Middle ear

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 cochlear window which lies posterosuperior to the promontory, and the vestibular 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 vestibular window of the internal 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.

Relating structures

The external ear is situated lateral to the middle ear, while the internal 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 vestibular window (fenestra vestibuli) and the cochlear 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.

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Vasculature

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.