Sound, Animal Hearing, Human Hearing
Hearing is the ability to collect, process and interpret sound. Sound vibrations travel through air, water, or solids in the form of pressure waves. When a sound wave hits a flexible object such as the eardrum it causes it to vibrate, which begins the process of hearing. The process of hearing involves the conversion of acoustical energy (sound waves) to mechanical, hydraulic, chemical, and finally electrical energy where the signal reaches the brain and is interpreted.
Outer ear and hearing
The pinna of the outer ear gathers sound waves from the environment and transmits them through the external auditory canal and eardrum to the middle ear. In the process of collecting sounds, the outer ear also modifies the sound. The external ear, or pinna, in combination with the head, can slightly amplify (increase) or attenuate (decrease) certain frequencies. This amplification or attenuation is due to individual differences in the dimensions and contours of the head and pinna.
A second source of sound modification is the external auditory canal. The tube-like canal is able to amplify specific frequencies in the 3,000 Hz region. An analogy would be an opened, half filled soda bottle. When you blow into the bottle there is a sound, the frequency of which depends on the size of the bottle and the amount of space in the bottle. If you empty some of the fluid and blow into the bottle again the frequency of the sound will change. Since the size of the human ear canal is consistent the specific frequency it amplifies is also constant. Sound waves travel through the ear canal until they strike the tympanic membrane (the eardrum). Together, the head, pinna and external auditory canal amplify sounds in the 2,000 to 4,000 Hz range by 10-15 dB. This boost is needed since the process of transmitting sound from the outer ear to the middle ear requires added energy.
Middle ear and hearing
The tympanic membrane or eardrum separates the outer ear from the middle ear. It vibrates in response to
pressure from sound waves traveling through the external auditory canal. The initial vibration causes the membrane to be displaced (pushed) inward by an amount equal to the intensity of the sound, so that loud sounds push the eardrum more than soft sounds. Once the eardrum is pushed inwards, the pressure within the middle ear causes the eardrum to be pulled outward, setting up a back-and-forth motion which begins the conversion and transmission of acoustical energy (sound waves) to mechanical energy (bone movement).
The small connected bones of the middle ear (the ossicles—malleus, incus, and stapes) move as a unit, in a type of lever-like action. The first bone, the malleus, is attached to the tympanic membrane, and the back-and-forth motion of the tympanic membrane sets all three bones in motion. The final result of this bone movement is pressure of the footplate of the last (smallest) bone (the stapes), on the oval window. The oval window is one of two small membranes which allow communication between the middle ear and the inner ear. The lever-like action of the bones amplifies the mechanical energy from the eardrum to the oval window. The energy in the middle ear is also amplified due to the difference in surface size between the tympanic membrane and the oval window, which has been calculated at 14 to 1. The large head of a thumbtack collects and applies pressure and focuses it on the pin point, driving it into the surface. The eardrum is like the head of the thumb tack and the oval window is the pin point. The overall amplification in the middle ear is approximately 25 dB. The conversion from mechanical energy (bone movement) to hydraulic energy (fluid movement) requires added energy since sound does not travel easily through fluids. We know this from trying to hear under water.
Inner ear and hearing
The inner ear is the site where hydraulic energy (fluid movement) is converted to chemical energy (hair cell activity) and finally to electrical energy (nerve transmission). Once the signal is transmitted to the nerve, it will travel up to the brain to be interpreted.
The bone movements in the middle ear cause movement of the stapes footplate in the membrane of the oval window. This pressure causes fluid waves (hydraulic energy) throughout the entire two and a half turns of the cochlea. The design of the cochlea allows for very little fluid movement, therefore the pressure at the oval window is released by the interaction between the oval and round windows. When the oval window is pushed forward by the stapes footplate, the round window bulges outward and vice versa. This action permits the fluid wave motion in the cochlea. The cochlea is the fluid filled, snail shell-shaped coiled organ in the inner ear which contains the actual sense receptors for hearing. The fluid motion causes a corresponding, but not equal, wave-like motion of the basilar membrane. Internally, the cochlea consists of three fluid filled chambers: the scola vestibuli, the scola tympani, and the scala media. The basilar membrane is located in the scala media portion of the cochlea, and separates the scala media from the scala tympani. The basilar membrane holds the key structure for hearing, the organ of Corti. The physical characteristics of the basilar membrane are important, as is its wave-like movement, from base (originating point) to apex (tip). The basilar wave motion slowly builds to a peak and then quickly dies out. The distance the wave takes to reach the peak depends on the speed at which the oval window is moved. For example, high frequency sounds have short wavelengths, causing rapid movements of the oval window, and peak movements on the basilar membrane near the base of the cochlea. In contrast, low frequency sounds have long wavelengths, cause slower movements of the oval window, and peak movements of the basilar membrane near the apex. The place of the peak membrane movements corresponds to the frequency of the sound. Sounds can be "mapped" (or located) on the basilar membrane; high frequency sounds are near the base, middle frequency sounds are in the middle, and low frequency sounds are near the apex. In addition to the location on the basilar membrane, the frequency of sounds can be identified based on the number of nerve impulses sent to the brain.
The organ of Corti lies upon the basilar membrane and contains three to five outer rows (12,000 to 15,000 hair cells) and one inner row (3,000) of hair cells. The influence of the inner and outer hair cells has been widely researched. The common view is that the numerous outer hair cells respond to low intensity sounds (quiet sounds, below 60 dB). The inner hair cells act as a booster, by responding to high intensity, louder sounds. When the basilar membrane moves, it causes the small hairs on the top of the hair cells (called stereocilia) to bend against the overhanging tectorial membrane. The bending of the hair cells causes chemical actions within the cell itself creating electrical impulses (action potentials) in the nerve fibers attached to the bottom of the hair cells. The nerve impulses travel up the nerve to the temporal lobe of the brain. The intensity of a sound can be identified based on the number of hair cells affected and the number of impulses sent to the brain. Loud sounds cause a large number of hair cells to be moved, and many nerve impulses to be transmitted to the brain.
Each of the separate nerve fibers join and travel to the lowest portion of the brain, the brainstem. Nerves from the vestibular part (balance part) of the inner ear combine with the cochlear nerves to form the VIII cranial nerve (auditory or vestibulocochlear nerve). Once the nerve impulses enter the brainstem, they follow an established pathway, known as the auditory pathway. The organization within the auditory pathway allows for a large amount of cross-over. "Cross-over" means that the sound information (nerve impulses) from one ear do not travel exclusively to one side of the brain. Some of the nerve impulses cross-over to the opposite side of the brain. The impulses travel on both sides (bilaterally) up the auditory pathway until they reach a specific point in the temporal lobe called Heschl's gyrus. Crossovers act like a safety net. If one side of the auditory pathway is blocked or damaged, the impulses can still reach Hes chl's gyrus to be interpreted as sound.
See also Neuron.
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