Hearing - How the Ear Works
- Outer Ear
- Middle Ear
- Inner Ear
- Eighth Nerve
How the Ear Works
Your hearing is a vital part of your life. Conversations with family, friends and even strangers give us information we need to get through our daily lives. On top of these regular interactions, we rely on our hearing for safety alarms and alerts. Our auditory system is incredibly complex, requiring many different parts to work properly. The normal function includes:
- Environmental sounds produce sound waves which travel through the air.
- These sound waves are funneled into the ear canal by the shape of the outer ear.
- Sound waves bounce off of the eardrum causing it to vibrate.
- The vibrations are passed through the middle ear space by three small bones (ossicles) that attach to the eardrum.
- The stapes pushes against the entrance to the snail shell shaped, fluid filled cochlea (inner ear) causing pressure waves.
- These pressure waves move the tiny "hair cells" which turn the sounds into electrical impulses.
- The electrical impulses are sent through the eighth nerve to the brain stem, and up to the auditory region of the brain where it is interpreted as sound.
The Outer Ear
The portion of the ear that can be seen from the outside is called the auricle or pinna. The distinct shape of the pinna evolved to capture and amplify sounds in the 2,000 - 3,000 Hz range, the range where much of the understanding in speech comes from. This shape is as distinct as a fingerprint, and is actually being used as a means of identification in some places.
The other portion of the outer ear is the external auditory meatus, or ear canal. The ear canal is made up of a thin, highly vascularized layer of skin, meaning there is an abundant flow of blood to the ear. The outer half has a fatty layer where cerumen (earwax) is produced. This helps to remove debris, contaminants and bacteria from the ear to keep it healthy. Cerumen is normal, but if it builds up in excess it can block the ear canal causing discomfort and a mild conductive hearing loss. If this occurs, be sure to see a professional to have it safely removed. The inner half of the canal is skin on bone, meaning no cerumen is produced down near the eardrum.
The Middle Ear
The tympanic membrane or eardrum, is the skin at the bottom of the ear canal that leads to the middle ear. It is extremely thin and flexible, but is made up of three layers for strength.
The middle ear space is a hollow, air filled cavity that starts behind the eardrum. Sounds are transmitted through the space by three tiny bones known as ossicles. The ossicular chain attaches to the eardrum by the malleus, which is embedded in the eardrum. The sound vibrations travel through the malleus, the incus, then finally the stapes. As the sound moves from the eardrum to the foot of the stapes, the sounds are amplified due to the relative size difference between the two. The stapes presses in on the fluid filled inner ear.
The eustachian tube allows our bodies to equalize the pressure in the middle ear space with the air around us to allow the eardrum to move efficiently. Because the middle ear is surrounded by bone and the sealed eardrum, the only interaction with the outside atmosphere is through this tube. The eustachian tube runs down from the ear to the back of the throat, and is normally closed unless forced open by yawning, chewing or swallowing. Rapid pressure changes in the atmosphere, or from ascending or descending quickly can cause the middle ear space to feel plugged. This may cause the ears to 'pop' when the eustachian tube equalizes the pressure. This is why some people recommend chewing gum while flying in an airplane.
The Inner Ear
The inner ear is actually a hollow space located within the temporal bones of the skull, which are located on the sides of the head near the outer ears. There are two main parts to the inner ear, the cochlea and the semicircular canals. The semicircular canals are used to determine your balance, specifically the position of your head in space.
The cochlea is a snail shaped, fluid filled space also contained in the temporal bones. As the stapes presses in on the cochlea, pressure waves are created. These waves move the tiny stereocilia (hair cells) which are embedded in the walls of the cochlea. As the fluid waves move the stereocilia they bend, triggering electrical impulses which are transmitted to the eighth nerve.
The Brain Stem
Once the electrical signals are sent to the eighth nerve, they are transmitted up the brain stem through a series of transmission stations. These are known (in order) as the cochlear nucleus, superior olivary complex, lateral lemniscus, inferior colliculus and medial geniculate body. Each of these pieces not only transmit information to the brain, they do some recoding and processing along the way, and also send signals back down to the eighth nerve to trigger certain reflexes based on the sound inputs. As with many of our systems, there is decussation, meaning that signals crossover and the left side handles information from the right ear, and vice versa.