Classically, the ear is divided into the outer ear, the middle ear and the inner ear. The outer ear includes the external part (the pinna or auricle, not shown), the ear canal (the oblong yellow space at the left of the diagram) and the ear drum, shown as a diagonal blue slash above. The medical name for the ear canal is external auditory canal; the medical term for the ear drum is tympanic membrane.
A note about directions: as used here, lateral means "towards the outside of the head," while medial means "towards the inside of the head." In this diagram, lateral is to the left, medial is to the right.
The middle ear is medial to the ear drum. The middle ear bones, or ossicles, are shown in black and grey. They are, from left to right (lateral to medial), the malleus, incus and stapes (hammer, anvil and stirrup to some elementary school students!) The Eustachian tube (shown above in pink) is generally considered part of the middle ear. It connects the middle ear space (the tympanic cavity, or tympanum) to the highest part of the throat, the nasopharynx.
The inner ear (shown here in blue and green) is a fluid-filled space. The cochlea, shown in green, contains sensory cells that are sensitive to sound. The vestibular system, shown here in blue, contains sensory cells that are sensitive to rotational motion, linear motion, and changes in the position of the head with respect to the ground (gravitational sensors, in essence).
So, how does this thing work?
Both the external ear and middle ear are shown here in yellow, to indicate that both are air-filled spaces. The key function of the external ear and middle ear is to convert air pressure waves into liquid pressure waves. "Air pressure waves" is just another way of saying "sound waves." And if you have ever tried to hear someone speaking to you while your head is under water, you will certainly appreciate the fact that very little sound penetrates water. Liquid pressure waves within the inner ear stimulate highly-specialized nerve cells in the cochlea; next, nerve impulses are sent to the brain, and the mind then perceives sound. (My ENT colleagues will cringe at this over-simplification, but what I have said is fundamentally correct.)
Let's focus on the relationship between the ear drum, middle ear bones and cochlea. Air pressure waves (sound waves) cause the tympanic membrane (ear drum) to vibrate. These vibrations are transmitted through (and are actually amplified by) the ossicles. The most medial ossicle, the stapes, acts as a plunger. As it vibrates, it alternately pushes at and pulls away from the fluid in the inner ear. Thus, an air pressure wave is converted into a liquid pressure wave within the inner ear fluid compartment.
Think of it as a flow of energy. You know, like: power company (electrical energy) to wall socket (electrical energy) to light bulb (light energy). In the ear, sound transmission can be broken down in the same way:
Air pressure waves in the ear canal (sound energy) causes...
Ear drum motion (vibrational energy), which causes...
Motion of the middle ear bones (vibrational energy), which causes...
Inner ear fluid pressure waves (vibrational energy), which causes...
Nerve impulses within the cochlea, which are then carried via a series of nerve connections to the portion of the brain that perceives "sound," the auditory cortex (electrochemical energy).
Anything that disrupts this flow of energy results in hearing loss. Traditionally, hearing loss is described as either conductive (the flow of energy down to the cochlea) or sensorineural ("sensory" refers to the nerve cells in the cochlea which "fire" in response to the fluid pressure wave; "neural" refers to nerve impulses beginning with the auditory nerve, which exits the cochlea and enters the brainstem, and all other nerves within the brain ultimately leading to the auditory cortex).
A brief description of some common causes of conductive or sensorineural hearing loss might help clarify things a bit.
CONDUCTIVE HEARING LOSS: This is the term that is used when there are problems which the flow of air pressure waves down the ear canal, across the ear drum, or through the ossicles. Here are some examples of problems that cause conductive hearing loss:
wax impaction in the canal (an ear plug accomplishes the same thing).
infection in the canal, causing the canal to swell shut ("swimmer's ear," for which the medical term is otitis externa).
infection, scarring or perforation of the ear drum.
disruption of the ossicles (due to trauma or infection).
fluid in the middle ear space (e.g., in acute middle ear infection, or otitis media, this fluid is usually pus; fluid can also be present without infection -- this is known as a sterile effusion).
impaired mobility of the ossicles. If the ossicles can't vibrate, sound waves cannot very well be transmitted through them. Chronic infection can cause scars or adhesions to impair the mobility of the ossicles. In a condition known as otosclerosis, the stapes becomes "frozen in place" by bony scarring.
SENSORINEURAL HEARING LOSS: This is the term that is used when there are problems in the cochlea, the auditory nerve, or any other of the nerves linking the cochlea to the auditory cortex of the brain. Here are some examples of problems that cause sensorineural hearing loss:
Drug-induced damage of the cochlea (examples: antibiotics, such as gentamicin; certain diuretics; chemotherapy drugs, such as cisplatin).
Traumatic damage of the cochlea (noise is a form of trauma; a blow to the head, or penetrating injury of the inner ear, can also cause sensorineural hearing loss).
Age-related damage of the cochlea (presbyacusis).
A tumor on the auditory nerve (acoustic neuroma or schwannoma).
Certain infections, such as meningitis.
