Middle ear (tympanic cavity)

Auris media; cavitas tympani

Also known as: tympanic cavity, tympanum, epitympanic recess, attic, tensor tympani, stapedius, chorda tympani, acoustic reflex, mastoid antrum

A narrow, air-filled, mucosa-lined cavity in the temporal bone between the tympanic membrane and the inner ear; it contains the ossicles and two muscles, and connects to the nasopharynx (auditory tube) and the mastoid air cells.

Show on body map Ear — Coronal Section

Overview

Height × AP depth≈15 × 15 mm

Width≈2 mm (umbo) to ≈6 mm (attic)

Pressure gain≈22× (≈25–30 dB)

Acoustic reflex threshold≈70–90 dB above hearing threshold

The tympanic cavity is shaped like a biconcave disc: ≈15 mm high and ≈15 mm anteroposteriorly, but only ≈2 mm wide at the umbo, ≈4 mm below and ≈6 mm above. It comprises the tympanic cavity proper (opposite the drum) and the epitympanic recess (attic) above the drum, which houses the head of the malleus and body of the incus and opens posteriorly through the aditus to the mastoid antrum and air cells. It is lined by a mucous membrane (mostly simple squamous to cuboidal; ciliated respiratory-type near the auditory tube) that also covers the ossicles, muscles and nerves.

It is described as a box with six walls (see parts). The cavity contains: the ossicles (malleus, incus, stapes); two muscles — tensor tympani (from the cartilaginous auditory tube and greater wing of the sphenoid, in a canal above the tube; its tendon turns around the processus cochleariformis to insert on the handle of the malleus; supplied by V3, via the nerve to medial pterygoid) and stapedius (from the hollow pyramidal eminence on the posterior wall to the neck of the stapes; supplied by the facial nerve) — the smallest skeletal muscle in the body; the chorda tympani (branch of VII), which crosses the medial surface of the drum between the handle of the malleus and long process of the incus; and the tympanic plexus (mainly IX) on the promontory.

The middle ear's job is impedance matching: by concentrating force from the large tympanic membrane onto the small stapes footplate (area ratio ≈17:1) and the lever action of the ossicles (≈1.3:1) it increases pressure ≈22× (≈25–30 dB). Without it, ≈99.9% of acoustic energy would be reflected at the air–perilymph interface. The acoustic (stapedial) reflex — contraction of stapedius (and tensor tympani) in response to loud sound (≈70–90 dB above threshold), bilaterally — stiffens the chain and attenuates low-frequency transmission, but its latency (tens of milliseconds) is too long to protect against sudden impulse noise.

Location: Within the petrous temporal bone between the tympanic membrane (laterally) and the bony labyrinth (medially).

Parts & subdivisions

Roof (tegmental wall)
Tegmen tympani — thin plate of petrous bone separating it from the middle cranial fossa and temporal lobe (route for otogenic meningitis and temporal lobe abscess).
Floor (jugular wall)
Thin bone separating it from the superior bulb of the internal jugular vein; pierced by the tympanic nerve (IX).
Lateral (membranous) wall
Tympanic membrane, and above it the lateral wall of the epitympanic recess (scutum).
Medial (labyrinthine) wall
Promontory (basal turn of the cochlea) with the tympanic plexus; oval window (stapes footplate) above and behind it; round window below and behind; prominence of the facial canal above the oval window; prominence of the lateral semicircular canal above that.
Anterior (carotid) wall
Thin bone separating it from the internal carotid artery; openings of the auditory tube and the canal for tensor tympani.
Posterior (mastoid) wall
Aditus to the mastoid antrum above; pyramidal eminence (stapedius); opening for the chorda tympani; the vertical part of the facial canal descends within it.
Tensor tympani
Pulls the malleus handle medially, tensing the drum; supplied by V3 (nerve to medial pterygoid); first-arch derivative.
Stapedius
Pulls the stapes posteriorly, tilting the footplate; supplied by facial nerve (VII); second-arch derivative. Paralysis (e.g. Bell palsy) → hyperacusis.
Chorda tympani
Branch of VII carrying taste from the anterior two-thirds of the tongue and parasympathetic fibres to submandibular and sublingual glands; crosses the drum; exits via the petrotympanic fissure.
Tympanic plexus
On the promontory; formed by the tympanic nerve (Jacobson, IX) and caroticotympanic sympathetic nerves; gives the lesser petrosal nerve (parasympathetic to the parotid via the otic ganglion) and sensation to the middle ear and tube.

Anatomy

Innervation: Mucosa: tympanic plexus (glossopharyngeal nerve, IX). Muscles: tensor tympani — mandibular nerve (V3); stapedius — facial nerve (VII).
Arterial supply: Anterior tympanic (maxillary), stylomastoid (posterior auricular), petrosal (middle meningeal), superior tympanic, inferior tympanic (ascending pharyngeal) and caroticotympanic arteries.
Relations: Above: middle cranial fossa/temporal lobe. Below: internal jugular vein. Front: internal carotid artery. Behind: mastoid antrum and facial nerve. Medial: inner ear. Lateral: external meatus.
Development: First pharyngeal pouch (tubotympanic recess) expands to form the tympanic cavity and auditory tube; it envelops the developing ossicles, which become covered with endodermal mucosa. Pneumatisation of the mastoid continues after birth.

Function

Impedance matching — transmit and amplify airborne vibration to the cochlear fluids (≈25–30 dB gain)
Protect the inner ear from loud low-frequency sounds (acoustic reflex)
Ensure sound reaches the oval window preferentially, with the round window out of phase
Equalise pressure with the atmosphere via the auditory tube

Physiology: Pressure gain ≈ area ratio (≈55/3.2 ≈ 17) × lever ratio (≈1.3) ≈ 22×, i.e. ≈27 dB. Acoustic reflex: afferent VIII → cochlear nucleus → superior olivary complex → facial motor nuclei bilaterally (and trigeminal motor nucleus). Testing it (tympanometry) helps localise lesions of VII and VIII.

Clinical significance

Acute otitis media and complications

Infection ascends the auditory tube; pus under pressure bulges and may perforate the drum. Spread through the aditus gives acute mastoiditis (postauricular swelling pushing the auricle forward); through the tegmen, meningitis or temporal lobe abscess; medially, labyrinthitis; to the facial canal (dehiscent in some people), facial palsy.

Otosclerosis

Abnormal bone remodelling of the otic capsule fixes the stapes footplate in the oval window, causing progressive conductive hearing loss in young adults (see stapes).

Chorda tympani and facial nerve injury

Middle-ear surgery can damage the chorda tympani (altered taste on the anterior tongue). Loss of the stapedius reflex in a facial palsy indicates a lesion proximal to the nerve to stapedius.

Mnemonics

Muscles and nerves

"Tensor Tympani — Trigeminal; Stapedius — Seventh."

Compiled from standard anatomy and physiology references — see the Special Senses sources. For education only; not medical advice.