MIDDLE EAR

The middle ear transmits acoustic energy from the air-filled external auditory canal to the fluid-filled cochlea. It functions as an impedance-matching device inasmuch as it couples the low impedance of air to the high impedance of the fluid-filled cochlea. The impedance match is achieved in three ways. The first and most important factor is that the effective vibratory area of the tympanic membrane is approximately 17 to 20 times greater than the effective vibratory area of the stapes footplate (Fig. 129.3). A second factor involves the lever action of the ossicular chain. The arm of the long process of the incus is shorter, by a factor of 1.3, than the length of the manubrium and neck of the malleus. A third and minor factor is the shape of the tympanic membrane. The combined result of these three factors is a pressure gain of approximately 25 to 30 dB. The variance in published measurements of the transformer ratio is noteworthy. With the exception of studies of acoustic impedance of the ear, most data are from studies of human cadavers, with all of their shortcomings, or of animals, usually cats. In addition to its role in the transfer of power to the inner ear, the tympanic membrane protects the middle ear space from foreign material of the ear canal and maintains the air cushion that prevents insufflation of foreign material from the nasopharynx through the eustachian tube.

FIGURE 129.3. Schematic of ossicular chain and tympanic membrane shows the differences in area and vibratory pattern of the ossicles. (From Relkin EM. Introduction to the analysis of middle ear function. In: Jahn HF, Santos-Sacchi J, eds. Physiology of the ear. New York: Raven Press, 1988:103, with permission.)

The vibratory behavior of the ossicular chain is described in Fig. 129.3. The transformer action of the tympanic membrane and ossicular chain provides for relatively efficient transfer of power to the inner ear, and the fidelity of sound transmission across the middle ear is outstanding. In other words, distortion of signals (peak clipping, intermodulation distortion) does not occur in the middle ear even for input signals with sound levels greater than 130 dB sound pressure level (SPL).

The middle ear, including the tympanic membrane, ossicular chain with supporting ligaments, and middle ear space, can be viewed as a passive mechanical system with both mass and compliant elements and therefore resonant properties. This linear system is coupled to the cochlea, which contributes a large resistance. The result is a middle ear system that is highly damped and linear and has a wide frequency response. The input-output function or transfer function of the middle ear is shown in Fig. 129.4A. The ratio of the volume velocity of the stapes to sound pressure at the tympanic membrane increases in humans to approximately 800 to 900 Hz, which is the resonant frequency of the middle ear, and decreases at higher frequencies. Phase shift or time lag between movement of the tympanic membrane and the stapes generally increases with frequency (Fig. 129.4B). Although the middle ear is an impressive system in terms of frequency response, linearity, and transformer properties, considerably less than half of the power entering the middle ear actually reaches the cochlea. As shown in Fig. 129.5, the human middle ear is particularly inefficient at frequencies greater than 2 kHz, especially in comparison with the ears of cats and chinchillas. It is also important to recall that a 50% loss of power is a loss of only 3 dB.

FIGURE 129.4. A: Transfer function of the middle ear in chinchilla, cat, and human. Ordinate is the ratio of the volume velocity of the stapes to sound pressure at the tympanic membrane. B: Phase shift of stapes footplate in relation to tympanic membrane. (From Rosowski JH. The effects of external and middle ear filters on noise-induced hearing loss. J Acoust Soc Am 1991;90:124, with permission.)

FIGURE 129.5. Efficiency of the transfer of power through the middle ear. For all species shown, less than half the power that enters the middle ear actually reaches the cochlea. Energy loss is caused by absorption by the tympanic membrane, ossicular ligaments, and middle ear. (From Rosowski JH. The effects of external and middle ear filters on noise-induced hearing loss. J Acoust Soc Am 1991;90:124, with permission.)

The combined effects of the acoustic properties of the head, external ear, and middle ear, as well the input impedance of the cochlea, have a profound effect on auditory function. For example, these factors determine the shape of the audibility curve and therefore the frequency range of human hearing (Fig. 129.6). In other words, one reason humans do not detect and recognize sounds greater than approximately 20 kHz is that such high-frequency sounds are not transmitted efficiently through the middle ear to the cochlea. A second example of this sound transformation is shown in Fig. 129.7, in which the spectrum of a cannon measured in a sound field is compared with the spectrum of the cannon by the time it is transformed and shaped by the acoustic properties of the external ear, head, middle ear, and input impedance of the cochlea. Low-frequency energy is not transmitted to the cochlea, and the frequency region of greatest energy concentration is 3 to 4 kHz. Thus these acoustic properties are primarily responsible for the ability of intense low-frequency sounds (measured in a sound field) to produce high-frequency hearing losses and injuries in the basal region of the cochlea.


FIGURE 129.6. Comparison between overall outer and middle ear transfer function {circles, lower curve) and median threshold of audibility. (From Zwislocki JJ. The role of the external and middle ear in sound transmission. In: Turner D, ed. The nervous system, human communication and its disorders, vol 3. New York: Raven Press, 1975:45, with permission.)

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FRFOKENCY CHlJ


FIGURE 129.7. A comparison of the relative power spectra of impulses produced by a cannon and measured in a free field with the power that actually reaches the cochlea of a cat. (From Rosowski JH. The effects of external and middle ear filters on noise-induced hearing loss. J Acoust Soc Am 1991;90:124, with permission.)

Two striated muscles, the tensor tympani and the stapedius muscle, are located in the middle ear. The former attaches to the malleus and is innervated by the trigeminal nerve. The stapedius muscle attaches to the stapes and is innervated by the stapedial branch of the facial nerve. Both the stapedius and tensor tympani muscles are noteworthy not only because they are the smallest striated muscles in the body but also because they have a high innervation ratio, that is, nerve fibers per muscle fiber. Although there is no question that contraction of these muscles affects sound transmission through the middle ear, the details of the effect and the extent of the influence of the middle ear muscles are still not fully understood. A number of disparate functions have been attributed to the middle ear muscles.

One function of the middle ear muscles is to protect the cochlea from loud sounds (2). When sounds louder than approximately 80 dB SPL are presented monaurally or binaurally, consensual (bilateral) reflex contraction of the stapedius muscle occurs. This contraction increases the stiffness of the ossicular chain and tympanic membrane, attenuating sounds less than approximately 2 kHz. Although the tensor tympani contracts as part of a startle response, acoustic reflex data from human subjects with neurologic involvement of cranial nerves V and VII suggest that the tensor tympani does not normally respond to intense acoustic stimulation. Laboratory and field studies of noise-induced hearing loss have shown convincingly that the stapedial reflex protects the cochlea, particularly from low-frequency (less than 2 kHz) sounds in excess of 90 dB. Inasmuch as the latency of the acoustic reflex is greater than 10 ms, the cochlea may be unprotected from short-duration, unanticipated impulsive sounds.

The following functions have been attributed to the middle-ear muscles: providing strength and rigidity to the ossicular chain; contributing to the blood supply of the ossicular chain; reducing physiologic noise caused by chewing and vocalization; improving the signal-to-noise ratio for high-frequency signals, especially high-frequency speech sounds such as voiceless fricatives, by means of attenuating high-level, low-frequency background noise; functioning as an automatic gain control and increasing the dynamic range of the ear; and smoothing out irregularities in the middle-ear transfer function.

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