The Nervous System, Ear, Hearing and Balance — Page 67, Lesson 99

I want to walk you through the topic of hearing loss and balance, starting with Noise Induced Hearing Loss, or NIHL. Loud noises can damage the very sensitive membrane inside the cochlea, as well as the fine structures on that membrane. The cochlea is the spiral-shaped part of the inner ear that converts sound vibrations into nerve signals. At first, the hearing loss may be temporary, but if you are exposed to loud noise in excess of 90 decibels—that's dB for short—on a continued basis, the result will be permanent hearing loss. The early symptom of NIHL is an inability to hear high-pitched notes. Why? Because those high frequencies are normally detected by the finer cells in the cochlea, and those finer cells suffer the greatest damage from loud noise. Helicopter pilots and military jet pilots tend to suffer from NIHL. And the excerpt notes that with the advent of personal stereos, there has been an alarming increase of this impairment appearing in youth. Environmental noise pollution is now a significant factor in how common NIHL is. For those of us in the aviation industry, NIHL is an occupational hazard. It is strongly recommended that you use ear plugs conscientiously whenever possible. The most dangerous type of noise to the ear is noise of high frequency. Now let's talk about presbycusis, which is hearing loss through ageing. Hearing deteriorates as you advance in age. In old age, the frequency range of hearing falls to between 50 and 8000 cycles per second, or less. A cycle per second is the same as a hertz. Some loss of hearing is natural as you grow older, but if that is combined with NIHL, there may be sufficient impairment to lead to the loss of a flying licence. It is worth noting that aircraft engineers are warned always to use hearing protection when exposed to noise in excess of 115 dB. As a rough guide, such levels occur when normal speech cannot be clearly heard at a distance of 2 metres. Intermittent and sudden noise is generally considered to be more disruptive than continuous noise. In addition, high frequency noise generally has a more adverse effect on performance than lower frequency noise. Now let's move to the ear and balance. As well as distinguishing sound, the ear is used to detect both angular and linear movement, as well as accelerations. Our primary source of spatial orientation is sight, but the ear provides a secondary system, particularly if vision is restricted. Within the inner ear are three semicircular canals. They are filled with liquid and arranged in three planes at 90 degrees to each other. They detect angular accelerations greater than 0.5 degrees per second squared. Inside the semicircular canals are fine hair-like cells. These cells bend as the liquid in the canals moves in relation to the walls of the canals. The movement of these hairs generates small electric currents, which are passed to the cerebellum. The cerebellum is the second smaller division of the brain. In fact, the cerebellum has the ability to predict the loss of balance and compensate. For example, as you step onto an escalator, muscles will work to push the body forward instinctively to avoid losing balance. So the cerebellum has a major part to play in both balance and coordination.