Tuesday, April 25, 2017

The Basics : Middle Ear

Hey Awesomites

In this post, I will be talking about the middle ear structures and its relations with its neighbors ( just a summary ).

The Middle Ear is an air filled and bilaterally compressed/ concaved cavity lined by mucous membrane located in between the external and internal parts of ear. It is divided into:
- Epitympanum or the Attic ( 6mm ) - lies  above pars tensa and medial to pars flaccida
- Mesotympanum ( 2mm ) - lies opposite to pars tensa
- Hypotympanum ( 4mm ) - lies below the level of pars tensa


BOUNDARIES of the middle ear ( homologous to structure of a cube ) :-

Roof : Tegmen tympani - a thin bony plate that is a part of petrous part of temporal bone, separates the middle ear cleft from middle cranial fossa.
- Infection in the middle ear may spread superiorly and lead to formation of abscess in the meninges ( especially Extradural abscess ), meningitis or if severe, it may even lead to abscess formation in the temporal lobe.

Floor : Jugular bulb - The middle ear cavity is separated from jugular bulb by a thin piece of bone that if deficient may lead to formation of a layer of fibrotic tissue and mucous membrane in between. The contents of jugular bulb are:
- Internal Jugular vein
- Glossopharyngeal nerve ( IX )
- Vagus ( X )
- Accessory nerve ( XI )

The tympanic branch of glossopharyngeal nerve enters the middle ear at the junction of the floor and medial wall to play an important role in formation of tympanic plexus.

Anterior wall : The upper part of the narrow anterior wall has two openings or tunnels for - ( mnemonic : TEA )
- Canal for Tensor tympani muscle
- Pharyngotympanic ( or Eustachian ) tube

The lower part of anterior wall is separated from the Internal Carotid Artery by a thin plate of bone. The ICA is surrounded by a plexus of sympathetic nerves that enter middle ear through openings in this bony plate to form tympanic plexus.

Posterior wall : Posteriorly, it is related to middle ear cleft ( Aditus, Antrum and mastoid air cells )
- Infection in this region may spread posteriorly into the sigmoid sinus ( in posterior cranial fossa ) and cause thrombophlebitis !!

Medial wall : Medially the middle ear cavity is related to the promontory, oval and round window

Lateral wall : Tympanic membrane separates the middle ear from the external ear.



A brief about the functions of middle ear:
On the incoming of sound waves, the tympanic membrane oscillates and these oscillations are sensed by the strongly attached and faithful middle ear ossicle, the Malleus. The sound energy is transmitted as such by the ossicles ( Malleus - Incus - Stapes ) to the internal ear for further processing.

The major function of these ossicles is amplification of sound waves - Tympanic membrane is 17 times larger than the oval window - So that means the sound energy is picked up by the larger area ( TM ) and impinged over a much smaller area ( oval window ) thus amplifying it 17 times.

In addition, the lever action of the ossicular chain is approx. 1.3 units. Thus the intensity ( force ) of sound waves/ vibrations changes ( increased by ~20 times ) and not the frequency !! If the sound waves are not amplified ( in case OC is removed ), the Air Conduction would be lost. So BC > AC and thus hearing would then be poor.


Thats all
Hope this helped :)
Stay Awesome!

- Jaskunwar Singh

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