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Hearing and balance: The outer and middle ear

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VOL: 103, ISSUE: 38, PAGE NO: 24

Marion Richardson, BD, DipN, Cert Ed, RNT, RN, is principal lecturer, University of Hertfordshire

This article is the first in a four-part series on hearing and balance, continuing our exploration of the senses. I…

 

This article is the first in a four-part series on hearing and balance, continuing our exploration of the senses. It discusses the outer and middle chambers of the ear, whose primary function is to transmit sound to the auditory receptors in the inner ear (Allan, 2005).

 

 

SOUND AND SOUND WAVES

 

The human ear is sensitive to a wide range of sounds - young children can detect sounds from 20-20,000Hz (Martini, 2006), although this reduces with age. The components of the ear can transform the characteristics of sounds into nerve signals that are transmitted to the brain, where they are interpreted.

 

 

Sound is a pressure disturbance produced by mechanical vibration of an object - the stronger and faster the vibrations, the more audible the sound (Allan, 2005). It is transmitted as a result of a ‘knock-on’ effect - the disturbed molecules of air around the sound source in turn disturb the adjacent molecules.

 

 

The sound wave arrives at the two eardrums at slightly different times, depending on the position of the source. This means the brain receives the information from each ear at slightly different times, helping it to determine where the sound came from.

 

 

Sound waves travel through air at approximately 1,235km/h (768mph) and the number of cycles (or waves) per second (hertz or Hz) is the ‘frequency’ of the sound. The brain interprets frequencies as pitch, so a high-pitched sound may have 15,000Hz or more, a low-pitched sound 100Hz or less (Martini, 2005). In old age there is a loss of sensitivity to high-frequency sounds.

 

 

Sound also requires energy and the greater the strength or intensity of the vibration, the louder the sound - this is measured in decibels (dB). Normal conversation is about 50dB while a noisy restaurant might measure 70dB. Severe hearing loss occurs with frequent or prolonged exposure to sounds of over 90dB (Marieb, 2006) and ear defenders should be worn (as a guide, amplified rock music can measure 120dB and more).

 

 

When sound waves strike an object their energy is a physical pressure. Sound waves produce higher intensity vibrations on the ear closest to them and again, the difference in impulses from the ears helps the brain to determine the source of the sound. Most people can locate a sound source fairly precisely.

 

 

In order to be heard, sound waves must reach the thin, flexible eardrum (tympanic membrane), which must vibrate in resonance with those waves.

 

 

THE OUTER EAR

 

The outer ear is basically a funnel which channels sounds to the eardrum or tympanic membrane. The pinna or auricle (Fig 1) helps to protect the external auditory canal or meatus.

 

 

The auditory canal is approximately 25mm long and ‘S’ shaped, ending at the eardrum. This shape helps to prevent the entry of debris and air currents, which might damage the eardrum. Ceruminous glands line the canal and produce ear wax (cerumen), which keeps the tissues moist, traps foreign bodies and repels insects. Excess production of wax can impair sound transmission and cause partial deafness, especially if it becomes stuck to the eardrum (Allan, 2005).

 

 

The eardrum separates the inner and middle ear. It is made of connective tissue and is shaped like a flattened cone with its apex protruding into the inner ear - this helps the funnelling of sound. Airborne sounds entering the external auditory canal, strike the eardrum and set it vibrating at the same frequency. The greater the intensity of the sound, the more the membrane is displaced.

 

 

THE MIDDLE EAR

 

The middle ear is an air-filled cavity within the temporal bone of the skull and contains the three smallest bones in the body - the malleus, incus and stapes (Fig 2), which are collectively known as the ossicles. The handle of the malleus is secured to the eardrum while the base of the stapes fits into the oval window - a membrane that separates the middle and inner ear.

 

 

The ossicles transmit sound energy from the eardrum to the oval window. When the eardrum vibrates, the malleus and incus are displaced and push and pull on the stapes, which acts as a plunger on the oval window. This lever system ensures the same force hitting the eardrum is transferred to the oval window. Because the eardrum is about 20 times bigger, the pressure exerted on the oval window is about 20 times that on the eardrum, so the sound is amplified. The vibration of the oval window sets the fluid in the inner ear into wave motion (see part 2).

 

 

Two tiny muscles attached to the ossicles contract when sound is loud (and potentially damaging to the inner ear). As a result, the ossicles are stiffened and function less well - this ‘damps down’ the transmission of sound to the inner ear.

 

 

A small tube (the pharyngotympanic or auditory tube) links the middle ear with the nasopharynx. Normally it is flattened and closed but yawning or swallowing opens it briefly to equalise pressure in the inner ear with external air pressure. This is important because the eardrum does not vibrate freely unless there is equal pressure on both of its surfaces. When the pressures are unequal, for example during air flight, the eardrum bulges, causing hearing problems and sometimes pain (Marieb, 2006).

 

 

HEARING LOSS

 

Otitis media or inflammation of the middle ear is the most common cause of hearing loss in children. The condition often follows a sore throat or may occur as a result of a food allergy.

 

 

Complete deafness is relatively rare. Some children are born deaf, in a significant proportion of cases this is because their mother contracted rubella (German measles) during the first trimester of pregnancy.

 

 

However, adults who are labelled ‘deaf’ are more likely to have a markedly reduced sense of hearing. This often develops gradually as part of the ageing process and is often well advanced before it is recognised (Allan, 2005).

 

 

Conduction deafness is the most common type of deafness and occurs if sound conduction through the outer and middle ears is impaired. It has a number of possible causes, including:

 

 

- Impacted earwax blocking the auditory canal;

 

 

- Perforated or ruptured eardrum;

 

 

- Middle ear infections (otitis media);

 

 

- Age-related damage (loss of flexibility in the eardrum, ossification of the round window or otosclerosis of the ossicles - when the ossicles become rigid, sound is conducted to the ear through vibrations of the skull bones, which is much less satisfactory) (Marieb, 2006; Martini, 2005).

 

 

Part 2 of this series will explain how sounds are transmitted through the inner ear to the brain and interpreted.

 

 

- This article has been double-blind peer-reviewed.

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