A Tour around the Ear

(Date of publication 14 March 2005)

The intensity range of the human ear is approximately 130 decibels. Doesn't sound very impressive, does it? But this means that the maximum tolerable volume is 10,000,000,000,000 times louder than the lowest audible sound, the energy of which is equivalent to that given off by a 50 watt light bulb 3,000 miles away! The displacement of the eardrum which results is about one-tenth the diameter of a hydrogen atom. Other statistics are almost as impressive; the ear can detect a difference of just 2 degrees in the direction of a sound source, and can recognise at least 400,000 different sounds.

A labelled diagram of the outer, middle and inner ear can be found on this page and the Macromedia Flash presentation at the bottom of the page gives a simple outline of the sensation of hearing. The Physics Classroom has a more detailed but easily understandable description of the way in which sound energy is converted into a nerve impulse. Essentially, the outer ear collects and channels sound to the eardrum, which vibrates when the sound – in the form of pressure waves – reaches it. As the eardrum is connected to the hammer, or malleus, this vibration sets the bones of the middle ear in motion, transmitting energy via the stirrup, or stapes, to the fluid of the inner ear in the form of compression waves. The bones of the middle ear (see the photograph here) act as an amplifier. A mechanical advantage means that the displacement of the stirrup is greater than that of the hammer, and the area of the stirrup in contact with the inner ear is much smaller than that of the eardrum, so that the force transmitted is multiplied almost 15 times. The inner surface of the cochlea is lined by over 20,000 hair-like nerve cells, of microscopically different lengths, that resonate in response to different frequencies of compression wave. When a cell resonates it sends an electrical impulse along the auditory nerve to the brain, where the pattern of impulses is interpreted.

These diagrams from Washington University show that the inner ear has a vestibular system, in addition to the cochlear, which is concerned with balance. The three interconnected semicircular canals act as gyroscopes and are positioned in three planes perpendicular to each other. Each is filled with endolymph and contains a motion sensor, which responds to angular acceleration by sending an impulse to the brain via the vestibular nerve.

The most common medical problem affecting the ear is otitis media, or infection of the middle ear; 80% of children in the US have at least one episode before their third birthday. Acute otitis media (AOM) generally follows an upper respiratory tract infection and starts suddenly with pain, irritability, fever and hearing loss. Examination of the affected ear reveals a reddened, bulging eardrum owing to the accumulation of pus; compare the affected and normal membranes in this photograph. Oral antibiotics are usually all that is required, but it is important for clinicians to distinguish between AOM and otitis media with effusion – the presence of fluid in the middle ear without signs or symptoms of infection – to avoid antibiotics being prescribed unnecessarily. If the infection is not eliminated and becomes chronic, hearing loss may occur as a result of perforation of the eardrum or erosion of the bones of the middle ear. In children this may impair the development of speech and language skills.

By contrast, the hearing loss caused by otosclerosis is so gradual that 90% of sufferers do not notice the deterioration and consequently do not seek treatment. The characteristic feature of this condition is a change in the texture of the bones in the middle ear, which become spongy. An excess of bone tissue builds up around the stapes, restricting its movement, and the auditory nerve may also be affected, possibly by toxic enzymes released into the cochlear. Although the cause is unknown, approximately 60% of cases are genetic in origin, and an individual whose parents both have the disorder stands a 50% chance of developing it. Sodium fluoride has been shown to stabilise the hearing loss, but most patients are treated by a surgical technique known as stapedectomy, which involves the removal of the affected stapes and its replacement by a prosthesis. This 10-minute video of the procedure is most impressive given the surgeon's restricted working area.

Sufferers from tinnitus hear sounds that are not produced by an external source. These may be of any type or pitch, intermittent or continuous, and they afflict up to 20% of the population. Exposure to loud noise is often to blame, but almost any disorder affecting the ear can be responsible, from an accumulation of wax to hypothyroidism, infection to hypertension. If symptoms remain after the treatment of any underlying cause, patients may be helped by a hearing aid if their hearing is impaired, or by special devices which use white noise to 'mask out' the problem sounds.

Exposure to loud noise may also lead to noise-induced hearing loss, which initially affects the higher frequencies (3,000 to 6,000 Hz), but is totally preventable. The sheer volume damages the hair cells of the inner ear by the magnitude of the pressure waves generated. Limited exposure produces a temporary hearing loss, but the result of chronic exposure to 85 decibels or more for eight hour periods is likely to be cell death and permanent deafness.

It might be salutary to conclude with a quick look at iatrogenic problems that affect the ear. Many drugs are known to be ototoxic, interfering with the patient's sense of balance, producing hearing loss, or both. Among them are certain antibiotics (especially aminoglycosides), chloroquine, anti-cancer drugs and diuretics. This page is on a personal site and evidence of ototoxicity is not firmly established for all the drugs mentioned, but as the author is a professor of pharmacology, his opinion should be well informed.

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