Chapter Fourteen. The Hearing Mechanism. 1. Introduction.

Transcription

1 Chapter Fourteen The Hearing Mechanism 1. Introduction. 2. Hearing. 3. The Ear. 4. The External Ear. 5. The Inner Ear. 6. Frequency Discrimination. 7. The Organ of Corti. 8. Tests and Exrecises. 9. References. 1. Introduction. This chapter highlights the importance to humans of the sense of hearing and details the mechanism by which it works, including the role played by the different parts of the ear and the brain. Hearing is one of the most important of the human senses, needed for protection from danger, communication and enjoyment of surroundings. Noise is defined as unwanted or damaging sound - that is, sound which has an adverse effect on health or safety or which interferes with what people are trying to do. The ability to hear can be affected by noise. To understand how noise causes these effects we first need to know about the mechanism of hearing and its structure, including the role played by the different parts of the ear and the brain. This chapter describes the three main parts of the ear - the external, middle and inner ear, and their role in hearing sounds. More detail is provided about the organ of Corti in the inner ear, where the sound is transformed into nerve impulses which are sent to the brain. This is the structure which is damaged by excessive noise. The Almond Tree Effect 1

2 2. Hearing. Hearing is one of the most important of the human senses. It is essential for the location of sounds that may warn of danger, the enjoyment of pleasant sounds such as music and the natural environment and, most importantly for humans, the development of speech and language for communication. 2.1 The Importance of Hearing. Figure x: The ear. Helen Keller, the well-known campaigner for the blind once said: "...after a lifetime in silence and darkness that to be deaf is a greater affliction than to be blind...i have imagination, the power of association, the sense of touch, smell and taste, and I never feel blind, but how can I replace the loss of hearing?" (Gasaway, 1997) 2.2 Hearing System Is Easily Overloaded. The hearing mechanism is our only sense which never shuts off and which receives input from all directions, giving us a continual connection with and warning of our surroundings. Unlike vision, which can be shut off by simply closing our eyes, hearing can only be partially protected from continual high intensity noise by complex central nervous system (CNS) mechanisms. Our ears just weren't designed to be able to cope with some of the noises which have been introduced to our environment since the industrial revolution. The result for many people is partial deafness. A continual overloading of the system by noise also hinders the more complex function of comprehension - the understanding of what it is we are hearing. Continual attempts by the auditory parts of the CNS to produce ordered thought processes in the presence of excessive noise can lead to stress. 2.3 Sensitivity of the Ear. The human ear is capable of detecting sounds over a wide range of frequencies and sound intensities. A young, healthy ear can respond over a frequency range of 20 Hz to Hz, as shown below. The minimum sound pressure level perceptible to the ear at a particular frequency is called the threshold of hearing at that frequency. This is Figure x: Threshold of hearing. The Almond Tree Effect 2

3 different for each individual, even between people with 'normal' hearing capacities. It is also age related, with a progressive loss in sensitivity at the high frequencies occurring with increasing age. This is called presbyacusis. To understand how noise affects our hearing ability, we first need to understand how the hearing mechanism works, and this in turn requires an understanding of its structure. 3. The Ear The ear is situated in the temporal bone of the skull. The diagram below shows its three main parts: the external ear - which collects the airborne sound waves the middle ear - which transmits the sound waves as a vibration the inner ear - which changes the sound waves to electrical impulses and sends them via the auditory nerve to the brain. Figure x: The external ear 4. The External Ear. The external ear consists of the outer ear (also known as the auricle or pinna), the ear canal (or external auditory meatus) and the eardrum (or tympanic membrane). 4.1 Outer Ear. The outer ear, the visible part, collects the sound waves and, due to one being located on each side of the head, assists in the detection of the direction of the sound source. This is done by modifying the frequency spectrum of the impinging sound. 4.2 Ear Canal. The sound waves then travel through the ear canal, which is a tube about 25 to 30 mm in length and slightly angled in the upwards and backwards directions. The outer third consists of cartilage covered by a thin lining of skin in which there are hairs and wax glands. These normally allow the ear canal to clean itself. However, sometimes there can be a build-up of wax which may block the ear canal and reduce hearing ability. The inner two thirds of the canal is bony and covered by skin without hairs or glands. The main function of the ear canal is to protect Figure x: The ear structure. the delicate eardrum from mechanical damage, The Hearing Mechanism 3

4 but it also has an effect on the magnitude of the sound transmitted. Because it is basically a tube open at one end and closed at the other, it acts like an open-ended organ pipe, resonating at frequencies around 2 to 4 khz and giving an increased sound pressure level at these frequencies of up to 20 db. This partly explains why noise damages our hearing most at these frequencies. 4.3 Eardrum. The eardrum is situated at the end of the ear canal. It is a very thin cone shaped membrane, about 7 mm in diameter, positioned at an angle of 55 to the canal floor. Perforation or rupture of the eardrum may cause a hearing loss of 5 to 50 db. (This may be caused by air blasts, sharp objects or disease.) The eardrum vibrates in synchrony with the sound pressure changes of the sound waves which have travelled down the Figure x: The ear structure. canal. Attached to the inner side of the eardrum is the first of the three middle ear bones.4.4 The Middle Ear 4.4 Ossicles. The middle ear is a box-like cavity about the size of the nail of the index finger. Suspended in the upper part of this cavity are three small bones, collectively called the ossicles. These are: the hammer (malleus) attached to the eardrum; the anvil (incus); and the stirrup (stapes) attached to the oval window of the inner ear. This chain of bones conducts the vibration of the eardrum to the oval window. On the way the vibration is amplified about 25 times due to the mechanical advantage of the lever action of the ossicles and the difference in area of the eardrum and oval window. This amplification is needed to compensate for the approximate 40 db transmission loss that would occur if airborne sound was introduced directly to the fluid filled cochlea. The transfer of energy of this system is most efficient in the 1 to 4 khz frequency range. Interruption of the ossicle chain (by disease or injury) can result in a hearing loss of 60 db. 4.5 Tensor Tympani & Stapedius Muscles. The ossicles are suspended by ligaments and two small muscles. The tensor tympani muscle is attached to the hammer and the stapedius muscle to the stirrup. In the presence of intense sound these muscles contract and introduce Figure x: The middle ear. The Almond Tree Effect 4

5 Figure x: The middle ear. resistance to the transmission of vibrations below 2000 Hz. Sound intensity is reduced by about 10 to 30 db, partially protecting the inner ear from damage. However, the ability of the muscles to stay contracted is limited. Hence this is not effective protection against long term intense noise exposure. Also, as it takes approximately 25 milliseconds for the muscles to respond, impulsive noises such as hammering, which have rise times less than this, will not be attenuated. 4.6 Eustachian Tube. Also in the middle ear is the opening to the eustachian tube, which connects with the back of the nose. Its purpose is to maintain equal air pressure on both sides of the eardrum so this can vibrate freely. The eustachian tube is normally closed, but opens with swallowing or yawning - you have probably noticed this when you have changed altitude in a plane or going up or down a mountain. Inflammation or infection of the nose or throat may cause blockages of the eustachian tube with a fall in pressure or even infection of the middle ear leading to hearing loss. Figure x: The inner ear.` The Hearing Mechanism 5

6 5. The Inner Ear. This is the most important part of the ear for hearing and contains the structures which are damaged by excessive noise. It consists of three cavities in the petrous part of the temporal bone, containing: the vestibule - next to the oval window; the three semi-circular canals - which are the body's sensor for balance and orientation; the cochlea - a bony spiral organ, about 35 mm long, shaped like a snail shell of 2 1/2 turns. (Cochlea means "snail" in ancient Greek.) This is where mechanical vibrations transmitted from the middle ear are transformed into nerve impulses to be perceived by the brain as sound. Each division contains an incompressible fluid called perilymph. 5.1 Cochlea. The cochlea is itself divided lengthwise into three chambers: the scala vestibuli - which has the oval window at its base; the scala tympani - which ends in the round window (a simple membrane which acts as a pressure release); and the scala media - which contains the true hearing sensory structure - the organ of Corti. The dividing membranes are called the basilar membrane and Reissner's membrane. The organ of Corti, which contains the sensory hearing cells, is supported on the basilar membrane in the scala media which is filled with endolymph fluid. The scala vestibuli and scala tympani are connected at the apex of the cochlea by an opening called the helicotrema, and are filled with perilymph fluid. The scala media is at a slightly higher electrical potential than the other two chambers (+80 mv). This potential difference is important for the correct functioning of the cochlea. 5.2 Organ of Corti. Vibration of the stirrup and oval window sends a travelling wave through the perilymph fluid in Figure x: The Cochlea. Figure x: Cross-Section of the Cochlea. The Almond Tree Effect 6

7 the scala vestibuli and scala tympani, causing both the round window and the basilar membrane to move. The amount of displacement of the membrane depends on the amplitude of the wave at a particular point. This movement is detected by the sensory hair cells of the organ of Corti, which rests on the basilar membrane. The organ of Corti is a complicated system of cells extending along the basilar membrane. There are about 30,000 hair cells placed in four rows - one inner row and three outer rows, supported by other cells and tissues. The hair cells transform the movement into nerve impulses. 5.3 Auditory Nerve. Nerve fibres carry the impulses from the hair cells. They pass through the spiral ganglia, to join together to become the auditory nerve. This connects to the cochlea nuclei in the brain stem and hence to the higher auditory centres in the temporal lobe of the brain. Here the messages, received and analysed by the ear, are interpreted. 6. Frequency Discrimination. 6.1 Basilar Membrane. The ear is able to detect different frequencies in sound due to the characteristics of the basilar membrane. The basilar membrane is one of the most important structures in the cochlea. It has the mechanical properties of elasticity (springiness), damping (friction) and mass (inertia), the first two of which change along its length. At the basal end (near the oval window), the membrane is narrow and rigid, while at the apex, it is wider and floppier. The elasticity interacts with the inertia of the fluids in the cochlea to support a wave-like motion travelling from the basal end to the apex. At each point along the membrane the ratio of stiffness to mass varies, and this determines an upper frequency above which a wave will not travel. At the basal end, where the stiffness is high, this cut off frequency is also high, and most frequencies in the auditory range will travel as a wave. Towards the apex, the stiffness decreases, and so the cutoff frequency is less and high frequency waves will not travel in this region. Figure x: Position of the peak vibration of the basilar membrane for sounds of different frequency The Hearing Mechanism 7

8 Hence the basilar membrane acts to sort the incoming sound waves into different frequency components - high near the oval window and low near the apex. This frequency discrimination is essential to good hearing. 7. The Organ of Corti. 7.1 Basilar Membrane. The organ of Corti contains the sensory hair cells which are embedded in supporting cells attached to the basilar membrane. There are two types of hair cells - inner and outer. The inner hair cells, of which there are about 10000, form a single row along the inside spiral of the cochlea. The outer hair cells, of which there are about 20000, are in three parallel rows towards the outside of the spiral. 7.2 Stereocilia. Each hair cell has a cluster of hair-like structures, called stereocilia, on its upper surface. The stereocilia are arranged in "w" or "v" formations. Stereocilia are rigid and composed of actin (a protein commonly found in the muscles of the body) enclosed in a plasma membrane. They vary in length depending on their position along the basilar membrane from 2.1 µm (micrometre) at the base to 4.7 µm at the apex and are about 0.3 µm in diameter. The hairs pivot or bend at their base, but the shaft remains rigid and may break if pushed beyond a stress point. Above the hair cells is the tectorial membrane, which is attached to the lining of the cochlea wall and may be attached to the outer hair cell stereocilia. When a travelling wave displaces the basilar membrane, a shearing movement of the stereocilia occurs. These function much like a microphone - small back and forth movements of the cilia change the flow of electric current through the hair cells. (Remember the small electric potential in the scala media?) 7.3 Function of the Inner and Outer Hair Cells. The inner hair cells are the primary sensory cells. They directly connect to individual nerve fibres of the auditory nerve. The sound-induced Figure x: Section of basilar membrane with organ of corti Figure x: Stereocilia of outer hair cells. The Almond Tree Effect 8

9 voltage changes within the inner hair cells lead to electrical activity in the nerve, which is sent to the brain. The outer hair cells appear to serve a different, mechanical purpose, only recently discovered. Experiments have shown that they are likely to lengthen and shorten in sympathy with the electrical signals passed by their hairs. We saw earlier how the wave travelling along the basilar membrane stops once it reaches a particular place, depending on the frequency. As the wave slows to a halt, its amplitude increases (as the power flow remains constant), resulting in a sharp peak just before the wave falls away abruptly to nothing. However, experimental observations of the amplitude increase show that it is actually greater than expected on the basis of constant power flow alone. This has led to the theory that the outer hair cells are actually injecting more mechanical energy into the system. This process is known as the cochlear amplifier. The precise mechanism by which the outer hair cells do this is still uncertain: it may be by the lengthening and shortening activity or by twitching of their hairs. In summary, the process is a feedback loop illustrated below: acoustic energy enters the cochlea via the motion of the stirrup; the stirrup and oval window vibration induces a wave which travels towards the apex on the basilar membrane; as each frequency component approaches the cut-off point along the membrane it slows down; at the same time, the outer hair cells sense the basilar membrane motion and inject energy with the correct timing to enhance the vibrations (like timing pushes of a child on a swing); the wave vibrations reach a peak and then fall away. 7.4 Damage to Hair Cells. When a person acquires a hearing loss during their lifetime, eg by loud noise, certain drugs and the ageing process, it is most usually because the outer hair cells have been damaged resulting in a greatly reduced amplitude of vibration of the basilar membrane near the cut-off point. The Figure x: Schematic diagram showing relative movement of the basilar membrane and stereocilia. Figure x: Feedback loop. The Hearing Mechanism 9

10 result is an inability to hear softer sounds (but not louder ones, since the outer hair cells can only inject a limited amount of energy and have little influence on large amplitude vibrations). 7.5 Nerve Fibres. The nerve fibres attached to both the inner and outer hair cells are either afferent (to the brain) or efferent (from the brain). More than 90% of the afferent fibres are connected to the inner hair cells - about 20 to each cell. The more numerous outer hair cells only connect with about 9% of the afferent fibres. This means that there is only a weak connection with the central nervous system, probably passing control information, rather than information about sounds. Conversely, most efferent fibres terminate on outer hair cells, with fewer attached to inner hair cells. The role of the efferent fibre system is still under research, but it probably oversees the operation of the inner ear. 8. Tests and Exercises. 9. References. Callender, J.H. (1974). Time-Saver Standards for Architectural Design Data. McGraw-Hill Book Company. Evans, M. (1980). Housing, Climate and Comfort. The Architectural Press, London. Givoni, B. (1976). Man, Climate And Architecture. Second Edition. Applied Science Publishers Ltd., London. Koenigsberger, O.H., Ingersoll, T.G., Mayhew, A. and Szokolay, S.V. (1974). Manual of Tropical Housing And Building, Part I, Climatic Design. Longman, London. Markus, T.A. and Morris, E.N. (1980). Buildings, Climate and Energy. Pitman International, London. National Universities Commission (1977). Standards Guide for Universities. National Universities Commission, Lagos. Olgyay, V. (1963). Design With Climate - Bioclimatic Approach To Architectural Regionalism. Princeton University Press, Princeton, New Jersey. United Nations (1971). Design of Low Cost Housing and Community Facilities, Volume I, Climate and House Design. Department of Economic and Social Affairs, New York. The Almond Tree Effect 10