of the tympanic cavity and also supplies the meatal (external) side of the tympanic membrane. There is also an anastomosing branch of the meningeal accessory artery (branch of maxillary), called the tubal artery that travels through the auditory tube and communicates with the other arteries of the middle ear.

The anterior tympanic artery (arising from the maxillary artery) provides blood supply to the auditory ossicles. It is also involved in a circular anastomosis with the posterior tympanic branch of the stylomastoid artery (arising from either the occipital or posterior auricular divisions of the external carotid artery) that also supplies the tympanic membrane. The stylomastoid artery goes on to perfuse the mastoid air cells and the posterior tympanic cavity. Additionally, it gives a branch to the stapedius muscle, called the stapedial artery, and also a branch to the mucosal (internal) side of the tympanic membrane. The tensor tympani muscle is supplied by the superior tympanic artery, which is a branch of the middle meningeal branch of the maxillary artery. The middle meningeal artery also provides petrosal and superior tympanic branches that supply the tympanic cavity.

The tympanic plexus is formed by contributions of the inferior tympanic artery, a branch of the ascending pharyngeal artery, and the caroticotympanic arteries. The crura of the stapes are wrapped by anterior and posterior crural arteries, which are derived from the tympanic plexus.

Venous drainage

The tympanic veins drain deoxygenated blood to the superior petrosal sinus and the pterygoid venous plexus.

Innervation

The auriculotemporal nerve (branch of CN V3) and the vagus nerve (CN X) provide general sensory innervation to the meatal side of tympanic membrane. The mucosal side of the tympanic membrane transmits its general sensory impulses via the tympanic branches of the glossopharyngeal nerve (CN IX). The mucous membrane that lining the walls as well as the contents of the tympanic cavity is innervated by the tympanic nerve, a branch of the glossopharyngeal nerve (CN IX) via the tympanic plexus.

Sympathetic innervation is achieved by the caroticotympanic nerves from the carotid sympathetic plexus. The stapedius muscle receives motor innervation from a branch of the facial nerve called the nerve to stapedius, while tensor tympani muscle is supplied via the medial pterygoid branch of the mandibular nerve (CN V3).

Conclusion

Philosophers are known to coin thought provoking sayings that leave individuals in awe. One such question that has stood the test of time is “If a tree falls in a lonely forest, but there are no animals around, does it make a sound?” In 1884, the authors of Scientific American answered a different form of this question stating that sound is the perception of vibratory impulses transmitted via air or other media.

So although the tree will generate vibration, if there are no ears to perceive it, then there is no sound. However the reader chooses to interpret this question, there is one undeniable fact: the ear is an essential component in the perception of sound.

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