Middle Ear

Middle Ear

• Consists of the tympanic cavity with its ossicles and is located within the petrous portion of the temporal bone.
• Transmits the sound waves from air to auditory ossicles and then to the inner ear.

A. Tympanic (middle ear) cavity

• Includes the tympanic cavity proper (the space internal to the tympanic membrane) and the
• epitympanic recess (the space superior to the tympanic membrane that contains the head of the malleus and the body of the incus).
• Communicates anteriorly with the nasopharynx via the auditory (eustachian) tube and posteriorly with the mastoid air cells and the mastoid antrum through the aditus ad antrum.
• Is traversed by the chorda tympani and lesser petrosal nerve.

1. Boundaries of the tympanic cavity

• Roof : tegmen tympani.
• Floor : jugular fossa.
• Anterior : carotid canal.
• Posterior : mastoid air cells and mastoid antrum through the aditus ad antrum.
• Lateral : tympanic membrane.
• Medial : lateral wall of the inner ear, presenting the promontory formed by the basal turn of the cochlea, the fenestra vestibuli (oval window), the fenestra cochlea (round window) , and the prominence of the facial canal.

2. Oval window (fenestra vestibuli)

• Is pushed back and forth by the footplate of the stapes and transmits the sonic vibrations of the ossicles into the perilymph of the scala vestibuli in the inner ear.

3. Round window (fenestra cochlea or tympani)

• Is closed by the secondary tympanic (mucous) membrane of the middle ear and accommodates the pressure waves transmitted to the perilymph of the scala tympani.

B. Muscles

1. Stapedius muscle

• Is the smallest of the skeletal muscles in the human body.
• Arises from the pyramidal eminence, and its tendon emerges from the eminence.
• Inserts on the neck of the stapes.
• Is innervated by a branch of the facial nerve.
• Pulls the head of the stapes posteriorly, thereby tilting the base of the stapes.
• Prevents (or reduces) excessive oscillation of the stapes and thus protects the inner ear from injury from a loud noise.
• Its paralysis results in hyperacusis.

2. Tensor tympani muscle

• Arises from the cartilaginous portion of the auditory tube.
• Inserts on the handle (manubrium) of the malleus.
• Is innervated by the mandibular branch of the trigeminal nerve.
• Draws the tympanic membrane medially and tightens it (in response to loud noises), thereby increasing the tension and reducing the vibration of the tympanic membrane.

C. Auditory ossicles

• Consist of the malleus, incus, and stapes.
• Form a bridge by synovial joints in the middle ear cavity, transmit sonic vibrations from the tympanic membrane to the inner ear, and amplify the force.

1. Malleus (hammer)

• Consists of a head, neck, handle (manubrium), and anterior and lateral processes.
• Its rounded head articulates with the incus in the epitympanic recess.
• Its handle is fused to the medial surface of the tympanic membrane and serves as an attachment for the tensor tympani muscle.

2. Incus (anvil)

• Consists of a body and two processes (crura).
• Its long process descends vertically, parallel to the handle of the malleus, and articulates with the stapes.
• Its short process extends horizontally backward to the fossa of the incus and provides the attachment for the posterior ligament of the incus.

3. Stapes (stirrup)

• Consists of a head and neck, two processes (crura), and a base (footplate).
• Its neck provides insertion of the stapedius muscle.
• Has a hole through which the stapedial artery is transmitted in the embryo; this hole is obturated by a thin membrane in the adult.
• Its base (footplate) is attached by the annular ligament to the margin of the oval window (fenestra vestibuli). Abnormal ossification between the footplate and the oval window (otosclerosis ) limits the movement of the stapes, causing deafness.

D. Auditory (pharyngotympanic or eustachian) tube

• Connects the middle ear to the nasopharynx.
• Allows air to enter or leave the middle ear cavity and thus balances the pressure in the middle ear with atmospheric pressure, allowing free movement of the tympanic membrane.
• Has cartilaginous portion that remains closed except during swallowing or yawning.
• Is opened by the simultaneous contraction of the tensor veli palatini and salpingopharyngeus muscles.

E. Sensory nerve and blood supply to the middle ear

• Is innervated by the tympanic branch of the glossopharyngeal nerve, which forms the tympanic plexus with caroticotympanic nerves from the internal carotid plexus of sympathetic fibers. The tympanic nerve continues beyond the plexus as the lesser petrosal nerve, which transmits preganglionic parasympathetic fibers to the otic ganglion.
• Receives blood from the stylomastoid branch of the posterior auricular artery and the anterior tympanic branch of the maxillary artery.

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