Describe how pitch and loudness are processed by the cochlea.
Firstly, you should know that the sensation of sound is caused by the vibration of basilar membrane and by the bending of stereocilia of certain hair cells in Corti organ. The bending of stereocilia is causing depolarisation of these hair cells, and transduction of sound into neural signals.
1)How is loudness processed
Loudness is determined by the auditory system in at least three ways.
First, as the sound becomes louder, the amplitude of vibration of the basilar membrane and hair cells also increases, so that the hair cells excite the nerve endings at more rapid rates.
Second, as the amplitude of vibration increases, it causes more and more of the hair cells on the fringes
of the resonating portion of the basilar membrane to become stimulated, thus causing spatial summation of impulses—that is, transmission through many nerve fibers rather than through only a few.
Third, the outer hair cells do not become stimulated significantly until vibration of the basilar membrane reaches high intensity, and stimulation of these cells presumably apprises the nervous system that the sound is loud.(Guyton and Hall Physiology Ch. 52 p.656-657)
2)How is pitch/ frequency processed
Determination of Sound Frequency— The “Place” Principle
From earlier discussions ***(see bellow) in this chapter, it is apparent that low-frequency sounds cause maximal activation of the basilar membrane near the apex of the cochlea, and high-frequency sounds activate the basilar membrane near the base of the cochlea. Intermediate frequency sounds activate the membrane at intermediate distances between the two extremes.(Note: Depending on the region of the basilar membrane that is activated/vibrating, cochlear
nerve fibers respond differently. That is basically how we understand different frequencies.
I think the above is a sufficient answer, but you can read the rest of the text too:
Furthermore, there is spatial organization of the nerve fibers in the cochlear pathway, all the way from the cochlea to the cerebral cortex. Recording of signals in the auditory tracts of the brain stem and in the auditory receptive fields of the cerebral cortex shows that specific brain neurons are activated by specific sound frequencies. Therefore, the major method used by the nervous system to detect different sound frequencies is to determine the positions along the basilar membrane that are most stimulated. This is called the place principle for the determination of sound frequency.
Yet, referring again to Figure 52–6, one can see that the distal end of the basilar membrane at the helicotrema is stimulated by all sound frequencies below 200 cycles per second. Therefore, it has been difficult to understand from the place principle how one can differentiate between low sound frequencies in the range from 200 down to 20. It is postulated that these low frequencies are discriminated mainly by the socalled volley or frequency principle. That is, lowfrequency sounds, from 20 to 1500 to 2000 cycles per second, can cause volleys of nerve impulses synchronized at the same frequencies, and these volleys are transmitted by the cochlear nerve into the cochlear nuclei of the brain. It is further suggested that the cochlear nuclei can distinguish the different frequencies of the volleys. In fact, destruction of the entire apical half of the cochlea, which destroys the basilar membrane where all lower-frequency sounds are normally detected, does not totally eliminate discrimination of the lower-frequency sounds.
(Guyton and Hall Physiology Ch. 52 p.692)
*** Extra information about basilar membrane and response to different frequensies
Basilar Membrane and Resonance in the Cochlea.
The basilar membrane is a fibrous membrane that separates the scala media from the scala tympani. It contains 20,000 to 30,000 basilar fibers that project from the bony center of the cochlea, the modiolus, toward the outer wall. These fibers are stiff, elastic, reedlike structures that are fixed at their basal ends in the central bony structure of the cochlea (the modiolus) but are not fixed at their distal ends, except that the distal ends are embedded in the loose basilar membrane. Because the fibers are stiff and free at one end, they can vibrate like the reeds of a harmonica.
The lengths of the basilar fibers increase progressively beginning at the oval window and going from the base of the cochlea to the apex, increasing from a length of about 0.04 millimeter near the oval and round windows to 0.5 millimeter at the tip of the cochlea (the “helicotrema”), a 12-fold increase in length. The diameters of the fibers, however, decrease from the oval window to the helicotrema, so that their overall stiffness decreases more than 100-fold.
As a result, the stiff, short fibers near the oval window of the cochlea vibrate best at a very high frequency, while the long, limber fibers near the tip of the cochlea vibrate best at a low frequency. Thus, high-frequency resonance of the basilar membrane occurs near the base, where the sound waves enter the cochlea through the oval window. But lowfrequency resonance occurs near the helicotrema, mainly because of the less stiff fibers but also because of increased “loading” with extra masses of fluid that must vibrate along the cochlear tubules.
(Guyton and Hall Physiology Ch. 52 p.653)
I hope you found the sources useful.
I also tried to deal with your other question, but I can hardly find anything relative on the web. I will try again tomorrow.
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