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             Tympanic membrane is the partition between external auditory canal & middle ear.

Measurement-  * 9-10 mm tall

                         * 8-9 mm wide

                         * 0.1 mm thick

 

                       

 

Parts-   

  1. Pars Tensa: * Forms most of the membrane.

                                * Periphery thickened to form a fibrocartilaginous ring called “Annulas tympanicus”, fits in the tympanic sulcus.

                                * Central part tented inwards at the level of tip of malleus, called “umbo”.

                                * Cone of light seen in antero inferior quadrant.

  1. Pars Flaccida (Sharpnell’s Membrane): * Situated above the lateral process of malleus between the notch of Rivinus & the anterior & posterior malleal folds.

Layers-  Three layers-

  1. Outer epithelial- Continuous with skin lining meatus.
  2. Inner mucosal- Continuous with mucosa & middle ear.
  3. Middle fibrous- Encloses handle of malleus & has three types of fibres- radial, circular & parabolic.

Nerve Supply-

  1. Anterior half of lateral surface: Auriculotemporal nerve.
  2. Posterior half of lateral surface: Auricular branch of Vagus nerve.
  3. Medial surface: Tympanic branch of Glossopharyngeal nerve.

 

 *** ROLL OF TYMPANIC MEMBRANE IN SOUND TRANSMISSION:

       Tympanic membrane helps in mechanical conduction of sound.

                        A sound signal in the environment

                                                    ↓

                                    Collected by pinna

                                                    ↓

                                 Pass through the EAC

                                                    ↓

                                        Strikes the TM

                                                    ↓

Vibrations of TM are transmitted to Stapes foot plate through a chain of ossicles coupled to TM.

 

Conduction is accomplished by-

  1. Lever action of ossicles- Handle of Malleus 1:3 times longer than long process of Incus, providing the mechanical advantage of 1.3.
  2. Hydraulic action of TM: The area of TM> the area of Stapes foot plate.

                                          

Effective vibratory area of TM- 40-45 mm2.

Foot plate area 3.2 mm2.

So, Effective area ratio is 14:1.

Lever ratio of ossicles is 1.3:1

So, total transformer ratio is (14*1.3) : 1 ≈ 18:1

This is the mechanical advantage provided by TM.

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