How the Ear Works
Image Courtesy of Cochlear Americas
The ear is divided into three parts: the outer, middle, and inner ear. Each part performs an important function in the hearing process.
The outer ear consists of the pinna (the part you see on the outside) and the ear canal. The outer ear serves to collect sound and funnel it to the middle ear. The eardrum (tympanic membrane) is the dividing structure between the outer and middle ear. Sounds reach the eardrum causing it to vibrate. The middle ear consists of an air-filled cavity containing the three middle ear bones, called ossicles. These three bones are the malleus, incus, and stapes. The ossicles are connected and move together in a lever action to amplify sounds. The vibration of the eardrum moves these bones which are connected to the inner ear by the stapes. The inner ear contains two parts, the cochlea (hearing part) and the vestibular (balance) part. The cochlea is a fluid filled organ that contains tiny hair cells. When the stapes moves it puts the fluid in the cochlea in motion. The motion of this fluid stimulates the tiny hair cells at particular points which stimulate nerve endings and convert the signal into an electrical impulse. The nerves transmit the impulses to the brain via the acoustic (8th) nerve. The brain interprets these signals as sound. The cochlea is considered a tonotopic organ, meaning that it consists of frequency (pitch) specific regions. Depending on where within the cochlea the stimulation occurs the person will perceive different sounds.
Problems or damage can occur at any point within this system which will often result in hearing loss. The outer ear can collect an abundance of earwax (cerumen). If cerumen fills the ear canal it will block the sounds from collecting. A foreign body of any type in the ear canal can do this as well. A hole in the eardrum (tympanic membrane perforation) can occur which reduces the eardrum’s ability to vibrate properly in response to sound waves entering the ear. The middle ear is most often problematic in children. Fluid can develop in what should be the air-filled portion of the middle ear. This will impede the movement of the ossicles and reduce their ability to transmit sounds properly to the inner ear. Other medical problems can occur within the middle ear including breaks or fixations of the ossicles, or growths within the middle ear, that will degrade the ossicles ability to transmit sound to the inner ear. All of these problems can result in a conductive hearing loss, meaning something is preventing the sound from be properly “conducted” through the ear system. These are often medically treatable problems.
Problems occurring in the inner ear or the acoustic nerve involve the nerves. If the tiny hair cells of the inner ear are damaged, they will not allow the signal to trigger the electrical nerve impulses that the brain needs to perceive sound. There is often damage to a particular region of the cochlea which results in loss in the ability to understand speech sounds properly. This often occurs as a function of age, or as a result of excessive noise exposure. Hearing loss that occurs within the cochlea or acoustic nerve is referred to as a sensorineural hearing loss. These are generally not medically treatable and often require the use of an amplifying device, such as a hearing aid.
Problems can also co-occur and cause a conductive and a sensorineural hearing loss at the same time. This is referred to as a mixed hearing loss. In these cases, the conductive portion is often medically treatable, but the sensorineural portion is not.
There are many additional causes of all three types of hearing losses, including trauma, infection, heredity, noise exposure, medications, and illnesses.