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Family
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Preview of Understanding Deafness and Tinnitus
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The structure of the ear |
The human ear is one of the more remarkable
parts of the human body, not only because of the beauty and unlikelihood
of its structure, but also because of its remarkable sensitivity to sounds.
From an anatomical point of view, the ear is conventionally and conveniently
divided into three parts: the outer, the middle and the inner ears.
The ear
is a remarkable part of the body sensing sound and balance. It
is divided into three parts: the outer, middle and inner ears. |
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The outer ear
The outer ear comprises the pinna (auricle), which
is made of a convoluted plate of flexible cartilage that extends as a
nearly closed tube one-third of the way down the ear canal. This outer
third, which is about eight millimetres (one-third of an inch) long,
has small hairs that point outwards to form a line of defence against
small animals creeping in. The roots of the hairs produce oils and these
mix with the secretions from nearby sweat-like glands to form the basis
of wax. The deep two-thirds of the ear canal (16 millimetres/two-thirds
of an inch long) has a bony wall lined with thin and rather fragile skin
which is devoid of glands. At the far end of the ear canal and stretched
across it is the eardrum (tympanic membrane), which forms the boundary
between the outer and middle ears.
The middle ear (tympanum)
The eardrum is a circle of thin skin about eight to
nine millimetres (one-third of an inch) in diameter. Despite its name,
it is not flat like the skin of a drum, but is slightly conical with
the curved sides sloping inwards. The eardrum has three layers.
The outer layer
In contact with the deep ear canal, the outer layer
is covered with a thin layer of skin.
The inner layer
The inner layer is in continuity with the lining of
the middle ear, and consists of rather flat cells that have the ability
to transform into the type of cells that line the nose and sinuses. Following
infection, chemical irritation – such as tobacco smoke – or
allergy, these cells alter and produce mucus which flows into the middle
ear.
The middle layer
The middle layer of the eardrum is very important and
consists of elastic fibres arranged both like the spokes of a wheel (radial
fibres) and in circles (circumferential fibres), so that this layer is
like a sprung trampoline net. The larger lower three-quarters of the
eardrum (the pars tensa) is tense and absorbs sound. The smaller upper
portion of the membrane is more floppy because the fibres of the middle
layer are not organised in regular patterns, and this region is called
the pars flaccida. The middle ear itself (the tympanum) lies deep to
the eardrum and is an air-filled space that holds three small bones (ossicles),
which connect the eardrum to the inner ear.
These bones are called the
hammer (malleus), anvil (incus) and stirrup (stapes) because of their resemblance
to these objects. The hammer has a handle and a head and the handle lies
within the layers of the eardrum. The head of the hammer sits in the upper
part of the middle-ear space called the attic (epitympanum) and is connected
by a joint – just the same as any other joint in the body – to the
rather bulky body of the anvil. From the anvil, a long strut (the long
process) descends back into the middle ear proper and is connected to
the head of the stirrup. The two arches (crura) of the stirrup join the
footplate, which sits in a small (3 mm x 2 mm) hole in the skull called
the oval window (fenestra ovalis). This is the opening into the fluid-filled
space of the inner ear. Just below the oval window is another small hole
into the inner ear called the round window (fenestra rotunda). A thin
membrane closes this and, when the footplate of the stirrup moves ‘in
and out’, the round window membrane moves ‘out and in’ because
the fluid in the inner ear transmits the pressure changes.
The hammer
and anvil are supported in the middle ear by several membranes and ligaments,
which minimise their weight, allow them to move easily and bring them
a blood supply. Unfortunately, this leaves only a small space for the
passage of air from the middle ear to the attic.
Running through the middle
ear is the facial nerve (nerve VII or the seventh nerve). This nerve
leaves the brain and has to pass through the skull on its way to supply
the muscles of facial expression, that is, muscles for frowning, winking,
smiling, scowling, and so on. The nerve lies in a thin bony tube and
runs horizontally from the front to the back of the middle ear just above
the oval window and stirrup, before it turns downwards to leave the base
of the skull. The nerve then turns forwards to reach the face. The facial
nerve is therefore relatively vulnerable in diseases of the middle ear
and, indeed, in middle-ear surgery itself. A facial palsy results in
one side of the face being paralysed, so that the face droops and fails
to move. Smiling results in a scowl and drinking in dribbling, and the
eye fails to close on blinking.
Running through the eardrum is the nerve
that carries taste from the front two-thirds of the tongue (the chorda
tympani nerve). This nerve is on its way to join the facial nerve in
the middle ear where it ‘hitch-hikes’ a lift back to the brain.
Finally,
there are two small muscles in the middle ear. The one at the front (tensor
tympani) is attached at the top of the handle of the hammer and tenses
up the eardrum when swallowing activates it. The function of this muscle
is not clear but it may be to make eating and swallowing a less noisy
event.
The muscle at the back of the middle ear (stapedius)
arises near the facial nerve, is supplied by it and attaches to the head
of the stirrup. It responds to loud sounds by contracting and stiffening
the chain of small bones, and possibly reduces transmission of prolonged
and potentially damaging, loud sounds to the inner ear.
The inner ear (labyrinth)
The inner ear is probably the most remarkably intricate
piece of the body. It makes hearing possible by converting sound into
electrical impulses that then travel along the hearing nerve (the acoustic
nerve or auditory nerve) to the brain. The inner ear also plays a major
role in balance. The balance portions of the inner ear (vestibular labyrinth)
can detect acceleration of the head in any direction whether in a straight
line (linear) or twisting and turning (angular). The electrical signals
that arise in response to head movement pass along the balance nerve
(vestibular nerve), which in due course joins with the hearing nerve
to form a single bundle (stato-acoustic, vestibulo-acoustic or eighth
nerve, nerve VIII) which then enters the brain.
The portion of the inner ear that actually hears is
the cochlea. This is a hollow coiled tube set in the very dense bone
called the bony labyrinth (part of the petrous [rock-like] temporal bone).
This tube is filled with fluid, which is much the same as general body
fluid (lymph) and that which surrounds the brain (cerebrospinal fluid – CSF).
This inner-ear fluid is called perilymph. Inside the perilymph is another
coiled triangular-shaped tube called the cochlear duct (scala media),
which contains the all-important ‘hair cells’ – these
convert sound into electricity. These hair cells are arranged in two
groups that follow the coils of the cochlear duct and spiral upwards
from base to apex. There is a single row of inner hair cells (IHCs),
which lie closer to the core of the cochlea (modiolus), and three or
four rows of outer hair cells (OHCs), which are further away. In a healthy
young human ear there are about 3,500 IHCs and about 12,000 OHCs. Each
hair cell has a cluster of small rigid hairs (stereocilia), which project
from the thicker upper surface of the cell into the special fluid that
fills the cochlear duct. This fluid is called endolymph and is remarkable
in that it has a strongly positive electrical charge associated with
it – about 80 millivolts – and is rich in potassium, a metallic
element.
The hair cells in their rows are grouped together with
their supporting cells in the organ of Corti. This is a small ridge that
sits on a thin, very flexible membrane called the basilar membrane. The
basilar membrane forms the floor of the triangular cochlear duct. The
sloping roof is another very thin membrane (Reissner’s membrane)
and the side wall is a thickened region rich in blood vessels (the stria
vascularis). This structure is responsible for maintaining the composition
of the rather unusual and very important endolymph.
Adjacent to the base
of the hair cells are the nerves that carry impulses to the brain (the
afferent nerves). At least 90 per cent of these nerves come from the
inner hair cells, despite their smaller number. Each inner hair cell
has about 10 nerve endings attached to it and there are, therefore, about
30,000 nerve fibres in the acoustic nerve.
The outer hair cells have nerves
attached to them but most are nerves coming from the brain (the efferent
nerves), whose function is described later.
The hearing nerves travel
inwards, along with the balance and facial nerves, through a canal in
the inner part of the skull (variously called the internal auditory meatus
[IAM], internal auditory canal [IAC] or porus acousticus) to reach the
brain stem. This part of the brain deals with lots of automatic functions
such as pulse, blood pressure, general alertness, balance, and so on.
About
half of the hearing nerves from each ear cross over to the other side
of the brain stem and then, on both sides, the nerves pass up the brain
stem through the mid-brain, eventually to reach ‘conscious-
ness’ in what is called the cortex of the brain. For hearing, this
conscious region is located in the temporal lobe portion of the brain,
which lies on each side of the head just above the ear.
Sound and how the ear works
Sound
Sound travels as small waves of pressure through the
air at a speed of about 343 metres per second (740 miles per hour). The
waves of sound are rather like ripples on the surface of a pond spreading
out after a stone has been thrown in. These waves have pitch (frequency)
and that is the number of crests that pass a point in a second. Pitch
is measured as ‘cycles per second’ (cps) which is now more
commonly written as hertz (Hz) after Heinrich Rudolf Hertz (1857–1894),
a pioneering scientist who worked on theories of light and electricity;
261 Hz is equivalent to middle C on the piano. One thousand cycles per
second (1,000 cps) is one kilohertz (1 kHz).
Sound waves also have intensity and, when the comparison
is made with ripples on the pond, this equates to the volume of the wave.
In real life, it is easier to measure the pressure of the wave rather
than its intensity and this pressure is measured in units called pascals
(Blaise Pascal [1623–1662] was, among other things, a mathematician
and physicist of genius working on statistics, probability and geometry,
and atmospheric pressure). One pascal is rather large for sound pressure
measurements so that micropascals (µPa), that is, one-millionth
of a pascal, tend to be used.
The quietest sound that the average healthy
18 year old, without previous ear problems and with normal eardrums,
can hear has a pressure of 20 micropascals (20 µPa). This level
forms the basis for measuring the pressure of other commonly heard sounds
in our environment.
The human
ear is very versatile and able to differentiate a wide variety
of sounds by intensity (volume) and frequency (pitch). |
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The range of pressures that the ear can hear is enormous.
The quietest, just detectable sound may be 20 µPa, but a jet engine
heard close by has a level of 20,000,000 µPa. For convenience these
levels are recorded as decibels (dB) after Alexander Graham Bell (1847–1922),
teacher of the deaf and inventor of the telephone, the audiometer and
gramophone, who derived a convenient way of expressing this huge range
of sound pressures.
Sound
travels as waves through the air rather like ripples on the surface
of a pond. These waves have pitch (number of wave crests) and
intensity (height or volume of the wave). |
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How hearing works
Sound waves are partly collected by the pinna, which
in humans has only a limited function. You will have noticed how dogs
prick up their ears in response to an interesting sound; this enables
them not only to hear better but also to localise the source of a sound
more accurately. In humans, the convolutions of the pinna do help a little
in both respects, but complete loss of the pinna only reduces the hearing
by a few decibels, although sound localisation is impaired.
The ear canal not only protects the eardrum from direct
damage, but also has a role in hearing. The resonance properties of a
tube that is open at one end and closed at the other result in sounds
being enhanced over a certain frequency range at the closed end of the
tube. The common example of resonance happens when you blow across the
top of an empty bottle to produce a note. If the bottle is then partly
filled with liquid, the note changes as the resonance properties change.
For the dimensions of the human ear, this enhancement is most marked
in the range 1,500–6,000 Hz which just happens to include most of
the frequencies used for speech and sorting out one complex sound from
another – for example, speech from background noise.
The large area
of the eardrum, which is not rigid but flexible and buckles slightly
to help absorb energy, now collects the sounds. The hammer, anvil and
stirrup transfer this sound energy to the relatively small area of the
oval window.
This system, which comprises the large flexible eardrum
linked by a chain of bones with a small lever action to the inner ear,
is really quite efficient in converting airborne sound waves into sound
waves in the fluids of the inner ear.
Normally, when sound hits the surface
of a liquid, 99.5 per cent or more is reflected. The operation of the
middle-ear mechanism results in about 50 per cent of the sound reaching
the eardrum being transferred to the inner ear.
As sound waves hit the
perilymph beneath the footplate, they create a wave that travels up and
around the cochlea. This travelling wave builds up to a maximum for each
particular pitch and then rapidly falls away to nothing. The location
of the peak of the wave varies at different pitches: for high-pitched
sounds, the wave peaks near the base of the cochlea, whereas for low-pitched
sounds this peak is near its apex.
As this pressure wave passes through
the cochlea, there is movement of the thin basilar membrane and, along
with it, the organ of Corti containing the hair cells. Overlying the
hair cells is a gelatinous membrane called the tectorial membrane. One
edge of this is attached to the bony core at the centre of the cochlea
(the modiolus); the other is loosely attached to the organ of Corti outside
the outermost outer hair cell. The tips of the hairs of the OHCs are
lightly embedded in the under-surface of the tectorial membrane whereas
the tips of the IHCs (which, as mentioned earlier, give rise to most
of the nerve fibres) do not reach the tectorial membrane and stand free
in the endolymph.
As the travelling wave reaches its peak, the OHCs near
this peak give a small, physical ‘kick’ to enhance the movement
of the basilar membrane. This internal amplifier causes the endolymph
to squirt towards the hairs of the IHCs. If the movement of fluid is
great enough, the hairs are deflected and very small channels open up
somewhere near the tips of the hairs. The potassium in the endolymph
can now flood down through these small channels, being propelled by the
very strong positive electrical charge of the endolymph into the bodies
of the IHCs. Here the influx of potassium alters the hair cell membrane
and small parcels of chemicals are released from the base of the hair
cell, causing the nearby nerves to become active and send pulsed signals
towards the brain.
The signals pass from one relay station to the next
in a regular fashion and have complex interactions in the brain stem.
About one-fifth of a second after detection, electrical signals reach
the auditory areas of the brain (auditory cortex of the temporal lobes)
and sounds are perceived.
An inner
hair cell found within the organ of Corti. When the endolymph
is disturbed by sound deflecting the basilar membrane, small
channels open up somewhere near the tips of the inner hair cells.
This allows the positive charge into the body of the inner hair
cell, stimulating the nerve cells at its base to send an impulse
to the brain. |
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At each step, the system is set to maximise sensitivity
to sound. There is the very well-balanced middle-ear mechanism, which
generates the pressure changes in the cochlea that result in the complex
travelling wave; this, in turn, depends on the delicate fine structure
of the cochlea. There is the highly unusual fluid named the endolymph
and a quite remarkable internal cochlear amplifier. Why so? Quite simply,
hearing is an important and efficient early warning system. Without good
hearing most mammals would find it difficult to survive.
The middle ear, eustachian tube and mastoid
In order to hear, creatures living on land need to
have an eardrum with air on both sides to collect airborne sound and
transfer it onwards to the inner ear. Reptiles, birds and mammals all
have the same system, although the numbers of bones are different in
the three groups and birds have only one strut-like bone between the
eardrum and inner ear.
In mammals, the middle ear is lined with a tissue
that is rather like the lining of the nose, with mucus-producing glands
and cells with a surface covered by moving flexible hairs or cilia – not
to be confused with the stereocilia of the hair cells, which are different
structures altogether and which are not mobile. The middle ear is, therefore,
an air-filled space, lined with living tissue capable of producing both
debris from dead surface cells and mucus from the glands. This creates
two problems: first, clearing the debris and mucus and, second, a more
subtle but very important problem. Oxygen is absorbed from the air in
the middle ear into the blood vessels running through its lining, in
much the same way that oxygen is absorbed in the lungs. Some carbon dioxide
is given off from those same blood vessels into the air in the middle
ear, but overall the effect is a drop in middle-ear pressure as more
oxygen is removed than carbon dioxide produced. With atmospheric pressure
outside the eardrum, something has to ‘give’ and the only thing
that can move is the eardrum. This would be pushed inwards by the external
pressure and the eardrum would then stop working normally. Eventually
the whole middle ear would collapse and a significant hearing loss would
develop.
The middle
ear is connected to the nasal cavity via the eustachian tube.
This tube allows the middle ear to maintain an equal pressure
with the outside and replenish the cells of the middle ear with
oxygen. |
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When working properly, the eustachian tube prevents
both these problems. This tube runs forwards and inwards from the front
wall of the middle ear to open into the back of the nasal cavity above
the soft palate (the nasopharynx). The end nearer the nose is soft and
flexible, and opens when you swallow or yawn. Although we do not know
precisely how this mechanism works, when the eustachian tube opens, enough
air enters the middle ear to replenish the oxygen that has been absorbed
and so keep the middle-ear pressures close to atmospheric. It has been
calculated that only one or two millilitres of air per ear per day – that
is, less than half a teaspoon of air – are necessary to maintain
proper ventilation of the middle ear, but without this the middle ear
fails to perform properly.
The eustachian tube is also the conduit along
which the cilia move the mucus produced in the middle ear to the back
of the nose, where it can be swallowed. This thin film of mucus, carrying
the debris produced in the middle ear, is moved along the floor of the
eustachian tube with air passing above it to reach the middle ear from
the nose. Thus, the two functions of ventilation and self-cleansing are
achieved when the system is working properly. Unfortunately, in humans,
the mechanism is rather fragile and often fails to work adequately, possibly
because of the shape of the skull that is needed to accommodate the large
brain.
There is also an extension of the air-filled spaces
of the middle ear backwards into the mastoid bone. You can feel this
as a rounded bump if you put your hand to the back of your ear. The mastoid
bone should be hollow, with the air-filled spaces broken up by small
and incomplete bony partitions rather like a honeycomb. The average mastoid
has an air volume of about 15–20 millilitres (three to four teaspoons)
and this helps to buffer pressure changes in the middle ear and reduce
adverse effects on the tympanic membrane. People with small mastoid air
spaces seem to be at a much greater risk of developing middle-ear and
mastoid disease. As yet, we do not fully understand whether it is middle-ear
and mastoid disease that cause the failure of the mastoid to develop,
or whether a small size reduces the pressure buffer and therefore causes
the development of disease. The probable answer is that it will be a
bit of both.
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KEY POINTS
- The ear comprises outer, middle and inner ears
- The outer ear collects sounds and enhances the
speech frequencies at the eardrum
- The middle ear conducts the sound energy to the
fluid of the inner ear
- The inner ear contains the sensory hair cells
that convert sound energy into electrical messages that pass to the
brain
- The outer hair cells act as an internal amplifier
and also provide clarity and discrimination
- The whole system is incredibly sensitive and therefore
very delicate
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