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. Most auditory afferents synapse on inner hair cells. Movement of the cilia opens potassium channels. The influx of potassium causes a subsequent influx of calcium and a receptor potential that can cause an action potential in the afferent dendrites.
Jeholodens jenkinsi is a tricondant that is very similar to what they looked like. Well, Wikipedia , quoting the inestimable source of Purves , says …. I have no argument with that, but as a cognitive scientist, I take a reflex to be an action that is beyond my favorite parcel of cognition, namely learning. Which brings me to outer hair cells. If what rocks your world is rock and roll, watch this short video. If your taste is more classical, watch this one. Either way, you should see how the top of the cigar-shaped object in the center of the screen vibrates in time to the music.
What you are seeing is an isolated outer hair cell. It moves. In fact in moves in such a way as to exaggerate the movement of the basilar membrane, much like a person on a swing trying to make it swing higher:. The following image attempts to put this into a cycle of static pictures. This pumping makes faint sounds easier to register on the basilar membrane. Reptiles do not have outer hair cells, so they appear to be an innovation in the mammalian lineage.
This chapter sketches the transduction of sound to action potentials in the ear. It can be divided into two sequences, the mechanical transmission of acoustic energy and then its transduction to electrical energy:. Think back to the concept of tonotopy: the long axis of the basilar membrane encodes frequency, so its position on the basilar membrane endows each inner hair cell with the ability to respond to a different frequency, and this response is carried forward by the auditory nerve.
Since the stereocilia only open potassium pores as long as they are bent, a hair cell only produces neurotransmitters for as long as the frequency lasts. In this way, the duration of the sound is also carried forward by the auditory nerve. Finally, recall that I asked you in reference to Fig. Imagine the difference in contact between a faint sound and a loud sound.
For a faint sound, the basilar membrane barely moves, so only the tallest stereocilia brushes up again the tectorial membrane and so only opens its few potassium pores and ultimately producing just a handful of action potentials. On the other hand, for a loud sound, the basilar membrane moves brusquely, forcing all the stereocilia up again the tectorial membrane and so opening all the potassium pores.
This produces a cascade of action potentials. This is how the intensity of a sound can be transduced to the auditory nerve. The inner hair cells are the primary auditory receptors and exist in a single row, numbering approximately 3, The outer hair cells are arranged in three or four rows. They number approximately 12,, and they function to fine tune incoming sound waves.
The longer stereocilia that project from the outer hair cells actually attach to the tectorial membrane. All of the stereocilia are mechanoreceptors, and when bent by vibrations they respond by opening a gated ion channel. As a result, the hair cell membrane is depolarized, and a signal is transmitted to the cochlear nerve. Intensity volume of sound is determined by how many hair cells at a particular location are stimulated.
The hair cells are arranged on the basilar membrane in an orderly way. The basilar membrane vibrates in different regions, according to the frequency of the sound waves impinging on it.
Likewise, the hair cells that lay above it are most sensitive to a specific frequency of sound waves. Hair cells can respond to a small range of similar frequencies, but they require stimulation of greater intensity to fire at frequencies outside of their optimal range.
The difference in response frequency between adjacent inner hair cells is about 0. Compare that to adjacent piano strings, which are about six percent different. Place theory, which is the model for how biologists think pitch detection works in the human ear, states that high frequency sounds selectively vibrate the basilar membrane of the inner ear near the entrance port the oval window.
Lower frequencies travel farther along the membrane before causing appreciable excitation of the membrane. The basic pitch-determining mechanism is based on the location along the membrane where the hair cells are stimulated. Hence, no one ever guesses they have a major hearing loss. A profile of hearing ability is recorded on a chart called an Audiogram.
Typically, an audiogram has numbers across the top which show each frequency pure tone that will be sampled. For example they might include: , , , , , and Hz.
On the left, going down the side, would be the intensity, marked off in decibels relative to threshold, required for a person to hear each pure tone at threshold. A series of pure tone are presented with increasing intensity.
The individual raises his hand when he finally hears it. Most audiograms use an "X" to show the threshold for the right ear and an "O" for the left ear. If one connects the points with a line, a profile of hearing is apparent.
In the following audiograms you can see the high frequency loss in the profile. See an audiogram showing a Hz loss and above. Speech is still intelligible but many fricatives are not heard.
See an audiogram showing a Hz loss in the graphic view. Speech is almost unintelligible. Most consonants are not heard. Speech is not intelligible because no consonants can be differented.
Only some prosodic features of speech are discriminable like rhythm and stress. This includes viral infections of the mother before birth or the baby after; blood incompatibilities between the baby and the mother; medical or pleasure drug effects; oxygen deprivation; physical trauma; genetic inheritance; and noise pollution to mention a few.
Young teenagers who play in rock concert bands are particularly at risk for noise pollution. A consequence of the unevenness the a sensory neural hearing loss is that one cannot guarantee that amplification will be beneficial in every case. In some cases it lowers the threshold level so that consonants can be discriminated and speech discrimination improves. In others, consonant discrimination remains poor and the person is blasted in the lower frequencies by the amplification.
In some cases the discomfort must be weighed against the gains. The best way to know if a hearing aid will be of benefit is to be tested with different makes and models by an agency which is not trying to sell one. Many major universities have Speech and Hearing Clinics. It is important to identify hearing loss among children as early as possible and to, if necessary, establish a program of amplification and training.
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