2. Describe the anatomy of the tympanic membrane and ossicles of the ear. Describe the role of these structures in the mechanism of hearing.

Tympanic membrane is also known as the ear drum. Anatomically it could be considered to be a part of the external ear since it is attached to the medial terminal end of the bony meatus. Functionally speaking it is part of the tympanic cavity.
It is more or less oval in shape (egg shaped). It is 9mm in diameter. Its broad portion lies superiorly. It is pearly white in color, thin and semitransparent. When viewed under illumination a trianglular cone of light (reflected light) is seen extending from the centre forwards and downwards. This reflection, or cone of light is due to the sectional shape of the membrane. The ear drum is set with an obliquity of about 55 degrees to the floor of the external meatus. The centre of the ear drum appears retracted, and is known as the umbo. This umbo lies at the apex of the cone of light. Visible as an ivory colored extension upwards from the umbo is the handle of the malleus. If the posterior portion of the membrane is transparent, then the image of the long process of the incus, and occasionally the stapedial tendon may also be seen.

Figure showing anatomy of ear drum

The ear drum is composed of 3 layers. The outer layer is formed by stratified squamous epithelium, and is continuous with that of the external auditory canal. Any condition affecting the skin of the external canal will also affect the outer layer of the ear drum. Common conditions like dermatitis involving the skin of the external canal can also involve the outer layer of the ear drum. Embryologically outer layer of the ear drum developed from the ectoderm. Myringitis granulosa a common condition affecting the ear drum affects only the outer layer of the tympanic membrane. The middle and inner layers are not involved in this condition. It is commonly caused by infections arising from the external canal. Constant irritation of the ear drum due to presence of wax may also predispose to this condition. Another condition which involves the outer layer of the ear drum is Bullous myringitis. In this condition blebs may be seen in the outer layer of the ear drum. It is commonly caused by viral infections, or mycoplasma pneumonia. It may also be associated with middle ear effusion.

The middle fibrous layer from which the ear drum derives its strength and resilience is derived from the mesoderm. This portion is infact sandwiched between the outer squamous lining derived from the ectoderm and inner mucosal lining of the middle ear cavity derived from the endoderm. The ectodermal and mesodermal components of the ear drum arise from the first branchial cleft, while the endodermal component is derived from the pharyngotympanic recess. The middle fibrous layer has two components: 1. radial and 2 circular fibres. The handle of the malleus lie between the middle fibrous layer and the inner mucosal layer of the ear drum. From the handle of the malleus the radial fibres of the middle fibrous layer radiate towards the circumferance of the ear drum. The circular fibres are more prominent and thickened along the circumference of the ear drum. The condensation of the circular fibres are fixed to the tympanic sulcus at the medial end of the external auditory canal. This middle firbous layer is absent in the attic area of the ear drum. The fibrocartilagenous ring and the fibrous layer of the ear drum are deficient superiorly. This deficient area is known as the notch of Rivinus. The attic portion of the ear drum which lack the middle layer is known as the pars flaccida, while the rest of the drum which has all the three layers is known as pars tensa. The chorda tympani nerve which is a branch of the facial nerve run between the middle fibrous and inner mucosal layers of the ear drum.

The skin of the external canal and the outer lining of the tympanic membrane are unique in a sense that they lack frictional and abrasive contacts which is common with the skin lining elsewhere in the body. Desquamated keratin does not accumulate on the surface of the tympanic membrane, or in the deep external meatus, because the skin lining here is endowed with a peculiar feature known as Migration. The surface layers of the skin of the ear drum, and the surface keratin move towards the periphery of the membrane, and then slowly along the external meatus to the exterior. Derangements of this unique feature is associated with some of the diseases of the external ear.

The inner layer of the ear drum derived from the endoderm of the pharyngotympanic recess is continuous with that of the mucosal lining of the middle ear cavity.

Blood supply:

The external surface of the ear drum receives its blood supply from the deep auricular branch of the maxillary artery. This small artery leaves the first part of the maxillary artery behind the neck of the mandible and gains access into the external canal by piercing the anterior wall behind the mandibular joint. It sends small branches into the membrane from the whole circumference of the pars tensa and one or more manubrial branches that descend on the handle of mandible from above. The internal surface of the ear drum is supplied from behind by the stylomastoid branch of the posterior auricular artery, and from the front by the tympanic branch of the maxillary artery. The superficial veins open into the external jugular vein; and those on the internal surface drain into the transverse sinus and veins of the dura mater, and partly into the venous plexus on the eustachean tube.

Nerve supply:

The innervation of the posterior half of the ear drum is by the auricular branch of the X nerve and the anterior half is by the auriculotemporal branch of the Vth nerve. The inner surface of the ear drum is supplied by the tympanic branch of the IXth nerve.

The middle ear contains three ossicles which play a very important role in sound transmission. These ossicles are:

1. Malleus
2. Incus
3. Stapes
   
   
   
   
   
   
   Malleus: This bone is shaped like a hammer hence the name. This is the largest of the three ossicles of the middle ear cavity. It has a head, neck and three processes arising from below the neck. The overall length of the malleus ranges between 7.5 - 9 mm. Its head lies in the attic region of the middle ear effectively dividing the attic into an anterior portion and a posterior one. The anterior portion lie anterior to the handle of the malleus, while the posterior portion lie behind the handle of the malleus. During surgical procedures for attic cholesteatoma clipping of this head will improve the exposure in the attic region. The head of the malleus on its posteriomedial surface has an elongated saddle shaped cartilage covered facet for articulation with the incus. This articular surface is constricted near its middle dividing the articular facet into a larger superior and a smaller inferior portions. The inferior portion of the articular facet lies at right angles to that of the superior portion. This projecting lower portion is also known as the cog or spur of the malleus. Below the neck the bone broadens and gives rise to the following: the anterior process from which a slender anterior ligament arises to insert into the petrotympanic fissure; the lateral process which receives the anterior and posterior malleolar folds from the annulus tympanicum, and the handle which runs downwards, medially and slightly backwards between the mucous and fibrous layers of the tympanic membrane. On the deep medial surface of the handle there is a small projection into which the tendone of the tensor tympani muscle inserts. Additionally the malleus is supported by the superior ligament which runs from the head to the tegmen tympani.
    
   
   
   
   
   
   
   

 

Figure showing the malleus

Incus: This bone is shaped like an anvil. It articulates with the malleus and has a body and two processes. The body lies in the attic and has a cartilage covered articular facet corresponding to that of the malleus. The short process projects backwards from the body to lie in the fossa incudis. It is infact attached to the fossa incudis by a short ligament. The long process of the incus descends into the mesotympanum behind and medial to the handle of the malleus. At its tip there is a small medially directed lenticular process which articulates with the stapes. The long process of the incus has precarious blood suppy. This portion of the incus is prone for undergoing necrosis in disease conditions.

Figure showing the incus

The stapes: The stapes consists of a head, neck, two crura and a base (footplate). The head of the stapes points laterally and has a small cartilage covered depression for articulation with the lenticular process of the incus. The tendon of the stapedius muscle attaches to the posterior part of the neck and the upper part of the posterior crura. The neck of the stapes gives rise to two crura, the anterior crura is thinner and less curved than the posterior crura. The two crura join the foot plate which closes the oval window during life. The average dimensions of the foot plate is 3mm x 1.4mm. The long axis of the foot plate is almost horizontal, with the posterior end being slightly lower than the anterior.

 

Figure showing the stapes

Role played by the ear drum and ossicles in hearing:

Sound which impinges on the ear drum sets it into vibrations. These vibrations are transmitted by the middle ear ossicles to reach the round window membrane which conduct it to the inner ear fluids. Initially the vibrations of the ear drum belong to the low pressure high displacement variety. These vibrations are not capable of vibrating the inner ear fluids. For vibrating the inner ear fluids the stimulus should be of low displacement high pressure variety. This change in the type of vibration is brought about by the impedance matching mechanism of the middle ear cavity.

Two processes are involved in the impedance matching mechanism of middle ear. They are:
1. The area of the tympanic membrane is larger than that of the stapes foot plate in the cochlea. The forces collected over the ear drum are concentrated over a smaller area, thus increasing the pressure over oval window. The pressure is increased by the ratio of these two areas i.e. 18.75 times.
2. The second process is the lever action of the middle ear bones. The arm of the incus is shorter than that of the malleus, and this produces a lever action that increases the force and decreases the velocity at the stapes. Since the malleus is 2.1 times longer than the incus, the lever action multiplies the force by 2.1 times. 

Buckling effect of ear drum: The ear drum curves from its rim

to its attachment to the manubrium. The buckling effect causes

greater displacement of the curved ear drum and less

displacement for the handle of the malleus. This causes high

pressure low displacement system. 

Role of ear drum in sound conduction:

The ear drum conducts sound from the external ear to the middle ear.

Bekesy postulated that the ear drum moved like a stiff plate up to

frequencies of 2 kHz. He also suggested that the inferior edge of the drum is

flaccid and moves the most. At frequencies above 6 kHz the vibrating

pattern becomes more complex and chaotic. This reduces the efficiency of

sound transfer mechanism.

The handle of the malleus is attached to the centre of the ear drum. This

allows sound vibrations on any portion of the ear drum to be transmitted to

the ossicles.