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The Sense of Hearing
This chapter describes the mechanisms whereby the ear receives sound waves, discriminates their frequencies, and transmits auditory information into the central nervous system, where its meaning is deciphered.
Tympanic Membrane and the Ossicular System
Conduction of Sound From the Tympanic Membrane to the Cochlea
Figure 53-1 shows the tympanic membrane (commonly called the eardrum ) and the ossicles , which conduct sound from the tympanic membrane through the middle ear to the cochlea (the inner ear). Attached to the tympanic membrane is the handle of the malleus . The malleus is bound to the incus by minute ligaments, so whenever the malleus moves, the incus moves with it. The opposite end of the incus articulates with the stem of the stapes , and the faceplate of the stapes lies against the membranous labyrinth of the cochlea in the opening of the oval window .
The tip end of the handle of the malleus is attached to the center of the tympanic membrane, and this point of attachment is constantly pulled by the tensor tympani muscle , which keeps the tympanic membrane tensed. This tension allows sound vibrations on any portion of the tympanic membrane to be transmitted to the ossicles, which would not occur if the membrane were lax.
The ossicles of the middle ear are suspended by ligaments in such a way that the combined malleus and incus act as a single lever, having its fulcrum approximately at the border of the tympanic membrane.
The articulation of the incus with the stapes causes the stapes to (1) push forward on the oval window and on the cochlear fluid on the other side of window every time the tympanic membrane moves inward; and (2) pull backward on the fluid every time the malleus moves outward.
“Impedance Matching” by the Ossicular System
The amplitude of movement of the stapes faceplate with each sound vibration is only three-fourths as much as the amplitude of the handle of the malleus. Therefore, the ossicular lever system does not increase the movement distance of the stapes, as is commonly believed. Instead, the system actually reduces the distance but increases the force of movement about 1.3 times. In addition, the surface area of the tympanic membrane is about 55 square millimeters, whereas the surface area of the stapes averages 3.2 square millimeters. This 17-fold difference times the 1.3-fold ratio of the lever system causes about 22 times as much total force to be exerted on the fluid of the cochlea as is exerted by the sound waves against the tympanic membrane. Because fluid has far greater inertia than air does, increased amounts of force are necessary to cause vibration in the fluid. Therefore, the tympanic membrane and ossicular system provide impedance matching between the sound waves in air and the sound vibrations in the fluid of the cochlea. The impedance matching is about 50% to 75% of perfect for sound frequencies between 300 and 3000 cycles/sec, which allows utilization of most of the energy in the incoming sound waves.
In the absence of the ossicular system and tympanic membrane, sound waves can still travel directly through the air of the middle ear and enter the cochlea at the oval window. However, the sensitivity for hearing is then 15 to 20 decibels less than for ossicular transmission—equivalent to a decrease from a medium to a barely perceptible voice level.
Attenuation of Sound by Contraction of the Tensor Tympani and Stapedius Muscles
When loud sounds are transmitted through the ossicular system and from there into the central nervous system, a reflex occurs after a latent period of only 40 to 80 milliseconds to cause contraction of the stapedius muscle and, to a lesser extent, the tensor tympani muscle . The tensor tympani muscle pulls the handle of the malleus inward while the stapedius muscle pulls the stapes outward. These two forces oppose each other and thereby cause the entire ossicular system to develop increased rigidity, thus greatly reducing the ossicular conduction of low-frequency sound, mainly frequencies below 1000 cycles/sec.
This attenuation reflex can reduce the intensity of lower frequency sound transmission by 30 to 40 decibels, which is about the same difference as that between a loud voice and a whisper. The function of this mechanism is believed to be twofold—to protect the cochlea from damaging vibrations caused by excessively loud sound and to mask low-frequency sounds in loud environments. Masking usually removes a major share of the background noise and allows a person to concentrate on sounds above 1000 cycles/sec, where most of the pertinent information in voice communication is transmitted.
Another function of the tensor tympani and stapedius muscles is to decrease a person’s hearing sensitivity to his or her own speech. This effect is activated by collateral nerve signals transmitted to these muscles at the same time that the brain activates the voice mechanism.
Transmission of Sound Through Bone
Because the inner ear, the cochlea , is embedded in a bony cavity in the temporal bone, called the bony labyrinth , vibrations of the entire skull can cause fluid vibrations in the cochlea. Therefore, under appropriate conditions, a tuning fork or an electronic vibrator placed on any bony protuberance of the skull, but especially on the mastoid process near the ear, causes the person to hear the sound. However, the energy available even in loud sound in the air is not sufficient to cause hearing via bone conduction unless a special electromechanical sound-amplifying device is applied to the bone.
Cochlea
Functional Anatomy of the Cochlea
The cochlea is a system of coiled tubes, shown in Figure 53-1 and in cross section in Figure 53-2 . It consists of three tubes coiled side by side: (1) the scala vestibuli; (2) the scala media; and (3) the scala tympani. The scala vestibuli and scala media are separated from each other by Reissner’s membrane (also called the vestibular membrane ), shown in Figure 53-2B ; the scala tympani and scala media are separated from each other by the basilar membrane . On the surface of the basilar membrane lies the organ of Corti , which contains a series of electromechanically sensitive cells, the hair cells . They are the receptive end organs that generate nerve impulses in response to sound vibrations.
Figure 53-3 diagrams the functional parts of the uncoiled cochlea for conduction of sound vibrations. First, note that Reissner’s membrane is missing from this figure. This membrane is so thin and so easily moved that it does not obstruct the passage of sound vibrations from the scala vestibuli into the scala media. Therefore, as far as fluid conduction of sound is concerned, the scala vestibuli and scala media are considered to be a single chamber. As discussed later, Reissner’s membrane maintains a special kind of fluid in the scala media that is required for normal function of the sound-receptive hair cells.
Sound vibrations enter the scala vestibuli from the faceplate of the stapes at the oval window. The faceplate covers this window and is connected with the window’s edges by a loose annular ligament so that it can move inward and outward with the sound vibrations. Inward movement causes the fluid to move forward in the scala vestibuli and scala media, and outward movement causes the fluid to move backward.
Basilar Membrane and Resonance in the Cochlea
The basilar membrane is a fibrous membrane that separates the scala media from the scala tympani. It contains 20,000 to 30,000 basilar fibers that project from the bony center of the cochlea, the modiolus , toward the outer wall. These fibers are stiff, elastic, reedlike structures that are fixed at their basal ends in the central bony structure of the cochlea (the modiolus) but are not fixed at their distal ends, except that the distal ends are embedded in the loose basilar membrane. Because the fibers are stiff and free at one end, they can vibrate like the reeds of a harmonica.
The lengths of the basilar fibers increase progressively, beginning at the oval window and going from the base of the cochlea to the apex, increasing from a length of about 0.04 millimeter near the oval and round windows to 0.5 millimeter at the tip of the cochlea (the “ helicotrema ”), a 12-fold increase in length.
The diameters of the fibers, however, decrease from the oval window to the helicotrema, so their overall stiffness decreases more than 100-fold. As a result, the stiff, short fibers near the oval window of the cochlea vibrate best at a very high frequency, whereas the long, limber fibers near the tip of the cochlea vibrate best at a low frequency.
Thus, high-frequency resonance of the basilar membrane occurs near the base, where the sound waves enter the cochlea through the oval window. However, low-frequency resonance occurs near the helicotrema, mainly because of the less stiff fibers but also because of increased “loading” with extra masses of fluid that must vibrate along the cochlear tubules.
Transmission of sound waves in the cochlea—“traveling wave”
When the foot of the stapes moves inward against the oval window, the round window must bulge outward because the cochlea is bounded on all sides by bony walls. The initial effect of a sound wave entering at the oval window is to cause the basilar membrane at the base of the cochlea to bend in the direction of the round window. However, the elastic tension that is built up in the basilar fibers as they bend toward the round window initiates a fluid wave that “travels” along the basilar membrane toward the helicotrema. Figure 53-4A shows movement of a high-frequency wave down the basilar membrane, Figure 53-4B shows a medium-frequency wave, and Figure 53-4C shows a very low-frequency wave. Movement of the wave along the basilar membrane is comparable to the movement of a pressure wave along the arterial walls, discussed in Chapter 15 ; it is also comparable to a wave that travels along the surface of a pond.
Vibration Patterns of the Basilar Membrane for Different Sound Frequencies
Note in Figure 53-4 the different patterns of transmission for sound waves of different frequencies. Each wave is relatively weak at the outset but becomes strong when it reaches the portion of the basilar membrane that has a natural resonant frequency equal to the respective sound frequency. At this point, the basilar membrane can vibrate back and forth with such ease that the energy in the wave is dissipated. Consequently, the wave dies at this point and fails to travel the remaining distance along the basilar membrane. Thus, a high-frequency sound wave travels only a short distance along the basilar membrane before it reaches its resonant point and dies, a medium-frequency sound wave travels about halfway and then dies, and a very low-frequency sound wave travels the entire distance along the membrane.
Another feature of the traveling wave is that it travels fast along the initial portion of the basilar membrane but becomes progressively slower as it goes farther into the cochlea. The cause of this difference is the high coefficient of elasticity of the basilar fibers near the oval window and a progressively decreasing coefficient farther along the membrane. This rapid initial transmission of the wave allows the high-frequency sounds to travel far enough into the cochlea to spread out and separate from one another on the basilar membrane. Without this rapid initial transmission, all the high-frequency waves would be bunched together within the first millimeter or so of the basilar membrane, and their frequencies could not be discriminated.
Vibration Amplitude Pattern of the Basilar Membrane
The dashed curves of Figure 53-5A show the position of a sound wave on the basilar membrane when the stapes is (a) all the way inward, (b) has moved back to the neutral point, (c) is all the way outward, and (d) has moved back again to the neutral point but is moving inward. The shaded area around these different waves shows the extent of vibration of the basilar membrane during a complete vibratory cycle. This is the amplitude pattern of vibration of the basilar membrane for this particular sound frequency.
Figure 53-5B shows the amplitude patterns of vibration for different frequencies, demonstrating that the maximum amplitude for sound at 8000 cycles/sec occurs near the base of the cochlea, whereas that for frequencies less than 200 cycles/sec is all the way at the tip of the basilar membrane near the helicotrema, the minute opening whereby the scala tympani and scala vestibuli communicate ( Figure 53-2 ).
The principal method whereby sound frequencies are discriminated from one another is based on the “place” of maximum stimulation of the nerve fibers from the organ of Corti lying on the basilar membrane, as explained in the next section.
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