Ross & Wilson Anatomy and Physiology in Health and Illness (92 page)

Figure 8.25 
Structure of taste buds. A
. A section of a papilla.
B.
A taste bud – greatly magnified.
C.
Coloured scanning electron micrograph of a taste bud (centre) on the tongue.

Physiology of taste

Four fundamental sensations of taste have been described – sweet, sour, bitter and salt. This is probably an oversimplification because perception varies widely and many ‘tastes’ cannot be easily classified. It is thought that all taste buds are stimulated by all ‘tastes’. Taste is impaired when the mouth is dry, because substances can only be ‘tasted’ when in solution.

The sense of taste triggers salivation and the secretion of gastric juice (see
Ch. 12
). It also has a protective function, e.g. when foul-tasting food is eaten, reflex gagging or vomiting may be induced.

Disorders of the ear

Learning outcomes

After studying this section you should be able to:

compare and contrast the features of conductive and sensorineural hearing loss

describe the causes and effects of diseases of the ear.

Hearing loss

Hearing impairment can be classified in two main categories:
conductive
and
sensorineural
. Common causes are shown in
Box 8.1
. Hearing impairment can also be mixed when there is a combination of conductive and sensorineural hearing loss in one ear.

Box 8.1
Common causes of hearing loss

Conductive hearing impairment

This is due to reduced transmission of sound waves when an abnormality of the outer or middle ear impairs conduction of sound waves to the oval window.

Otosclerosis

This is a common cause of progressive conductive hearing loss in young adults that may affect one ear but is more commonly bilateral. It is usually hereditary, more common in females than males and often worsens during pregnancy. Abnormal bone develops around the footplate of the stapes, fusing it to the oval window, reducing the ability to transmit sound waves across the tympanic cavity.

Sensorineural hearing impairment

This is the result of a disorder of the nerves of the inner ear or the central nervous system, e.g. the cochlea, cochlear branch of the vestibular nerve or the auditory area of the cerebrum. The individual usually perceives noise but cannot discriminate between sounds.

Risk factors for congenital sensorineural hearing impairment include:

•
family history

•
exposure to intrauterine viruses, e.g. maternal rubella

•
acute hypoxia at birth.

Presbycusis

This form of hearing impairment commonly accompanies the ageing process and is therefore common in older people. Degenerative changes in the sensory cells of the spiral organ (of Corti) result in sensorineural hearing loss. Perception of high-frequency sound is impaired first and later low-frequency sound may also be affected. The individual develops difficulty in discrimination, e.g. following a conversation, especially in the presence of background noise.

Ear infections

External otitis

Infection by
Staphylococcus aureus
is the usual cause of localised inflammation (boils) in the auditory canal. When more generalised, the inflammation may be caused by prolonged exposure to moisture, bacteria or fungi or by an allergic reaction to, e.g., dandruff, soaps, hair sprays, hair dyes.

Acute otitis media

This is inflammation of the middle ear cavity, usually caused by upward spread of microbes from an upper respiratory tract infection via the auditory tube. It is very common in children and is accompanied by severe earache. Occasionally it spreads inwards from the outer ear through a perforation in the tympanic membrane.

Bacterial infection leads to the accumulation of pus and the outward bulging of the tympanic membrane. Sometimes the tympanic membrane ruptures and pus discharges from the middle ear (
otorrhoea
). The spread of infection may cause
mastoiditis
and
labyrinthitis
. As the petrous portion of the temporal bone is very thin, the infection may spread through the bone and cause meningitis (
p. 177
) and brain abscess.

Serous otitis media

Also known as
‘glue ear’
, or secretory otitis media, this is a collection of fluid (
effusion
) in the middle ear cavity. Causes include:

•
obstruction of the auditory tube by, for example, pharyngeal swelling, enlarged adenoids or tumour

•
barotrauma (usually caused by descent in an aeroplane when suffering from a cold)

•
untreated acute otitis media.

In adults the individual suffers from hearing loss and (usually painless) blockage of the ear, whereas young children show delay in speech development and behavioural disorders due to hearing impairment. Air already present in the middle ear cavity is absorbed and a negative pressure develops. At first there is retraction of the tympanic membrane, then fluid is drawn into the low-pressure cavity from surrounding blood vessels. Conductive hearing loss occurs and there may or may not be secondary infection. This is a common cause of hearing impairment in children.

Chronic otitis media

In this condition there is permanent perforation of the tympanic membrane following acute otitis media (especially when recurrent, persistent or untreated) and mechanical or blast injuries. During the healing process stratified epithelium from the outer ear sometimes grows into the middle ear, forming a
cholesteatoma
. This is a collection of desquamated epithelial cells and purulent material. Continued development of cholesteatoma may lead to:

•
destruction of the ossicles and conductive hearing loss

•
erosion of the roof of the middle ear and meningitis

•
spread of infection to the inner ear that may cause labyrinthitis (see below).

Ménière’s disease

In this condition there is accumulation of endolymph causing distension and increased pressure within the membranous labyrinth with destruction of the sensory cells in the ampulla and cochlea. It is usually unilateral at first but both ears may be affected later. The cause is not known. Ménière’s disease is associated with recurrent episodes of incapacitating dizziness (
vertigo
), nausea and vomiting, lasting for several hours. Periods of remission vary from days to months. During and between attacks there may be continuous ringing in the affected ear (
tinnitus
). Loss of hearing is experienced during episodes, and permanent hearing impairment may gradually develop over a period of years as the spiral organ is destroyed.