This section will provide an overview of the basic anatomy and function of the auditory system. It will include a discussion of how the sensory stimulus is translated into neural impulses, where in the brain that information is processed, how we perceive pitch, and how we know where sound is coming from.
The ear can be separated into multiple sections. The outer ear includes the pinna , which is the visible part of the ear that protrudes from our heads, the auditory canal, and the tympanic membrane , or eardrum.
The middle ear contains three tiny bones known as the ossicles , which are named the malleus or hammer , incus or anvil , and the stapes or stirrup. The inner ear contains the semi-circular canals, which are involved in balance and movement the vestibular sense , and the cochlea.
The cochlea is a fluid-filled, snail-shaped structure that contains the sensory receptor cells hair cells of the auditory system Figure 1. Figure 1. The ear is divided into outer pinna and tympanic membrane , middle the three ossicles: malleus, incus, and stapes , and inner cochlea and basilar membrane divisions. Sound waves travel along the auditory canal and strike the tympanic membrane, causing it to vibrate.
This vibration results in movement of the three ossicles. As the ossicles move, the stapes presses into a thin membrane of the cochlea known as the oval window. As the stapes presses into the oval window, the fluid inside the cochlea begins to move, which in turn stimulates hair cells , which are auditory receptor cells of the inner ear embedded in the basilar membrane.
The basilar membrane is a thin strip of tissue within the cochlea. The organ of Corti includes three rows of outer hair cells and one row of inner hair cells. The hair cells sense the vibrations by way of their tiny hairs, or stereocillia. The outer hair cells seem to function to mechanically amplify the sound-induced vibrations, whereas the inner hair cells form synapses with the auditory nerve and transduce those vibrations into action potentials, or neural spikes, which are transmitted along the auditory nerve to higher centers of the auditory pathways.
The activation of hair cells is a mechanical process: the stimulation of the hair cell ultimately leads to activation of the cell. As hair cells become activated, they generate neural impulses that travel along the auditory nerve to the brain.
Auditory information is shuttled to the inferior colliculus, the medial geniculate nucleus of the thalamus, and finally to the auditory cortex in the temporal lobe of the brain for processing. Watch the process of audition in the following video:. As mentioned above, the vibration of the tympanic membrane is what triggers the sequence of events that lead to our perception of sound.
Sound waves travel into our ears at various speeds and amplitudes. Like light waves, the physical properties of sound waves are associated with various aspects of our perception of sound. High-frequency sound waves are perceived as high-pitched sounds, while low-frequency sound waves are perceived as low-pitched sounds. The audible range of sound frequencies is between 20 and Hz, with greatest sensitivity to those frequencies that fall in the middle of this range. As was the case with the visible spectrum, other species show differences in their audible ranges.
For instance, chickens have a very limited audible range, from to Hz. Our pet dogs and cats have audible ranges of about 70— Hz and 45— Hz, respectively Strain, The loudness of a given sound is closely associated with the amplitude of the sound wave. The below micrograph is courtesy of Dr. Reprinted with permission. Physical characteristics of the basilar membrane cause different frequencies to reach maximum amplitudes at different positions.
Much as on a piano, high frequencies are at one end and low frequencies at the other. High frequencies are transduced at the base of the cochlea whereas low frequencies are transduced at the apex.
The cochlea codes the pitch of a sound by the place of maximal vibration. Note the position of the traveling wave at different frequencies. It may initially seem backwards that low frequencies are not associated with the base.
Select different frequencies by turning the dial. If audio on your computer is enabled, you will hear the sound you selected. Hearing loss at high frequencies is common.
The average loss of hearing in American males is about a cycle per second per day starting at about age 20, so a year old would likely have difficulty hearing over 10 kHz. If you can't hear the high frequencies, it may be due to the speakers on your computer, but it is always worth thinking about hearing preservation. As you listen to these sounds, note that the high frequencies seem strangely similar.
Think about cochlear-implant patients. These patients have lost hair-cell function. Their auditory nerve is stimulated by a series of implanted electrodes. Thus, cochlear implant patients probably experience something like high frequency sounds. Our absolute threshold, the minimum level of sound that we can detect, is strongly dependent on frequency. At the level of pain, sound levels are about six orders of magnitude above the minimal audible threshold.
Sound pressure level SPL is measured in decibels dB. Decibels are a logarithmic scale, with each 6 dB increase indicating a doubling of intensity. The perceived loudness of a sound is related to its intensity. Sound frequencies are measured in Hertz Hz , or cycles per second. Normally, we hear sounds as low as 20 Hz and as high as 20, Hz. The frequency of a sound is associated with its pitch. Hearing is best at about kHz.
Hearing sensitivity decreases at higher and lower frequencies, but more so at higher than lower frequencies. High-frequency hearing is typically lost as we age. The neural code in the central auditory system is complex. Tonotopic organization is maintained throughout the auditory system. Tonotopic organization means that cells responsive to different frequencies are found in different places at each level of the central auditory system, and that there is a standard logarithmic relationship between this position and frequency.
Each cell has a characteristic frequency CF. The CF is the frequency to which the cell is maximally responsive. A cell will usually respond to other frequencies, but only at greater intensities. The neural tuning curve is a plot of the amplitude of sounds at various frequencies necessary to elicit a response from a central auditory neuron.
The tuning curves for several different neurons are superimposed on the audibility curves in Figure The depicted neurons have CFs that vary from low to high frequencies and are shown with red to blue colors, respectively.
If we recorded from all auditory neurons, we would basically fill the area within the audibility curves. When sounds are soft they will stimulate only those few neurons with that CF, and thus neural activity will be confined to one set of fibers or cells at one particular place. As sounds get louder they stimulate other neurons, and the area of activity will increase. It may seem "backwards" but although the Cochlear duct seems to get smaller toward the apex, the basilar membrane actually gets wider.
High frequencies do not travel far along the basilar membrane. As an aside, low frequencies traverse the length of the Cochlea, and hence cause the most damage if they are sufficiently loud. Sound is transmitted to the fluid of the inner ear through vibrations of the tympanic membrane, malleus, incus and stapes.
Transduction, the change from mechanical energy to neural impulses, takes place in the hair cells, specifically through potassium channels at the tips of the stereocilia. Auditory afferents eventually reach the primary auditory cortex in Heschel's gyrus within insular cortex, and this area is tonotopically organized.
Stimulation of this area leads to conscious awareness of the sound, but the transduction from mechanical vibrations to neural activity occurs in the inner ear. Transduction occurs in both outer and inner hair cells. It consists of tiny hair cells that line the cochlea. These cells translate vibrations into electrical impulses that are carried to the brain by sensory nerves. In this cut-view, you can see the organ of Corti with its four rows of hair cells.
There is an inner row on the left and three outer rows on the right. Let's watch this process in action. First, the stapes rocks against the oval window. This transmits waves of sound through the cochlear fluid, sending the organ of Corti into motion.
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