There are two main sections to the ear the central and the peripheral hearing parts. The peripheral hearing parts consists of the outer, middle and inner ear. The outer ear contains the pinna/auricle, the eardrum and the ear canal.
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The middle ear is a small space filled with air, that contains 3 tiny bones (the malleus, incus and stapes which make up the ossicles). The malleus is joined to the eardrum joining it to the outer ear and the stapes, which is the smallest bone in the body, which in turn connects to the inner ear.
The inner ear holds both balance organs as well as hearing organs. The hearing part of the ear is called the cochlea (which is derived from the “Greek word for snail”) and has a distinctive coiled shape. It contains many thousands of sensory cells called hair cells and is linked to the central hearing system via the auditory nerve. The cochlea contains fluids necessary for us to hear.
The central hearing system consists of the auditory nerve and pathway through the brainstem and to the auditory cortex of the brain.
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How We Hear:
Sound waves are collected in the pinna and the sound waves are funnelled into the ear canals. The oscillations of the air particles that make up the sound wave collide with the ear drum, a delicate structure, that then makes the eardrum vibrate. This in turn moves the ossicles, a tiny chain of bones, transferring the oscillations of the sound wave to the cochlea of the inner ear. This occurs due to the fact that the last three bones in this chain of bones, the stapes, lies in a membrane – a sealed window in the bony wall which divides the cochlea of the inner ear from the middle ear. As the stapes vibrate, it makes the fluids in the cochlea, mentioned earlier, “move in a wave like manner”. This stimulates and activates the hair cells. These hair cells are designed to respond to varying sound pitches and frequencies. High-pitch stimulates hair cells in the lower cochlea and vice versa. Upon receiving a sound pitch/frequency that the hair cell is tuned to respond to, it creates several nerve impulses, which travel “instantaneously” along the auditory nerve. “We hear with our brain” which is a statement that is somewhat false, because it’s only when the nerve impulses from the auditory nerve reach the auditory cortex of the brain, where it’s converted into meaningful sound, do we hear anything. Without this final conversion, the sound waves would just be turned into impulses that couldn’t be recognised as sound by us.
This can be caused by various issues with the three sections of the ear or a combination of issues in several parts of them. Issues with the outer/middle ear can lead to a reduced volume of sound waves received by the brain as there is an inefficient transfer of sound to the cochlea in the inner ear so sound energy from the waves is lost and thus the volume is lower. An example of this would be too much earwax in the ear canal, which is referred to as conductive hearing loss because sound waves are conducted by the wax, so not enough sound waves reach the inner ear where the impulses are created by the hair cells and sent to the brain. Hence, the volume of the sound converted in the auditory cortex of the brain is lower than normal.
Sensorineural hearing loss is, as the name suggests, an issue with the transfer of nerve impulses from the hair cells in the inner ear to the area of the brain where sound is received. This can be caused by numerous things from birth defects in either the auditory nerve and/or the hair cells to damage sustained through life, from ageing or over exposure to loud sounds, to the delicate hair cells in the cochlea of the inner ear. This problem means that certain frequencies and pitches of sound waves cannot be processed by the specific hair cells, they’re damaged, so the nerve impulse is either not sent or sent incorrectly so isn’t processed correctly by the auditory cortex. This is characterised by a difficulty to hear speech with the presence of moderate background noise (dependent on the extent of damage to the hair cells or the auditory nerve) and general hearing difficulty.
A combination of both types/forms of hearing loss is called mixed hearing loss.
Treating Hearing Loss:
Hearing aids are used to help treat hearing loss caused by damage to the hair cells, Sensorineural hearing loss. A hearing aid helps to magnify sound vibrations which are converted to nerve impulses by the remaining hair cells. More amplification is required for people who have more severe damage sustained to the hair cells, so the increased amplification is used to “make up the difference”. However, if a person has sustained extensive damage to their hair cells then hearing aids may be ineffective as even large sounds won’t be converted into nerve impulses.
Some hearing loss problems in the outer ear such as excess wax in the ear canal can be removed by wax removal with a cotton bud.
Surgical processes can be used to remove/alter abnormalities in the structure of key bones in the ear which are used for the passage of sound waves to the cochlea. This is used in cases where there are severe deformities in the bones which prevent decent hearing in a person. Stem cell research, although not widely used because it’s still being researched, can be used to regrow new hair cells or auditory nerve cells from differentiation of adult stem cells or embryonic stem cells. But using embryonic stem cells has raised ethical issues in the past and still does to this day, not to mention the potential cause of long-term health complications, including cancer, to older patients or patients with underlying health issues.
Infections in the fluid of the cochlea can be treated by the insertion of small tubes into the cochlea to help with drainage as a lack of proper drainage can lead to an excess amount of fluid which provides an oxygenated, warm and moist environment for bacterial or fungal growth. This leads to infections.
Finally, for people with severe damage to the hair cells, i.e. sensorineural hearing loss, then cochlear implants can be used to bypass damaged areas of the cochlea and stimulates the hearing nerve with the appropriate response, a nerve impulse, that mimics that of a real hair cell. This allows for the nerve impulse to be converted in the brain to a meaningful sound as it would if hair cells were preserved.
How Hearing Helps Us Balance:
Muscles, ligaments and the ear all make up the vestibular system which sends impulses and signals to the brain which helps us keep balanced. Semi-circular canals in the inner ear help us to sense movement: one canal is used to sense up-and-down movement whilst one is used to sense side-to-side movement and another for left-to-right movement. Both the semicircular canals and the otolith organs are filled with fluid. When we move, that fluid moves causing hair cells, called ampullae, in the semicircular canal to be moved, which sends signals down the “acoustic” nerve to the brain which helps us locate when we are in space.
The otolith organs are situated under the semicircular canals and also contains hair cells. However, on these hair cells there are crystals that aren’t found on hair cells in the semicircular canals. These crystals are called otoliths or “ear rocks”. The otolith organs detect accelerations. These signals are sent to the brain which instructs various muscles and joints as well as the eyes to readjust to maintain balance. When in a car or travelling with a ship, the ears may not detect any movement, especially if the person is stationary, however other balance organs may send contradictory messages to the brain which causes us to feel nauseated or dizzy, commonly felt in car sickness.
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