Diagrammatic view of the sound-conducting mechanism. Note: As sound energy displaces the stapes footplate inwards, the wave of pressure passing through the scala vestibule to the scala tympani causes the round window membrane to bulge outwards and induces vibration of the basilar membrane.
The ear is a specialized sense organ with two basic functions. These are the perception of sound (hearing) and the detection of changes in posture and spatial orientation (balance). Both senses rely on adaptation of primitive vibration detectors, and share many similarities.
PHYSIOLOGY OF HEARING
For descriptive purposes the auditory portion of the ear can conveniently be divided into two parts
1. The sound-conducting apparatus
consisting of the external ear, the tympanic membrane, the ossicular chain and the labyrinthine fluids (which conduct the airborne sound vibrations to the inner ear with a minimal loss of energy).
2. The electromechanical transducer
which transforms mechanical sound energy (vibrations) into nerve impulses which are then transmitted to the auditory cortex of the brain, where they are perceived as hearing. This can be likened to a microphone, which converts (transducer) mechanical vibration into an electrical impulse.
The ear is a biological microphone; like a microphone it takes sound vibrations and converts them into an electrical signal-a nervous impulse. However it is a biological microphone, deep in the head, bathed in body fluids. It has therefore developed a complex mechanism to conduct sound from the surrounding air through the outer and middle ear to the inner ear with minimal energy loss.
Sound travels as a wave consisting of an alternating compression and rarefaction of the molecules in the medium through which it is propagated or travels, e.g. air or water. In the region of compression the pressure of the air is slightly higher than normal, whereas in the region of rarefaction the pressure is slightly lower than normal. When the source of the sound stops vibrating, the propagation of these pressure variations stops, and the medium returns to its normal pressure.
Sound waves may be physically quantified in terms of frequency and amplitude. The brain's subjective perception of these parameters is referred to as pitch and loudness respectively. The exact mechanism by which the inner ear and its central connections perform this function is not completely understood, and is outside the scope of this book.
The sound-conducting apparatus
The sound-conducting apparatus consists of the external ear, the tympanic membrane, the ossicular chain and the labyrinthine fluids.
The external ear consists of the pinna (auricle), external auditory canal and the epithelial layer of the tympanic membrane.
Although the pinna is not as prominent in man as in many other animals, it nevertheless has several important functions. The shell-like shape of the conchal bowl serves to collect, amplify and funnel sound down the external auditory canal. It has an important function in localizing the origin of sounds.
The external auditory canal conveys sound to the tympanic membrane and affords a considerable degree of protection to the tympanic membrane and the delicate contents of the middle ear.
The middle ear sound-conducting mechanism consists of the tympanic membrane and ossicular chain, which absorb airborne sound waves and transmit them with little loss of energy to the relatively dense fluids of the inner ear. Since air and water present greatly differing resistance to the passage of sound energy (impedance), the middle ear conducting mechanism has evolved as a transformer, thereby overcoming this impedance mismatch.
The pressure of sound waves arriving at the tympanic membrane is increased 18-fold at the stapes footplate. This is achieved by a combination of an areal difference and a lever effect.
The acoustic energy collected by the relatively large area of the pinna is in turn transmitted down the ear canal to the tympanic membrane. Here it is absorbed and transmitted to the smaller area of the stapes footplate (the hydraulic effect). In addition, the ossicular chain acts as a lever with a mechanical advantage of 1.3 to 1. The net effect of these two mechanisms (14 x 1.3) is a gain of approximately 18 to l.
The amplitude of vibration at the stapes footplate is reduced compared to that of the tympanic membrane, while the force per unit area transmitted to the labyrinthine fluids is increased by a similar ratio.
The intratympanic muscles
The stapedius muscle provides a degree of protection to the delicate hair cells of the cochlea, by contracting in response to loud sound. By limiting the excursion of the stapes footplate at the oval window, the amount of acoustic energy reaching the inner ear fluids is reduced. In normal individuals this contraction occurs reflexly at sound pressure levels of 80-90 dB above threshold.
The tensor tympani muscle dampens vibration of the ossicular chain, thereby preventing an uncomfortable acoustic reverberation.
The eustachian tube
The pressure changes produced by sound waves are so small that the tympanic membrane must be free to vibrate efficiently.
In order to do so, the air pressure on both sides of the membrane must be the same; think of the reduction of hearing produced by an unequal pressure differential when you have a cold. The eustachian tube provides the mechanism which equalizes the air pressure in the middle ear to that of the surrounding atmosphere
The eustachian tube is the only route by which air can enter or exit the normal middle ear. The eustachian tube is responsible both for the maintainance of normal middle ear ventilation and for pressure regulation.
The air within the tympanic cavity is constantly and slowly absorbed by the mucosa lining the middle ear cleft. The middle ear transformer mechanism is only able to operate at peak efficiency if the air pressure is identical on both sides of the tympanic membrane. If this absorbed air is not replaced, over time a negative pressure will develop in the middle ear and the tympanic membrane will be pulled inwards (retracted). If this negative intratympanic pressure persists, eventually the middle ear cleft will become filled with a serous fluid.
The eustachian tube is normally closed to prevent the sound of normal nasal respiration and that of one's own voice from passing up the eustachian tube and into the middle ear. The eustachian tube normally opens only on swallowing or yawning. These brief but repetitive periods of eustachian tube opening are