There are three main components of the human ear: the outer ear, the middle ear, and the inner ear.
The outer ear includes the pinna, the visible part of the ear, as well as the ear canal which terminates at the eardrum, also called the tympanic membrane. The pinna serves to focus sound waves through the ear canal toward the eardrum. The eardrum is an airtight membrane, and when sound waves arrive there, they cause it to vibrate following the waveform of the sound.
The middle ear consists of a small air-filled chamber that is located medial to the eardrum. Within this chamber are the three smallest bones in the body, known collectively as the ossicles (incus, malleus, and stapes). They aid in the transmission of the vibrations from the eardrum to the inner ear. While the middle ear may seem unnecessarily complex, the purpose of its unique construction is to overcome the impedance mismatch between air and water, by providing impedance matching.
Also located in the middle ear are the stapedius and tensor tympani muscles which protect the hearing
mechanism through a stiffening reflex. The stapes transmits sound waves to the inner ear through the oval window, a flexible membrane separating the air-filled middle ear from the fluid-filled inner ear. The round window, another flexible membrane, allows for the smooth displacement of the inner ear fluid caused by the entering sound waves.
The inner ear consists of the cochlea, which is a spiral-shaped, fluid-filled tube. It is divided lengthwise by the organ of Corti, which is the main organ of mechanical to neural transduction. Inside the organ of Corti is the basilar membrane, a structure that vibrates when waves from the middle ear spread out through the cochlear fluid. The basilar membrane is tonotopic, so that each frequency has a characteristic place of
resonance along it. Characteristic frequencies are high at the basal entrance to the cochlea, and low at the apex. Basilar membrane motion causes depolarization of the hair cells, specialized auditory receptors located within the organ of Corti. While the hair cells do not produce action potentials themselves, they release neurotransmitter at synapses with the fibers of the auditory nerve, which does produce action potentials. In this way, the patterns of oscillations on the basilar membrane are converted to spatio-temporal patterns of firings which transmit information about the sound to the brain stem.
As you can see, hearing is a complex process originating in the cochlea. The cochlea, a tiny snail shell shaped organ, is comprised of thousands of microscopic sensory cells. These sensory cells work like keys on a piano. Each sensory cell is organized and tuned to match a certain pitch. In a person with normal hearing, these cells respond to acoustic information in the environment and translate it into a neurological code that the brain can interpret. Understanding of sound happens in the brain; the ears are just the way in. The sensory cells have a very important role in this translation of acoustic information to a neurological code. If any of the sensory cells do not work properly, the information that arrives in the brain will be distorted and/or incomplete. The listener may have difficulty understanding what is said.
Speech is a complex acoustic signal. When a speech signal arrives at the cochlea, many sensory cells respond. Think of a sonata being played on a piano. Many keys are being played at once to make rich, full music. With hearing, many sounds are being “heard” at once to make sense of conversation, music, background noises, etc. When sensory cells are damaged and/or missing, incomplete and distorted sound arrives at the brain. Think about how a piece of music would sound when played on an out of tune instrument. This is comparative to speech coming through a cochlea with damaged and missing sensory cells. When the signal arrives at the brain, the music isn’t rich, full, or even recognizable. The listener has to work even harder to understand what he or she is listening to.
A cochlear implant is not a hearing aid. Rather, it is a neural prosthesis that may help to provide hearing to
people with severe to profound hearing loss by bypassing the damaged sensory cells of the cochlea. It is a procedure whereas a prosthetic electronic sensory device is utilized by surgical intervention. These implants normally consist of two components:
the sound processor and transmitter microphone, worn externally, normally behind the ear lobe (or is magnetically attached through the skull next to the implant); and
2) the implanted component is the impulse receiver, intercepting the impulses from the external component and conveying these electrical impulses onto the the cochlear
This acts as a receiver of sound waves to the interpreting area of the brain. The patient’s traditional means of hearing, “acoustic hearing,” is then replaced with “electric hearing” through the cochlear implant.
Receiving a cochlear implant is a surgery and, as with any surgical procedure, the decision to have surgery should be after all other avenues of remediation have been thoroughly examined and exhausted. Every surgery comes with risk and those risks should be weighed with concern, care, and sensitivity; never should the decision to have surgery be based on fear or unrealistic expectations.
Externally worn hearing aids amplify sound. In a person with damaged sensory nerves in the cochlea, a hearing aid would provide an amplified sound which would arrive at a damaged cochlea. The amplified sounds don’t overcome the damage of the sensory cells, however, property diagnosed hearing loss (even from damaged nerve cells) may be improved with the use of the proper hearing instrument designed and programmed for one’s specific hearing loss. Unlike a surgical procedure, the wearing of an external hearing instrument does not come with the same risks as a cochlear implant; in fact, the risk factors for negative effects of hearing aids are practically non-existent.
As with any health or medical question, seek the advice of one or more qualified professionals and make your decisions after seriously weighing all possible procedures along with their respective risks and benefits.
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