The Function of the Inner Ear
Sound waves entering the auditory canal are acoustical in nature. The actuation of the eardrum by this airborne wave sets the ossicles in motion, which in turn, move the oval window of the cochlea. The sound is now in mechanical form. As the oval window moves, it pushes against the fluid of the inner ear. Because this fluid is essentially incompressible, something has to give if the oval window is to move at all. This “give” takes place at the round window (see Fig. 7). When the oval window moves inward, the round window moves outward. As we shall see, this sets up orderly disturbances in the fluid, which through the hair cells, are translated to electrical currents and transmitted to the brain through neural impulses. The sound energy falling on the outer ear goes through the following stages: acoustical, mechanical, electrical, neural. In this section, we will look into what happens in the cochlea, or the inner ear.
In man, the cochlea is embedded deep within solid temporal bone. It is spiral shaped, but for our analytical purposes it is expedient to unroll the spiral to its full 32 millimeter length, or about 1¼ inches. A simplified cross section of the cochlea is shown in Fig. 6. Reissner’s membrane defines the chamber known as the scala vestibuli. The basilar membrane defines another chamber known as the scala tympani. Between these two major chambers is the organ of Corti which contains the hair cells responsible for the actual transduction of the mechanical movement of the membranes to electrical signals. A simplified but useful view of the cochlea is to imagine the two scala separated by what is known as the cochlear partition.
Figure 6. A cross-section of the human cochlea showing the scala vestibula and the scala tympani with the complex cochlear partition between them. The hair cells in the organ of Corti translate vibrations of the basilar membrane into neural impulses that are sent to the brain via the auditory nerve
It is with some hesitation that Fig. 7 is presented, but it may help fix in our minds just what takes place in the inner ear. The ossicles are replaced by an oversimplified lever arm, suggesting their function. As the stapes force the liquid inward, the round window bulges out into the middle ear space. The membranes constituting the cochlear partition separate the two chambers for the entire length of the cochlea, except for an opening (called the helicotrema) at the apex. Sound entering the cochlear fluid at the oval window sets up traveling waves on the cochlear partition. These traveling waves create localized peak intensities along the cochlear partition, the location of the peak being determined by the frequency of the sound. A complex wave would create numerous peaks with the fundamental and harmonic frequencies. High-frequency peaks occur near the oval and round windows, low-frequency peaks near the apex. In this over-simplified way, we see how the cochlea can be considered an analyzer of sound.
Figure 7. This diagram explains the relationship between the outer, middle, and inner sections of the human ear. The inner ear analyzes the sound. A simple sine wave exciting the ear results in a standing wave effect, causing the basilar membrane to exhibit a peak vibration at a certain place on the membrane. A complex wave would result in a peak for the fundamental and each harmonic
Sensitive hair cells, distributed throughout the length of the cochlear partition, respond to the vibratory peaks set up on the membranes. These hair cells are extensions of the nerve fibers of the auditory nerve.
At this point, we should consider the action of the membranes of the cochlear partition. It has been found that the movement of the basilar membrane dominates, hence our concentration on it. Models have been used extensively in the effort to understand the working of the cochlea. Fig. 8 portrays one of the early models. The case of the model is transparent plastic. The fluid used to simulate the cochlear fluid is a glycerin/water mixture carefully controlled for viscosity. Sandwiched between the two scala is the cochlear partition, a thin metal plate that gives support to the latex basilar membrane. When sound is injected through the membrane of the oval window, the peaks of vibration set up on the basilar membrane may be studied.
Figure 8. A model of the cochlea made of plastic and filled with a glycerin/water mixture to give the proper viscosity. When sound is introduced into the fluid through the oval window, vibratory peaks on the latex basilar membrane appear that can be studied Such models have been used for many years to study cochlea reaction (after Tonndorf)
An important aspect of such model study (as it is with studies centered on actual cochleas) is determining the shape of the vibratory peaks set up by the traveling waves. The sharpness of these peaks influences the preciseness of the analysis of the frequency’s dimension. A sharp peak excites fewer hair cells along the organ of Corti than would a broader peak.
Unit 3 demonstrates that it is possible to distinguish between a tone of 1000 Hz and one of 1003 Hz. This is a difference of 0.3 percent and calls for an extremely narrow peak response with very steep sides. The shape of the peaks observed on the basilar membrane appears far too wide to account for such discriminatory performance by the ear. Inserting microelectrodes into single nerve fibers and counting neural pulses as the frequency changes has revealed very sharp “tuning curves.” Recent work on the basilar membrane has revealed ever sharper peaks as experimental methods are improved. This is an active field of study, and the full picture of how the ear achieves its fine analysis is not yet fully known.