How the Ear Works
Picture courtesy of Cochlear Ltd.
How the ear works:
- Sound, which is transmitted as sound waves (vibration of the air), enters the ear canal and reaches the eardrum.
- The sound waves lead to the vibration of the eardrum, which also vibrates the small bones behind the ear drum.
- The vibration motion of the bones makes the fluid in the inner ear or cochlea to vibrate.
- The vibration waves in the inner ear fluid causes the sensory (hair) cells in the inner ear (cochlea) to bend. The hair cells change the movement into electrical signals.
- These electrical signals are transmitted through the hearing (auditory) nerve and up to the brain, where they are interpreted.
External Ear: The ear (external auditory) canal is covered by skin. The skin closer to the outside of the ear is thick, has hair, and produces ear wax (cerumen). This thick skin covers cartilage. The skin covering the ear canal that is further inside is very thin and overlies bone. The ear drum (tympanic membrane) is located at the end of the ear canal. The ear drum seals the outside (external ear canal) from the inside (middle ear).
Middle Ear: The middle ear (tympanic cavity) is the space behind the ear drum. It is covered by a lining that is similar to the lining of the nose. Normally, there is no skin inside of the middle ear space. The contents of the middle ear include the 3 little bones (ossicles) that take the sound from the ear drum to the inner ear (cochlea).
Chronic ear disease generally affects the middle ear. The middle ear space is connected to the back of the nose via a tube called the Eustachian tube. The Eustachian tube allows fluid/mucus to drain out of the ear and allows air to enter behind the ear drum. The middle ear space is also connected to the air space within the mastoid bone (the bone behind the ear). Due to this connection, diseases of the middle ear will generally affect the mastoid bone too.
The nerve that moves the face (the facial nerve) and the nerve that supplies the taste in the front part of the tongue (chorda tympani nerve) travel through the middle ear on their way to the face/mouth.
Mastoid Bone: The bone behind the ear drum has multiple air pockets within it. These air pockets are connected to the middle ear. The air pockets (called air cells) are lined with the same lining as the middle ear. Diseases of the middle ear will generally affect the mastoid as well.
Inner Ear: The inner ear includes the hearing (cochlea) and the balance organs (semicircular canals, utricle, and saccule). Therefore, any disease that affects the inner ear causes hearing and balance problems.
The inner ear is encased in bone and has two areas that are covered by membranes. These two "windows" are areas of potential communication with the middle ear. The two areas are called the round window and the oval window.
The inner ear is filled with two kinds of fluid, the endolymph and the perilymph. The leakage of perilymph as a result of trauma occurs from the oval or round windows and causes hearing and balance dysfunction. An increase in the endolymph pressure leads to the hearing and balance problems in Meniere's disease.
Anatomy Related to Acoustic Neuromas
The hearing nerve travels right next to the two balance nerves and the facial nerve (the nerve that controls the movement of the face). The tumors of the hearing or balance nerves generally start at the area where the nerves enter the temporal bone (bone surrounding the ear) and grow towards the brainstem. This area is called the cerebellopontine angle (CPA) (the angle between the cerebellum and the pons of the brainstem). Below is a sliced image of the head at the level of the hearing and balance nerve.