Human Hearing Mechanisms Physiology of the

The human ear is an organ of marvelous sensitivity, complexity, and robustness. For a person with acute hearing, the range of audible sound spans ten octaves, from 20 Hz to 20,000 Hz. The wavelengths corresponding to these frequencies vary from 1.7 centimeters (5/8 inch) to 17 meters (57 feet), a ratio of one thousand. The quietest sound audible to the average human ear, about zero dB at 1000 Hz, corresponds to an acoustic pressure of 20 x 10-6 N/m2 or Pa. Since atmospheric pressure is about 101,000 Pa (14.7 lb/sq in), it is clear that the ear is responding to extraordinarily small changes in pressure. Even at the threshold of pain, 120 dB, the acoustic pressures are still only about 20 Pa.

The excursion of the ear drum at the threshold of hearing is around 10-9 m(4 x 10-7 in) (Kinsler et al., 1982). Most atoms have dimensions of 1 to 2 angstroms (10-10 meters) so the ear drum travels a distance of less than 10 atomic diameters at the threshold of hearing. Were our ears only slightly more sensitive, we would hear the constant background noise due to Brownian movement, molecules set into motion by thermal excitation. Indeed, it is thermal motion of the hair cells in the cochlea that limits hearing acuity. In very quiet environments the flow of blood in the vessels near the eardrum is plainly audible as a disquieting shushing sound.

The anatomy of the ear, shown in Fig. 3.1, is organized into three parts, termed outer, middle, and inner. The outer and middle ear are air filled, whereas the inner ear is fluid filled. The outer part includes the pinna, the fleshy flap of skin that we normally think of as the ear, and a tube known as the meatus or auditory canal that conducts sound waves to the tympanic membrane or ear drum, separating the outer and middle ear sections. The pinna gathers the sound signals and assists in the localization of the height of a sound source. The 2.7 centimeter (one-inch) long auditory canal acts like a broadband quarter-wavelength tube resonator, whose lowest natural frequency is about 2700 Hz. This helps determine the range of frequencies where the ear is most sensitive—a more or less 3 kHz wide peak centered at about 3400 Hz. The auditory canal resonance increases the sound level at the ear drum around this frequency by about 10 dB above the level at the canal entrance. With the

Figure 3.1 A Schematic Representation of the Ear (Flanagan, 1972)

Outer Middle Inner Ear Ear Ear

Figure 3.1 A Schematic Representation of the Ear (Flanagan, 1972)

Outer Middle Inner Ear Ear Ear

Physiology Hearing Diagrams
THE COCHLEA SHOWN STRETCHED OUT (HISHLY SIMPLIFIED,)

diffraction provided by the pinna and the head, there can be as much as a 15 to 20 dB gain at certain frequencies at the ear drum relative to the free-field level.

The middle ear is an air-filled cavity about 2 cu cm in volume (about the same as a sugar cube), which contains the mechanisms for the transfer of the motion of the eardrum to the cochlea in the inner ear. The ear drum is a thin conical membrane stretched across the end of the auditory canal. It is not a flat drum head, as might be inferred from its name, but rather a tent-like sheath with its peak pointing inward. Near its center, the eardrum is attached to the malleus bone, which is connected in turn to two other small bones. These three, the malleus (hammer), incus (anvil), and stapes (stirrup) act as a mechanical linkage, which couples the eardrum to the fluid-filled cochlea. The stapes resembles a stirrup with its base pressed up against the oval window, a membrane that covers the entrance to the cochlea. Because of the area ratio of the eardrum to that of the oval window (about 20 to 1) and the lever action of the ossicles, which produces another factor of 1.5:1, the middle ear acts as an impedance matching transformer, converting the low-pressure, high-displacement motion of the ear drum into a high-pressure, low-displacement motion of the fluid of the cochlea. Atmospheric pressure in the middle ear is equalized behind the eardrum by venting this area to the throat through the eustation tube, which opens when we yawn or swallow.

The motion transfer in the middle ear is not linear but depends on amplitude. An aural reflex protects the inner ear from loud noises by tightening the muscles holding the stapes to reduce its excursion at high amplitudes, just as the eye protects itself from bright light by contracting the pupil. The contraction is involuntary in both cases and seldom is noticed by the individual. Pain is produced at high noise levels when the muscles strain to protect nerve cells. Unfortunately the aural reflex is not completely effective. There is a reaction time of about 0.5 msec so it cannot block sounds having a rapid onset, such as gunshots and impact-generated noise. A second reason is that the muscles cannot contract indefinitely. Under a sustained bombardment of loud noise they grow tired and allow more energy to pass.

The inner ear, shown in Fig. 3.2, contains mechanisms that sense balance and acceleration as well as hearing. Housed in the hard bone of the skull, the inner ear contains five

Figure 3.2 Structure of the Inner Ear (Hudspeth and Markin, 1994)

SEMICIRCULAR UTRICULUS

Figure 3.2 Structure of the Inner Ear (Hudspeth and Markin, 1994)

SEMICIRCULAR UTRICULUS

Hidden And Seek

separate receptor organs, each sensitive to a specific type of acceleration, as well as the cochlea, which detects the loudness and frequency content of airborne sound waves. The sacculus and utriculus include about 15,000 and 30,000 hair cells in planar sheets that react to vertical and horizontal linear accelerations respectively. These organs have the capability of encoding a unique signal for an acceleration in any given direction within a plane. Three semicircular canals are arranged to sense the orthogonal directions of angular acceleration. Each consists of a fluid-filled tube interrupted by a diaphragm containing about 7000 hair cells. They provide information on the orientation and acceleration of the human head. The bilateral symmetry of the ears gives us not only backup capability but extra information for the decomposition of motions in any direction.

The cochlea is a fluid-filled tube containing the hair cell transducers that sense sound. It is rolled up two and one-half turns like a snail and if we unroll the tube and straighten it out, we would find a narrow cavity 3.5 cm long, about the size and shape of a golf tee scaled down by two-thirds. At its beginning, called the basal end, it is about 0.9 cm in diameter and at the apical end it is about 0.3 cm in diameter. It has two thin membranes running down it near its middle. The thicker membrane is called the basilar membrane and divides the cochlea more or less in half, separating the upper gallery (scala vestibuli) from the lower gallery (scala tympani). Along the membrane lies the auditory nerve, which conducts the electrochemical impulses and snakes through a thin sliver of bone called the bony ridge to the brain.

The entrance to the cochlea, in the upper gallery, is the oval window at the foot of the stapes. At the upper end of the cochlea near its apex there is a small passageway connecting the upper and lower galleries called the helicotrema. At the distal end of the lower gallery near the oval window is another membrane, the round window. It acts like the back door to the cochlea, a pressure release surface for fluid impulses traveling along its length and back into the middle ear. The two membranes, the oval window and the round window, seal in the fluid of the cochlea. Otherwise the rest of the cochlea is completely surrounded and protected by bone.

Figure 3.2b shows a cross section of one of the spirals of the cochlea. The upper gallery is separated from a pie-shaped section called the middle gallery (scala media) by Reissner's membrane. Within this segment and attached to the basilar membrane is the organ of Corti, which includes some 16,000 small groups of hair cells (stereocilia), arranged in four rows, acting as motion transducers to convert fluid and basilar membrane movement into electrical impulses (Hudspeth and Markin, 1994). The stereocilia are cylindrical rods arranged in a row in order of increasing height and move back and forth as a group in response to pressure waves in the endolymphatic fluid. The hair cells are relatively stiff and only move about a diameter. Through this movement they encode the magnitude and the time passage of the wave as an electrochemical potential, which is sent along to the brain.

Each stereocilia forms a bond between its end and an area on the adjacent higher neighbor much like a spring pulling on a swinging gate (see Fig. 3.2d). When a gate is opened a nerve impulse is triggered and sent to the brain. If the bundle of stereocilia is displaced in the positive direction, toward the high side of the bundle, a greater relative displacement occurs between each stalk and more gates are opened. A negative displacement, towards the short side of the bundle reduces the tension on the biomechanical spring and closes gates. Orthogonal motion results in no change in tension and no change in the signal. The amplitude of the response to sound waves is detected by the number of gate openings and closings and thus the number of impulses sent up the auditory nerve.

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