Learning Objective: By the end of this session, the learner should be able to describe the anatomy of the middle ear, including the tympanic cavity, epitympanic recess, auditory ossicles, mastoid air cells, muscles, and auditory tube, and explain their clinical significance in conditions like otitis media and mastoiditis.
The middle ear is an air-filled cavity within the temporal bone, extending from the tympanic membrane to the lateral wall of the inner ear. Its main function is to transmit sound vibrations from the eardrum to the inner ear via the auditory ossicles (malleus, incus, stapes).
Parts of the Middle Ear
- Tympanic cavity – contains the auditory ossicles and lies medial to the tympanic membrane.
- Epitympanic recess – superior to the tympanic cavity, connected to mastoid air cells; the malleus and incus partially extend into it.
Borders of the Middle Ear
- Roof: Petrous temporal bone – separates from the middle cranial fossa
- Floor: Jugular wall – separates from internal jugular vein
- Lateral wall: Tympanic membrane and epitympanic recess
- Medial wall: Lateral wall of inner ear; bulge for facial nerve
- Anterior wall: Bony plate with openings for auditory tube and tensor tympani
- Posterior wall: Mastoid wall with aditus to the mastoid antrum
Auditory Ossicles
The ossicles transmit vibrations from the tympanic membrane to the oval window of the inner ear:
| Bone | Features & Articulation | Function |
|---|---|---|
| Malleus | Largest, attaches to the tympanic membrane; head articulates with the incus | Transmits vibrations from the tympanic membrane |
| Incus | The body articulates with the malleus; the long limb articulates with the stapes | Passes vibrations from the malleus to the stapes |
| Stapes | Smallest bone; stirrup-shaped; base fits oval window | Transmits sound to the inner ear fluid |
Mastoid Air Cells
Located posterior to the epitympanic recess, these air-filled spaces within the mastoid process act as a buffer system to maintain middle ear pressure.
- Connected to the middle ear via the aditus to the mastoid antrum
Middle Ear Muscles
Protective muscles reduce sound transmission via the acoustic reflex:
| Muscle | Origin & Insertion | Innervation | Action |
|---|---|---|---|
| Tensor tympani | Auditory tube → handle of malleus | Branch of the mandibular nerve (V3) | Pulls the malleus medially, dampens loud sounds |
| Stapedius | Posterior wall → stapes | Facial nerve (VII) | Dampens stapes vibrations |
Auditory (Eustachian) Tube
- Connects the middle ear → nasopharynx
- Equalises pressure in the middle ear and the external environment
- Shorter and straighter in children, predisposing them to middle ear infections
Clinical Relevance
- Otitis Media with Effusion (Glue Ear)
- Caused by auditory tube dysfunction → negative middle ear pressure → fluid accumulation
- The tympanic membrane appears inverted with visible fluid
- Mastoiditis
- Middle ear infections may spread to mastoid air cells, risking intracranial infection (meningitis)
- Drainage requires care to avoid facial nerve injury
