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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. 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. The eardrum is a circle of thin skin about eight to nine millimetres (one-third of an inch) in diameter.
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.
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. These bones are called the hammer (malleus), anvil (incus) and stirrup (stapes) because of their resemblance to these objects. 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. Running through the eardrum is the nerve that carries taste from the front two-thirds of the tongue (the chorda tympani nerve).
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. The hair cells in their rows are grouped together with their supporting cells in the organ of Corti.
Adjacent to the base of the hair cells are the nerves that carry impulses to the brain (the afferent nerves). 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.
Sound travels as small waves of pressure through the air at a speed of about 343 metres per second (740 miles per hour).
Sound waves also have intensity and, when the comparison is made with ripples on the pond, this equates to the volume of the wave. 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). Sound waves are partly collected by the pinna, which in humans has only a limited function.
The ear canal not only protects the eardrum from direct damage, but also has a role in hearing.
The large area of the eardrum, which is not rigid but flexible and buckles slightly to help absorb energy, now collects the sounds. 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.
As sound waves hit the peri­lymph beneath the footplate, they create a wave that travels up and around the cochlea. 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.
The signals pass from one relay station to the next in a regular fashion and have complex inter­actions in the brain stem.
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. 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 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. There is also an extension of the air-filled spaces of the middle ear backwards into the mastoid bone.
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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. Despite its name, it is not flat like the skin of a drum, but is slightly conical with the curved sides sloping inwards. The larger lower three-quarters of the eardrum (the pars tensa) is tense and absorbs sound. Unfortunately, this leaves only a small space for the passage of air from the middle ear to the attic. 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 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.


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. 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. This is a hollow coiled tube set in the very dense bone called the bony labyrinth (part of the petrous [rock­like] temporal bone).
This is a small ridge that sits on a thin, very flexible membrane called the basilar membrane. At least 90 per cent of these nerves come from the inner hair cells, despite their smaller number. This part of the brain deals with lots of automatic functions such as pulse, blood pressure, general alertness, balance, and so on. 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. The waves of sound are rather like ripples on the surface of a pond spreading out after a stone has been thrown in. 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).
This level forms the basis for measuring the pressure of other commonly heard sounds in our environment. 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. 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 hammer, anvil and stirrup transfer this sound energy to the relatively small area of the oval window.
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. This travelling wave builds up to a maximum for each particular pitch and then rapidly falls away to nothing.
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. 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. 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. 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 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). 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.
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 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 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 function of this muscle is not clear but it may be to make eating and swallowing a less noisy event. 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). 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.
These waves have pitch (frequency) and that is the number of crests that pass a point in a second. One pascal is rather large for sound pressure measurements so that micropascals (µPa), that is, one-millionth of a pascal, tend to be used.
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.
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 common example of resonance happens when you blow across the top of an empty bottle to produce a note. 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.
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.


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.
There is the highly unusual fluid named the endolymph and a quite remarkable internal cochlear amplifier. This creates two problems: first, clearing the debris and mucus and, second, a more subtle but very important problem. Thus, the two functions of ventilation and self-cleansing are achieved when the system is working properly. The mastoid bone should be hollow, with the air-filled spaces broken up by small and incomplete bony partitions rather like a honeycomb. 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. 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 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 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). If the bottle is then partly filled with liquid, the note changes as the resonance properties change.
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. 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.
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.
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. 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. 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 two arches (crura) of the stirrup join the footplate, which sits in a small (3 mm × 2 mm) hole in the skull called the oval window (fenestra ovalis). The facial nerve is therefore relatively vulnerable in diseases of the middle ear and, indeed, in middle-ear surgery itself. This structure is responsible for maintaining the composition of the rather unusual and very important endolymph. 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. 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. People with small mastoid air spaces seem to be at a much greater risk of developing middle-ear and mastoid disease. A facial palsy results in one side of the face being paralysed, so that the face droops and fails to move. These hair cells are arranged in two groups that follow the coils of the cochlear duct and spiral upwards from base to apex. 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. Just below the oval window is another small hole into the inner ear called the round window (fenestra rotunda).
Smiling results in a scowl and drinking in dribbling, and the eye fails to close on blinking. 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. 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.
Sounds of low frequency are said to have low pitch whereas sounds of high frequency are said to have high pitch. 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.



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