audiometry

Anatomy and physiology of the auditory system


Anatomy of the ear


This section deals with anatomy of the ear. This is one of the recurring themes in the audiometry units of competency that are aligned to the Diploma Course which is a qualification of the Health Training Package HLT02. The units of competency that include the theme of anatomy are:
HLTAU1A – Conduct screening hearing tests for children
HLTAU2A – Conduct screening hearing tests for adults
HLTAU3A – Conduct hearing assessments
HLTAU4A – Dispense hearing aids for adults

Anatomy of the ear is part of the required knowledge that underpins the development of competence. This knowledge will help you to understand the results of hearing assessments and how to communicate with clients about their results and options for rehabilitation. It will also help you in discussing results with other clinicians such as audiologists and doctors.

In your activities and assessments your teacher can reasonably ask you to:
• draw a diagram of the peripheral hearing mechanism with its parts labelled correctly
• use a diagram of the ear to explain its structure and function
• adjust your language and terminology to suit the person to whom you are giving the explanation
• draw a diagram of the inner ear with its parts labelled correctly
• explain the difference between the peripheral and central hearing mechanisms.


Introduction

The human ear is a complex structure of skin, cartilage, bone, mucous membrane, fluid and nerves. Its composition enables us to differentiate pitches of sound between 20Hz and 20,000Hz and to be able to hear a whisper to a roar. Every section of the ear contributes to our sense of hearing, although some more than others. Sound energy travels through the maze of our ear where its energy is transferred from acoustical energy to neural impulses to be interpreted by the brain into meaningful sound.

To understand how the hearing mechanism operates you must know the anatomy and physiology of the ear.

This topic provides essential knowledge for the practice of audiometry. When you have completed this topic you should be able to draw a diagram of the peripheral hearing mechanism with its parts labelled correctly and explain the structure and function of the ear.
At the end of the Learning guide there is a list of questions. You can use these questions to check that you have finished the topic.

This Learning guide will help you answer the questions but you cannot rely on it alone. You will need to read other material to fully understand the structure and function of the ear.


Suggested learning resources

There is not a required textbook for this topic but there are many that can help you. The following textbooks are all relevant and you may decide to refer to them as you study this topic. There are hundreds of internet sites that describe the anatomy of the ear. The sites that are listed below were accessed in September 2003. As internet sites and the information in them change, you may wish to perform your own search.

Relevant texts
TITLE The Speech Chain – The Physics and Biology of Spoken Language
AUTHOR Denes, P B, and Pinson, E N
PUB DATE 2nd edition, 1993
PUBLISHER W.H. Freeman and Company, New York, USA
ISBN 0716723441

TITLE Bases of Hearing Science
AUTHOR Durrant, J D, and Lovrinic, J H
PUB DATE 3rd edition, 1995
PUBLISHER Lippincott, Williams & Wilkins, Baltimore, USA
ISBN 0683027379

TITLE Clinical Audiology – An Introduction
AUTHOR Stach, B A
PUB DATE 1998
PUBLISHER Singular Publishing Group Inc, San Diego, USA
ISBN 156593346X

TITLE Handbook of Clinical Audiology
AUTHOR Katz, J, et al.
PUB DATE 4th Edition, 1994
PUBLISHER Williams & Wilkins, Baltimore, Md., USA
ISBN 0683006207

TITLE Audiology: the Fundamentals.
AUTHOR Bess, F H, and Humes, L E
PUB DATE 2nd Edition, 1995
PUBLISHER Williams & Wilkins, Baltimore. Md., USA
ISBN 0683006207

TITLE Introduction to Audiology
AUTHOR Martin, F N, and Clark, J G
PUB DATE 8th Edition, 2003
PUBLISHER Allyn & Bacon, Boston, USA
ISBN 0205366414

TITLE Audiologists Desk Reference Volume I - Diagnostic Audiology
Principles, Procedures and Protocols
AUTHOR Hall, J W, and Mueller, G H
PUB DATE 1997
PUBLISHER Singular Publishing Group, UK
ISBN 156593269

TITLE Basic Principles of Audiology Assessment
AUTHOR Hannely, M
PUB DATE 1991
PUBLISHER Prentice-Hall, USA
ISBN 0205135528

TITLE Audiology
AUTHOR Newby, Hayes
PUB DATE 1992
PUBLISHER Prentice-Hall, New York, USA
ISBN 013051921















The structure and function of the human auditory pathway


The main focus of this topic will be the peripheral auditory mechanism. It is comprised of the outer ear, the middle ear and the inner ear. It is essential that you understand the structure and function of the peripheral auditory mechanism so that you are able to draw a diagram of it and explain its structure and function. The way you explain the working of the ear will depend on whom you are explaining it to. You must be able to explain it using the correct terminology and also using common terms.
You need to have a general understanding of the central auditory mechanism. The central auditory mechanism involves the neural pathway and the brain.

The outer ear

The outer ear is the part of the ear closest to the outside world. It is the part of the peripheral hearing mechanism that gathers sounds from the environment. The sounds are channelled deep into the head.


The pinna

The most obvious part of our hearing system is perhaps of the least importance in the act of hearing. So why do we have this peculiar flap of skin on the outside of our head and what function does the pinna play in the hearing process?

The pinna, which is sometimes called the auricle, is a cartilaginous structure covered in skin, which is continuous with the face. The temporal muscles attach the pinna to the skull. Approximately 4% of humans can get these muscles to move, initiating ‘wiggling ears’! The size and structure of the pinna is individual, (in fact it is as unique in each of us as our fingerprints) and has a number of indentations and twists and turns. Important landmarks on the pinna include:
• the lobule or ear lobe which is at the base of each pinna
• the helix is the outer rim of the pinna
• the antitragus is found directly above the lobule
• the antihelix is an elevation running around the central part of the pinna
• the tragus is a small protrusion, often triangular in shape, pointing across the opening to the ear
• the concha is the middle portion of the pinna

ear.png

These particular landmarks in the anatomy of the pinna aid in accentuating high frequency sounds as they are funnelled into the ear. These frequencies (around 2000-4000Hz) are particularly important in our speech reception. The sounds are funnelled by the concha into the external auditory meatus or ear canal.


The external auditory meatus

The external auditory meatus, or ear canal, begins at the concha and forms a tunnel that extends inward and slightly upward in adults, downwards and at a more acute angle in children. It is lined with skin continuous with the pinna. The initial third of the external auditory meatus passes through cartilage and houses two sets of glands. These glands (sebaceous and ceruminous) produce substances that combine into a substance we commonly refer to as wax or cerumen. This section of the external auditory meatus also contains hair follicles. These hairs in conjunction with the wax help to keep foreign bodies and dirt from reaching the inner two thirds of the external auditory meatus. Many people think wax is dirty but as you can see it serves a very important function.

As our ears never stop growing it is not surprising to learn that the length of this tunnel is different in adults (2.5cm) and children (0.7 - 2.0cm).

The inner two thirds of the external auditory meatus pass through bone. There is a narrowing of the tunnel where the cartilage meets the bone, which is called the osseocartilaginous junction. This inner section of the external auditory meatus is filled with blood vessels and is very sensitive. Touching this part of the external auditory meatus can lead to excruciating pain.

The external auditory meatus serves as a tube resonator for frequencies between 2000 and 5500Hz and provides a protective mechanism for the middle ear section. Its shape, position, cerumen producing glands and hair follicles combine together to keep the ear clean and free of foreign objects, free from trauma and to keep the tympanic membrane, which is situated at the end of the external auditory meatus, at a constant temperature and humidity.

Middle ear

The middle ear is an air filled cavity that is about 2cm3 in volume. It is lined with mucous membrane and connected to the nasopharynx, ie, the back of the nose and throat, via the Eustachian tube. The middle ear is situated within the epitympanic recess of the mastoid process, lying beneath the meningial lining of the brain.

The mastoid process is part of the temporal bone but we usually refer to it as the mastoid bone. You can feel it as a bump when you rub the back of your ears. The mastoid bone has air pockets in it when it is healthy.

The middle ear is an impedance-matching transformer. It is called this because of its ability to transfer the sound energy from the air in the outer ear to the fluids in the inner ear, overcoming the loss of energy that usually occurs in such a transference. This is achieved directly by the pressure from the larger surface area of the tympanic membrane onto the smaller oval window in the inner ear. The tympanic membrane has a vibrating area seventeen times larger than that of the oval window. We see a pressure increase of 23 times, from the concentration of pressure applied by the tympanic membrane across the ossicular chain to the oval window. This approximates a 25dB increase in sound pressure. If the middle ear did not transfer energy in this way we could not hear as well.

The tympanic membrane

The tympanic membrane is important in the conduction of sound through to the middle and inner ears. What can its appearance tell us about the state of the middle ear?

The tympanic membrane or ear drum is a concave structure, ie it curves in slightly, consisting of three layers of tissue which connect the outer ear with the middle ear system. The tympanic membrane is semitransparent and sits at a 550 angle to the external auditory meatus.

The first layer of tissue is continuous with the skin in the external auditory meatus and is considered to be part of the outer ear. The second layer is a tough fibrous connective tissue, enabling the tympanic membrane to vibrate. The malleus (a part of the ossicular chain in the middle ear) is embedded in this section of the tympanic membrane. The malleus is attached through the centre of the upper section of the tympanic membrane and pulls it inward, giving the tympanic membrane its concave shape. The third layer of tissue is continuous with the mucous membrane in the middle ear cavity.

Landmarks can be seen on the tympanic membrane with the aid of an otoscope (also known as an auriscope). The visibility (or lack of) of these landmarks can indicate pathologies of the middle ear. It is therefore important to be able to distinguish a normal tympanic membrane from an abnormal one. A healthy tympanic membrane will be a pearly translucent colour and reflect back the light from the otoscope in a landmark known as the ‘cone of light’. This light reflex is viewed between three and six o’clock on the right tympanic membrane and on the left tympanic membrane between six and nine o’clock. If the light reflex is not clear or is not there, there may be a problem with the tympanic membrane or middle ear.

Another landmark important to see is the ‘handle of the malleus’, which is usually seen through the centre of the tympanic membrane as described above. The position of the malleus can vary due to the state of middle ear pressure. If it is difficult to view or appears to be sitting along the top of the tympanic membrane (retracted), the middle ear cavity may have abnormal pressure. Where the malleus attaches itself to the centre of the tympanic membrane is known as the greatest point of retraction or the ‘umbo’. The umbo’s position will vary if the malleus is retracting the tympanic membrane.

View of normal tympanic membrane (Right ear)
tympanicmembrane.png

The ‘annulus’ is a ring of tissue that holds the tympanic membrane in place at the end of the external auditory meatus.

You can see a photo of the TM here — with the annulus visible (barely):
http://www.ghorayeb.com/TympanicPerforation.html
If you can see the whole annulus you should be able to inspect all portions of the tympanic membrane.

The tympanic membrane is taut across two thirds of its surface area. This is known as the ‘pars tensa’. Above the malleus the tympanic membrane is looser and referred to as the ‘pars flaccida’ or attic. It is important to see both these sections to rule out the occurrence of a perforation. You should ask your client to see their doctor about any perforation, but if a perforation occurs in the attic area of the tympanic membrane, you should be extra careful. Attic perforations can result in squamous tumours.

The tympanic membrane is responsible for the transmission of sound waves through to the middle ear via the malleus. When sound waves hit the tympanic membrane it begins to vibrate causing the malleus to vibrate. The vibrations are then transmitted to the incus and the stapes.

The Ossicles

What are the parts of the ossicular chain and how do they aid in the transference of sound through the middle ear cavity?
ossicles.png

The peripheral hearing mechanism with an enlargement of the middle ear ossicles

The ossicular chain consists of the three smallest bones in the human body. It forms a chain across the middle ear cavity, joining the outer ear, via the tympanic membrane, to the inner ear through the oval window. These 3 bones collectively are referred to as the ossicles. Individually they are the malleus (hammer), incus (anvil) and stapes (stirrup). You know that the malleus is embedded in the tympanic membrane. Its head is connected to the incus, which in turn sits squarely on the head of the stapes. The base of the stapes is attached to the oval window, which is the beginning of the inner ear. The ossicles are held in place by a series of ligaments and muscles. The tensor tympani muscle is attached to the malleus and the stapedius muscle to the stapes.

Vibrations that are passed from the tympanic membrane are conducted along the ossicular chain to the oval window. The vibrations set up a rocking motion of the ossicles. The impedance matching system of the middle ear is assisted by the use of leverage to increase the force at the staples. The ‘lever action’ of the ossicles contribute approximately 2 dB ‘amplification’, whereas the advantageous difference in area between the tympanic membrane and oval window accounts for approximately 25 dB ‘amplification’.

This, with the ratio of the tympanic membrane to the oval window allows the sound to travel from air into liquid without significant loss of energy.

The Eustachian tube

You have probably had the experience of going down a mountain and your ears pop. This is the Eustachian tube performing its function.

While not contributing directly to the transference of sound across the middle ear, the Eustachian tube’s function is of vital importance to the health of the middle ear cavity. The Eustachian tube (ET) is part of the pressure equalisation system required to keep air pressure the same on both sides of the tympanic membrane. If the air pressure is not equal a feeling of ‘fullness’, pain and/or discomfort may occur in the ears and may lead to a temporary drop in hearing levels.

The ET is normally closed. It opens during swallowing, yawning or when excessive pressure is blown through the nose. In children the ET is shorter and in a more horizontal position. This positioning and size means it is harder for the ear to ‘pop’ and let any fluid build up drain away.

A difference in air pressure between the outside and the middle ear cavity most commonly occurs when there is a rapid change in altitude. For example, when we drive up or down a mountain or taking off or landing in an aeroplane. The air pressure at ground level is higher than at 10,000 feet, where a plane might fly. When our ears ‘pop’, air from the outside travels up or down the ET and equalises the pressure in the middle ear cavity.

The inner ear

Why do we need to understand of the complexities of the inner ear? It is made of many parts and performs two important roles. The inner ear system is responsible for transmitting signals to the brain about hearing and balance.

The oval window is the connecting link between the middle and inner ears. The oval window provides the opening to the vestibule, which is filled with fluid called perilymph. Connected to the vestibule we find the organs of balance - the semicircular canals.

The semicircular canals

The semicircular canals provide information regarding where we are in space, whether we are lying down or sitting up.

The semicircular canals arise from the utricle - a membranous sac within the vestibule. While the utricle is surrounded by perilymph, it is filled with another fluid called endolymph. The semicircular canals need to respond to movement from all directions in space, therefore these are three arranged at right angles. They are the superior semicircular canal, the posterior semicircular canal and the lateral semicircular canal. Each of these canals join at the utricle through an enlarged area called ampullae. Within these ampullae the crista (end organ for equilibrium) are placed.

With any change in angular body position, at least one semicircular canal is stimulated. The receptors within the semicircular canals tell us about motion and turning. Two other sense organs within the body contribute to our sense of balance. Our sense of sight and touch complement the semicircular canals, helping to keep us upright and steady! When there is a problem with the semicircular canal mechanisms a condition known as vertigo can occur. Vertigo can be a much more violent sensation than simple dizziness or light-headedness and can often be associated with hearing pathology.

The cochlea

The vestibule is also the beginning of the cochlea. The cochlea is a snail shaped shell that contains the organ of hearing. The cochlea turns on itself approximately two and-a-half times to form the snail shell. To discuss the cochlea and its components it is useful to ‘unroll’ the shell and investigate its canals.

The cochlea has three fluid-filled canals. Two of which (Scala Vestibuli and Scala Tympani) contain perilymph, while the middle canal (Scala Media) contains endolymph. Dividing these canals are membranes. Reissner’s membrane begins at the oval window, and separates the Scala Vestibuli from the Scala Media. The Scala Media is separated from the Scala Tympani by the basilar membrane. Resting on the basilar membrane is the organ of corti, which is often referred to as the ‘end organ of hearing’. A third membrane, the Tectorial membrane, is suspended above the organ of corti.

While the oval window is seen as the entrance to the cochlea, a second membrane provides an outlet, or pressure release, for the cochlear energy transference. This outlet is known as the round window. The round window is situated at the end of Scala Tympani. Both the round and oval windows are seated at the basal end of the cochlea. The Scalas Vestibuli and Tympani (and Media) come together at the point of the cochlea known as the apical end or helicotrema.

On the basilar membrane, within the organ of corti, thousands of tiny hair cells sit, arranged in three outer rows and one inner row. These hair cells, (approximately 16,000) are arranged in a frequency specific order, responding to incoming stimulus. At the basal end of the cochlea we will find the high frequency specific hair cells, which then scale down to the apical end where the low frequency hair cells reside. Connected the hair cells are nerve fibres or spiral ganglion, which join together to form the cochlear branch of the VIIIth cranial nerve, the auditory nerve.


Organ of corti

How then do these parts of the cochlea work together to transmit sound to the brain?

Sound coming in from the middle ear has been conducted along the ossicular chain to the oval window. The movement of the stapes pushing in the oval window, at the basal end of the cochlea, will displace the perilymph in the scala vestibuli. This movement will create a travelling wave in the cochlea, causing the round window to push outwards into the middle ear cavity. The two windows are thus ‘out of phase’.

The travelling wave causes the hair cells to move. The movement of the hair cells causes electrical potentials which are carried by the cochlear nerve, ultimately to the brain.

In the inner ear there is a small passage called the vestibular aqueduct. It is filled with endolymph fluid and consists of the endolymphatic duct and endolymphatic sac. Its function is to keep the cochlea and semicircular canals healthy by removing toxins and other debris.

Abnormalities of the vestibular aqueduct can result in hearing less. For information about the tonotopic response of the inner ear a good overview is given at: www.brainconnection.com/topics/?main=anat/auditory-phys


The central auditory pathway

The vestibulocochlear nerve (the VIIIth nerve)
We have discussed the peripheral hearing system and its mechanisms. Sound has been transferred, it now must be perceived. What is its path now?

The nerve that takes information from the ear to the brain is called the eighth nerve. It has different fibres that come from the semicircular canals and from the cochlea that join together in a bundle through the internal auditory meatus (IAM). The fibres from the cochlea form the auditory nerve and the fibres from the semicircular canals form the vestibular nerve. We use roman numerals when discussing the cranial nerves so the eighth nerve is written as the VIIIth nerve.

The VIIIth nerve is a sensory nerve, ie it carries the impulses of the sensation of hearing to the brain. It is one of the 12 cranial nerves that arise directly from the brain.
The auditory nerve must join with the vestibular portion of the VIIIth nerve and begin its journey through the internal auditory canal up into the brain. It is often referred to as the vestibulocochlear nerve.

Taking the ride with the VIIIth nerve will be the facial, or VIIth (seventh), nerve and the internal auditory artery. They will travel the 10mm of the internal auditory canal together.

Leaving the internal auditory canal and travelling a further 18mm, the VIIIth nerve will attach itself to the brain stem at the cerebellopontine angle, which is the junction of the cerebellum, medulla and pons. The midbrain, pons and medulla are collectively called the brain stem.

Separation of the auditory and vestibular nerves occur at this point. The auditory nerve itself will now divide, with one section descending to the dorsal cochlear nucleus, the other ascending to the ventral cochlear nucleus.

From the brain stem we move to the midbrain. Sound can be transferred here either ipsilaterally or contralaterally. The ipsilateral and contralateral superior olivary complexes are reached by the fibres of the ventral cochlear nucleus. Their ascending pathways reach along the lateral lemniscus on both sides of the brain. The contalateral inferior colliculus is reached by the fibres of the dorsal cochlear nucleus. The superior olivary complex, while being a relay station to the brain, also controls the reflex activity of the muscles in the middle ear. These muscles (the stapedial and tensor tympani) react to loud stimulus, giving both contralateral and ipsilateral responses. Their level of response and pattern of behaviour can be a useful diagnostic tool, in determining conductive, cochlear and retrocochlear pathology.

Auditory impulses then move onto their last subcortical relay station - the medial geniculate body, which is found within the thalamus. Most fibres arrive at this station from the ipsilateral inferior colliculus, some from the lateral lemniscus.

Fibres will now spread out and climb to the auditory cortex.


The primary auditory cortex

A large part of the cortex is involved in the interpretation of sound. The temporal, parietal and frontal areas of the cortex all interpret different aspects of sound. Latest thinking suggests the limbic system also plays a part in the emotional significance of sound. The main part of the brain involved with hearing is in the temporal part of the cerebrum and is referred to as the primary auditory cortex.

No matter how well the peripheral hearing mechanism functions, there is no perception until sound is interpreted in the brain. In some circumstances, the peripheral auditory mechanism functions as it should but the person still reacts like a hearing impaired person. This might be because the central auditory mechanism is not functioning well.

Much is involved in hearing. From the movement of the tympanic membrane to the cortex’s interpretation of that sound, about one third of a second of time has elapsed.

TOPIC QUESTIONS

If you can answer all of these questions with confidence (and without looking at your notes) then you should be ready for assessment events that will ask you to apply your knowledge about the anatomy of the ear to the practice of audiometry. Knowledge of anatomy will impact on your understanding of hearing assessments and how to communicate with clients about their results and options for rehabilitation. It will also help you in discussing results with other clinicians such as audiologists and doctors.

• What is anatomy?
• What is physiology?
• What are the peripheral and the central hearing mechanisms?
• What do the following abbreviations stand for - TM, EAM, IAM, ME?
• Can you draw a diagram of the peripheral hearing mechanism and label its parts?
• Can you explain the structure and function of the peripheral hearing mechanism to a hearing professional?
• Can you explain the structure and function of the peripheral hearing mechanism to a client?
• Can you draw a diagram of the tympanic membrane with the following landmarks labelled: cone of light, handle of malleus, umbo, annulus, pars tensa, pars flaccida?
• Can you describe the outer ear and its functions?
• What important role does the external auditory meatus play in maintaining the health of our ears?
• How does the external auditory meatus help in maintaining a healthy tympanic membrane?
• What is cerumen and where do you find it?
• Can you describe the middle ear and its functions?
• What is the function of the Eustachian tube?
• What are the ossicles?
• How is sound transferred from air borne to mechanical energy in the middle ear?
• Can you describe the structure and function of the inner ear?
• Can you draw the Organ of Corti and describe its function?
• What is the tonotopic response of the inner ear?
• What is the importance of the semicircular canals?
• Can you describe the passage of sound from the oval window to the IAM?
• What is the vestibular aqueduct and what is its function?