Organs - The Ear (Middle) Flashcards
Where is the middle ear located?
It lies within the temporal bone.
It extends from the tympanic membrane to the lateral wall of the inner ear.
What is the function of the middle ear?
To transmit vibrations from the tympanic membrane to the inner ear via the auditory ossicles.
The middle ear can be divided into two parts. These are…
1) Tympanic cavity - located medially to the tympanic membrane. It contains three small bones, known as auditory ossicles: the malleus, incus and stapes. They transmit sound vibrations through the middle ear.
2) Epitympanic recess - a space superior to the tympanic cavity, which lies next to the mastoid air cells. The malleus and incus partially extends upwards into the epitympanic recess.
What are the borders of the middle ear?
Roof - formed by a thin bone from the petrous part of the temporal bone. It separates the middle ear from the middle cranial fossa.
Floor - known as the jugular wall, it consists of a thin layer of bone, which separates the middle ear from the internal jugular vein.
Lateral wall - made up f the tympanic membrane, and the lateral wall of the epitympanic recess.
Medial wall - formed by the lateral wall of the inner ear. It contains a prominent bulge, produced by the facial nerve as it travels nearby.
Anterior wall - a thin bony plate with two openings; for the auditory tube and the tensor tympani muscle. It separates the middle ear from the internal carotid artery.
Posterior wall (mastoid wall) - it consists of a bony partition between the tympanic cavity and the mastoid air cells. Superiorly, there is a hole in this partition, allowing the two areas to communicate. This hole is known as the aditus to the mastoid antrum.
What are the bones of the middle ear?
These are known as the auditory ossicles.
From lateral to medial:
1) Malleus
2) Incus
3) Stapes
They are connected to each other, linking the tympanic membrane to the oval window of the internal ear.
What are the main features and location of malleus?
It is the largest and most lateral bone.
The head of the malleus lies in the epitympanic recess, where it articulates with the next ossicle, the incus.
What are the main features and location of incus?
The incus consists of a body and two limbs.
The body articulates with the malleus.
The short limb attaches to the posterior wall of the middle, and the long limb joins the last of the ossicles; the stapes.
What are the main features and location of stapes?
This is the smallest bone in the human body.
It joins the incus to the oval window of the inner ear.
It is stirrup-shaped, with a head, two limbs, and a base.
The head articulates with the incus.
The base joins the oval window.
Where is the mastoid air cells located? What is their purpose?
These air cells are located posterior to the epitympanic recess.
They are a collection of air-filled spaces in the mastoid process of the temporal bone.
The air cells are contained within a cavity called the mastoid antrum. The mastoid antrum communicates with the middle ear via the aditus to mastoid antrum.
The mastoid air cells acts as a ‘buffer system’ of air - releasing air into the tympanic cavity when the pressure is too low.
What are the muscles of the middle ear?
There are two muscles that have a protective function in the middle ear. These are:
1) Tensor tympani
2) Stapedius
Origin, insertion, action, and innervation of tensor tympani?
Origin - auditory tube
Insertion - handle of malleus
Action - pulls the handle of malleus medially when contracting
Innervation - tensor tympani nerve, a branch of the mandibular nerve
Origin, insertion, action and innervation of the stapedius?
Origin -
Insertion - stapes
Action -
Innervation - facial nerve
What is the auditory tube? What is its purpose?
The auditory tube (Eustachian tube) is a cartilaginous and bony tube that connects the middle ear to the nasopharynx.
It equalises the pressure of the middle ear to that of the external auditory meatus.
Where is the Eustachian tube located?
It extends from the anterior wall of the middle ear, in an anterior, medioinferior direction, opening on the lateral wall of the nasopharynx. In joining the two structures, it is a pathwy by which an upper respiratory infection can spread into the middle ear.
The tube is shorter and straighter in children, therefore middle ear infection tend to be more common in children than adults.
Clinical relevance: Otitis media with effusion
This is commonly known as glue ear.
It arises from persistend dysfunction of the auditory tube. If the auditory tube is unable to equalise middle ear pressure (due to blockage, inflammation, genetic mutation), a negative pressure develops in the middle ear.
The negative pressure draws out transudate from the mucosa of the middle ear, creating an environment suitable for pathogens to replicate and cause infection.
Upon inspection of a patient with otitis media with effusion, the eardrum will appear inverted, with fluid visible inside the ear.
