Auditory ossicles

Malleus

The most lateral and largest of the auditory ossicles is the malleus (hammer). It has several parts: handle, spatulate process, lateral process, anterior process, neck, and head.

The handle of the malleus (also known as the manubrium) is a downward extension embedded into the medial surface of the tympanic membrane. As the handle extends inferiorly, it narrows. At the end of the handle is the spatulate process of the malleus which attaches to the pars tensa of the tympanic membrane with ligaments. This attachment draws the tympanic membrane medially from the center forming a concavity called the umbo of the tympanic membrane. Superior to the handle, the lateral process projects laterally as a slight cone at the root of the handle. It attaches to the superior part of the tympanic membrane by the anterior and posterior malleolar folds. The anterior process is much longer than the lateral process. Superior to the lateral process, and inferior to the neck, it projects anteriorly like a spindle and attaches to the anterior wall of the middle ear. The anterior process is also called the Folian or Rau’s process.

Superior to the lateral and anterior processes is the neck of the malleus. It is quite narrow and lies on top of the pars flaccida of the tympanic membrane. Superior to the neck is the rounded head of the malleus which sits in the epitympanic recess. On its posterior side, it articulates with the body of the incus posterolaterally by a small synovial joint: the incudomalleolar joint.

The malleus is suspended in place by three ligaments:

• Anterior ligament of malleus (Casserio’s ligament) – attaches to the base of the neck of the malleus (superior to the anterior process) to the carotid (anterior) wall of the tympanic cavity.
• Superior ligament of malleus – attaches the head of the malleus to the tegmental wall/roof of the tympanic cavity.
• Lateral ligament of malleus – attaches the head or neck of the malleus to the posterior part of the tympanic notch (Rivinus' notch).

Incus

The incus (anvil) is the middle auditory ossicle. It is suspended medial to the malleus and lateral to the stapes and joins these ossicles together with synovial joints. It consists of the: body, short limb, long limb/process, and lenticular process.

The body of the incus articulates with the head of the malleus anterolaterally. Like the head of the malleus, it sits in the epitympanic recess. The short limb projects posteriorly from the body and is the site of attachment for the posterior ligament of the incus. The long limb/process projects inferiorly and anteromedially, parallel with the handle of the malleus. At its inferior end, it bends on a 90° angle to form the lenticular process which articulates with the head of the stapes by the incudostapedial joint.

Two ligaments suspend the incus in place:

• Superior ligament of incus – attaches the body of the incus to the tegmental wall/roof of the tympanic cavity. 
• Posterior ligament of incus – attaches the short limb of the incus to the mastoid (posterior) wall of the tympanic cavity.

Stapes

The most medial and smallest auditory ossicle is the stapes (stirrup). The features that can be identified on the stapes are the head (capitulum), anterior limb, posterior limb, and base (footplate).

On its lateral side, the head of the stapes (capitulum) articulates with the lenticular process of the long limb of the incus in the incudostapedial joint. The anterior and posterior limbs arise from the head, and attaches to the oval shaped base. The base of the stapes (footplate) then sits in the vestibular window of the labyrinthine (medial) wall of the tympanic cavity. 

There are two ligaments associated with the stapes:

• Stapedial membrane – a mucosal layer spanning across the space between the anterior limb, posterior limb, and base.
• Anular ligament of the stapes (stapedial annular ligament) – a ring of fibrous tissue around the vestibular window that connects it to the base of the stapes in the tympanostapedial syndesmosis.

More information about the anatomy of the ear and the process of hearing is provided below.

Articulations

There are three articulations of the auditory ossicles:

• Incudomalleolar joint – synovial joint between the head of the malleus (posterior side) and the body of the incus.
• Incudostapedial joint – synovial joint between the lenticular process of the long limb of the incus and the head of the stapes.
• Tympanostapedial syndesmosis – the connection between the base of the stapes and the vestibular window with the anular ligament of the stapes.

Muscles

Two skeletal muscles are associated with the auditory ossicles that contract in response to loud noises. This process lessens movements of the auditory ossicles and thus dampens vibrations caused by loud sounds to protect structures of the internal ear. These two muscles are the tensor tympani (Eustachian) muscle and stapedius muscle. 

The tensor tympani muscle lies inside a bony canal superior to the pharyngotympanic (Eustachian) tube, and passes posteriorly to insert onto the handle of the malleus. When it contracts, it pulls the handle of the malleus medially which tenses the tympanic membrane. Because the tympanic membrane is tense, it is not able to vibrate as much and the amplitude of oscillations is reduced. This is done in response to loud noises to protect the structures of the internal ear.

The stapedius muscle runs from its attachment on the inside of the pyramidal eminence of the mastoid (posterior) wall of middle ear, through a foramen at the apex of the pyramidal eminence, passes anteriorly, and inserts onto the posterior aspect of the neck of the stapes. When it contracts, the stapes is pulled posteriorly and its base is tilted inside the vestibular window. This action tightens the annular ligament of the stapes, reducing oscillation and preventing excessive movement of the stapes. Like the tensor tympani muscle, the stapedius muscle contracts in response to loud noises.

 Functions

The auditory ossicles transmit vibrations of the tympanic membrane through the middle ear to the vestibular window. At the vestibular window, a wave is generated to move the fluid in the internal ear, which will excite receptor cells and allow these mechanical vibrations to be transferred into electrical signals. Since the base of the stapes (attached to the vestibular window) is much smaller than the tympanic membrane, the vibratory forces at the base is 10 times greater than that at the tympanic membrane. This means that the ossicles increase the force of vibrations, but decrease the amplitude as the vibrations are transmitted across each ossicle. This converts large-amplitude low-force vibrations into small-amplitude, high-force vibrations.

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When we hear something, this sound must be converted from sound waves into electrical signals for the brain to process. First, sound waves enter the external acoustic meatus, or ear canal, and vibrate the tympanic membrane that is located at the end of this canal. On the opposite side of the tympanic membrane, the medial side, the malleus is attached with its handle. This is where a series of movements of the auditory ossicles begins.

First, movements of the medial portion of the tympanic membrane will move the handle of the malleus medially as well. In turn, this moves the head of the malleus laterally, which moves the head of the incus laterally as well because the head of the malleus and head of the incus articulate with each other. As the head of the incus moves laterally, its long process is moved medially. Since the long process of the incus articulates with the stapes, the stapes moves medially as well. The base of the stapes is attached to the vestibular window, and so the medial movement of the stapes means that the vestibular window is also moved medially. Movement of the vestibular window generates a wave in the fluid filled internal ear which stimulates receptor cells to transmit electrical signals to the brain via the cochlear part of the vestibulocochlear nerve (CN VIII).