The tympanic membrane, commonly known as the eardrum, plays a fundamental role in hearing. Located deep within the ear canal, this thin, cone-shaped sheet of tissue is the first structure to receive sound from the outside world. Its job is to capture airborne vibrations and convert them into mechanical energy that the rest of the ear can process.
Anatomy and Location of the Eardrum
The tympanic membrane forms a boundary, separating the outer ear canal from the middle ear cavity. It is stretched obliquely across the end of the external canal, resembling a flattened cone with its tip pointed inward. This concave shape is maintained because the handle of the malleus bone, the first of the three middle ear bones, is firmly attached to its inner surface.
The membrane’s physical structure is defined by three distinct layers of tissue that provide flexibility and tensile strength. The outermost layer, facing the ear canal, is epithelial tissue, a continuation of the skin lining the canal. The inner layer, facing the middle ear, is a mucosal membrane continuous with the lining of the middle ear cavity.
Between these two coverings lies the middle layer, known as the lamina propria, which provides the eardrum’s mechanical properties. This dense, fibrous connective tissue is made up of circular and radial fibers that grant the membrane stiffness and tension. This composition allows the membrane to be thin enough to vibrate with minute air pressure changes, yet strong enough to act as a partition for the middle ear.
The Mechanics of Hearing
Hearing begins when sound waves, which are fluctuations in air pressure, travel down the ear canal and strike the tympanic membrane. The membrane vibrates in sympathy with these incoming sound waves, effectively capturing the acoustic energy. The frequency and amplitude of the sound determine the speed and extent of the eardrum’s vibration.
Once the membrane is set in motion, it acts as a transducer, converting acoustic energy into mechanical energy. This action is immediately transferred to the malleus, the first bone of the middle ear, which is fused to the eardrum. The movement of the malleus then initiates a chain reaction through the other two ossicles: the incus and the stapes.
This chain of movement across the three bones amplifies the sound pressure. The eardrum is larger in area than the oval window, the membrane the stapes pushes against to enter the inner ear. This size difference, combined with the lever action of the ossicles, concentrates the force of the vibration. This concentration is necessary to move the fluid within the inner ear’s cochlea and transform the signal into electrical impulses that the brain interprets as sound.
Common Causes of Tympanic Membrane Damage
The tympanic membrane’s structure makes it susceptible to damage from various sources. One frequent cause of rupture, particularly in younger individuals, is infection within the middle ear, known as acute otitis media. When infection causes fluid and pus to accumulate, the resulting pressure behind the eardrum can become intense enough to force a perforation.
Damage can also occur due to sudden changes in pressure, a condition termed barotrauma. Activities like scuba diving or flying can create a pressure imbalance between the outer ear and the middle ear cavity, which can tear the membrane. Exposure to an extremely loud noise, such as a nearby explosion, can also generate a powerful pressure wave capable of causing a rupture.
Direct trauma is another common injury, often caused by foreign objects inserted into the ear canal. Insertion of items like cotton swabs or hairpins can inadvertently puncture the eardrum. Furthermore, a concussive force to the side of the head, such as a forceful slap, can generate enough force to cause a perforation.