Tympanic Membrane Of Ear/ EARDRUM

Tympanic Membrane Of Ear/ EARDRUM

EMBRYOLOGY:

Tympanic membrane derived from first pharyngeal membrane (which seperates 1st pharyngeal groove from 1st pharyngeal pouch)represents all three layers of embryonic disc.
It develops from three sources:

External very thin skin is derived from ectoderm of 1st pharyngeal groove
Medoderm of 1st & 2hd pharyngeal arches
Internal lining is derived from endoderm of tubotympanic recess, (a derivative of 1st pharyngeal pouch).

POSITION:

STRUCTURE:

Surface area of tympanic membrane 90 mm2.
Effective area is 55 mm2 (approximately 2/3 of the total area).
In a healthy ear it is shiny & pearly grey/ pale grey in colour.
Normal tympanic membrane is mobile with maximum mobility at periphery.
The tympanic membrane slants downward & inward obliquely, so that it forms an angle of 30-45 degree with the floor of horizontal canal.
Tympanic membrane consists of the following three layers:

1. Outer epithelial layer (Cuticular Stratum):

2. Middle fibrous layer (Fibrous Stratum):

It encloses the handle of malleus and consists of three types of fibers: radial, circular and parabolic.
In comparison to pars tensa, this layer is very thin in pars flaccida (consists of loose conn. tissue) and not organized into various fibers.

3. Inner mucosal layer (Mucous Stratum):

It is continuous with the middle ear mucosa.
Single layer of flat cells.
Cilliated collumnar cells are absent over medial aspect of TM.
The distance between tympanic membrane and medial wall of middle ear at the level of center is 2mm

PARTS:

The peripheral part of tympanic membrane is the most mobile part of the tympanic membrane
Tympanic membrane consists of two parts:

1. Pars tensa:

It forms most of tympanic membrane
Annulus tympanicus: TM is thickened in the periphery and forms a fibrocartilaginous ring called the annulus tympanicus that fits in the tympanic sulcus.
Umbo: The central part of TM near the tip of malleus is tended inwards and is called the umbo.
Cone of light: A bright cone of light radiating from the tip of malleus to the periphery in the anteroinferior quadrant is usually seen during otoscopy

2. Pars flaccida (Shrapnell’s membrane):

It is situated above the lateral process of malleus between the notch of Rivinus and the anterior and posterior malleal folds.
It is not as tense as pars tensa and may appear little pinkish.
Reissner’s membrane – Separates scala media from scala vestibuli in the inner ear
Basilar membrane – Seen in scala media and supports the organ of corti
Secondary Tympanic Membrane – Closes the scala tympani at the site of round window

NERVE SUPPLY:

Auriculotemporal nerve (CN V3): Anterior half of lateral surface of TM.
CN X (vagus nerve): Its auricular branch (Arnold’s nerve) supplies to posterior half of lateral surface of TM.
CN IX (glossopharyngeal nerve): Its tympanic branch (Jacobson’s nerve) supplies to medial surface of tympanic membrane.

Exam Question

Tympanic membrane represents all three layers of embryonic disc
Handle of malleus is attached to the center of the tympanic membrane
Nerve supply of the tympanic membrane is by Auriculotemporal
Tympanic membrane is Pearly white in color
The peripheral part of tympanic membrane is the most mobile part of the tympanic membrane
Shrapnell’s membrane is another name for Pars flaccida of the tympanic membrane
The distance between tympanic membrane and medial wall of middle ear at the level of center is 2mm
Distance of promontory from tympanic membrane 2mm
Surface area of tympanic membrane 90 mm2
Effective area is 55 mm2(approximately 2/3 of the total area).
Lateral wall of middle ear is formed by Tympanic membrane

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