The Physiology of Hearing

Hearing is the ability to receive and process sound waves or acoustic stimuli. Hearing is an essential process for communication and can be practically pleasurable as it provides people the chance to listen to music and to participate in good conversations. However, the process of hearing is not as simple as saying “I heard the sound of a car passing by. ” Rather, it is as complicated as anyone could ever imagine, and is indeed a series of complex processes that an acoustic stimulus undergoes from the moment it enters the external ear up to the point where the electrical version of it is interpreted by the brain.

The Physiology of Hearing The normal hearing process is called binaural hearing, or hearing using both ears. In binaural hearing, both ears hear in such a way that a sound that registers in one ear will become louder than one reaching the other ear after a split second has passed. When the brain analyzes and compares the information coming from both ears, the brain can detect whether the sound came from the right ear or the left. Furthermore, the brain has the ability to isolate sounds from a noisy background in order to maintain focus on only one conversation.

The first step in the hearing process is when sound enters the outer ear and the sound vibrations that make their way through the ear canal hit the taut membrane of the eardrum. This results in the vibration of the ossicles and the eardrum itself. The resulting vibrations are delivered with a relatively greater force to the inner ear as a result of the fact that the surface area of the eardrum is relatively larger than that of the oval window. In the process, the sounds may be amplified resulting in a corresponding increase in energy.

This energy is needed for the vibrations to be propelled through the fluid contained in the inner ear. As this fluid is actually relatively more resistant compared to air, a greater pushing force is required. Also, when sound is heard, the muscles in the middle ear may constrict depending on the intensity or loudness of the sound. The constriction that may happen is for reducing the volume of the sound before it reaches the sensitive inner ear. However, this phenomenon, called acoustic reflex, may at times not work in cases where the loud sound is heard so suddenly and hearing impairment may therefore result.

The second step in the hearing process is when the stirrup vibrates against the oval window thereby transmitting the sound wave pattern to the inner ear as well as to the fluid located in the upper and lower parts of the cochlea. The cochlea is known as “a hydromechanical frequency analyzer located in the inner ear [and whose] basilar membrane supports the sense organ of hearing, the spiral organ of Corti. ” (Dallos, 1992) The third step occurs when sound waves cause the hair cells of the Organ of Corti on the basilar membrane to move.

These hair cells are actually considered “the true sensory nerve endings for hearing” (Wever, 1933). The basilar membrane itself is directly not involved in the physiology of hearing yet “among the accessory structures, [it] plays the most prominent role” (Wever, 1933). In fact, the basilar membrane contains a number of “different frequency channels [which are activated by] amplitude fluctuations…of a common sound source” (Langner, 1991). In short, it is the natural periodic fluctuations of sound vibrations that actually trigger the different frequency channels of the basilar membrane.

This step may actually take place prior to the moving of the hair cells. The hair cell in the Organ of Corti is typically “a sensory receptor that responds to mechanical stimulation of its hair bundle” (Hudspeth & Jacobs, 1979) and this hair bundle is usually made up of “numerous large microvilli, or stereocilia, and a single true cilium [known as] the kinocilium” (Hudspeth & Jacobs, 1979). It is however the mechanical stimulation of the stereocilia that helps trigger the electrochemical stimulation that follows.

A high-frequency sound causes the basilar membrane to resonate with the cells near the cochlear base. However, in the case of a low-frequency sound, the resonation is closer to the cochlear tip. The fourth step would then be the vibration of the hair cells against the tectorial membrane after the sound wave has set the former in motion. The vibration against the tectorial membrane results in the displacement of the cilia on the hair cells resulting in an electrochemical reaction: there is a chemical reaction in the hair cells and this causes electrical impulses in the auditory nerve.

The number of impulses set off is directly proportional to the intensity or loudness of the sound. The next step would be for the electrical impulses to travel from the auditory nerve to the brain, specifically in different information-processing stations that process sounds in order to determine where they came from. The circuits that the impulses travel terminate at the auditory cortex, which is located in the symmetrical temporal lobes of the brain. After which, the electrical information is sorted and processed in the thin layer of tissue known as cortex or gray matter.

Moore (1991) states that in fact the whole process of “binaural interaction occurs primarily and almost simultaneously at three levels of the brain: the superior olivary complex (SOC), the nuclei of the lateral lemniscus (NLL) and the inferior colliculus (IC). ” Dallos (1992) also states that the production of electrical impulses by the Organ of Corti involves “matching particular frequencies with particular groups of auditory receptor cells and their nerve fibers.

” From the aforementioned statements, one can conclude that in truth the pathway of the electrical impulses in the auditory cortex is actually not linear but a group of branched complex pathways, that one would suppose to have each a distinct role in the interpretation of the sound. However, Moore (1991) states that “there is little evidence for strict, functional segregation in these binaural pathways, although subdivisions of the SOC appear to be predominantly involved in analyzing either interaural time or level differences (ITD, ILD).

” The binaural pathways theorized by Moore may be identical to “specialized auditory pathways [on which] sound identification and sound localization [depends]” (Alain et al. , 2001). This further strengthens the idea that there is no unique linear pathway for the interpretation of sounds by the auditory cortex. The presence of these so-called “specialized pathways” of hearing imply that certain sounds pass through different pathways in the auditory cortex as they are interpreted differently as to what and where they came from.

Far from what anyone can imagine, the auditory complex is indeed more complex than it actually seems and that the brain actually identifies and classifies sounds more intricately than our vocabulary can ever define. Moreover, Huttenlocher and Dabholkar state that these pathways “undergo a prolonged maturation sequence that extends into adolescence [specifically] between 3. 5 years and 12 years of age” (as cited in Ponton, Eggermont, Kwong & Don, 1999).

This statement simply means that perhaps at adolescence, most binaural pathways in the brain would become permanent, and that “in children, auditory skills such as speech recognition in noise that require cortical processes…may be limited [or underdeveloped]. ” (Ponton et al. , 1999) Back to the process of hearing, as the impulses travel to the cortex, the right and left temporal lobes communicate extensively in order for the signals to be compared. The auditory cortex also plays a role in the suppression of the noise in the background to allow the person greater focus on the sound he desires to hear.

The final step is the brain’s interpretation of the sound and the giving it of meaning with the use of language and speech. Conclusion With the recent studies conducted in the field of audiology, one can see that the physiology of hearing is indeed more complicated than it actually is. Hearing starts with the acoustic stimuli, or sound wave, entering the external ear. What follows is a series of steps where the actual sound wave makes its way through the cochlear fluid, the hair cells in the Organ of Corti, the tectorial membrane, the auditory nerve, and finally, the auditory cortex.

In the process, the actual sound wave becomes a vibration and is then converted later to an electrical impulse before it is interpreted by the cortex. It is also found out that the electrical impulse may make its way through a variety of neural pathways in the cortex, but whose individual functions are yet to be determined by further research. References Alain, C. , Arnott, S. R. , Hevenor, S. , Graham, S. & Grady, C. L. (9 Oct. 2001). “What” and “Where” in the Human Auditory System. Proceedings of the National Academy of Sciences of the United States of America, 98. 21, 12301-12306.

Retrieved May 31, 2010, from the Proceedings of the National Academy of Sciences of the United States of America database: http://www. pnas. org/content/98/21/12301. abstract Dallos, P. (Dec. 1992). The Active Cochlea. The Journal of Neuroscience, 12. 12, 4575-4585. Retrieved May 31, 2010, from The Journal of Neuroscience website: http://www. jneurosci. org/cgi/reprint/12/12/4575. pdf Hudspeth, A. J. & Jacobs, R. (1 Mar. 1979). Stereocilia Mediate Transduction in Vertebrate Hair Cells (Auditory System/Cilium/Vestibular System. Proceedings of the National Academy of Sciences of the United States of America, 76. 3, 1506-1509.

Retrieved May 30, 2010, from the Proceedings of the National Academy of Sciences of the United States of America database: http://www. pnas. org/content/76/3/1506. abstract Kemp, D. T. (Nov. 1978). Stimulated Acoustic Emissions from within the Human Auditory System. Journal of the Acoustical Society of America, 64. 5, 1386-1391. Retrieved May 29, 2010, from the Acoustical Society of America Digital Library database: http://scitation. aip. org/getabs/servlet/GetabsServlet? prog=normal&id=JASMAN000064000005001386000001&idtype=cvips&gifs=yes&ref=no Langner, G. (Jul. 1991). Periodicity Coding in the Auditory System.

Hearing Research, 60. 2, 115-142. Retrieved May 30, 2010, from the ScienceDirect database: http://www. sciencedirect. com/science? _ob=ArticleURL&_udi=B6T73-4864PB0-K7&_user=10&_coverDate=07%2F31%2F1992&_rdoc=1&_fmt=high&_orig=search&_sort=d&_docanchor=&view=c&_searchStrId=1353172862&_rerunOrigin=scholar. google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=b9ab4002662969c04720656b8a744cf6 Moore, D. R. (1991). Anatomy and Physiology of Binaural Hearing. International Journal of Audiology, 30. 3, 125-134. Retrieved May 31, 2010, from the Informa Health Care database: http://informahealthcare. com/doi/abs/10.

3109/00206099109072878 Ponton, C. W. , Eggermont, J. J. , Kwong, B. & Don, M. (2000). Maturation of Human Central Auditory System Activity: Evidence from Multi-channel Evoked Potentials. Clinical Neurophysiology, 111, 220-236. Retrieved May 30, 2010, from the Elsevier database: http://psyweb. psy. ox. ac. uk/oscci/ERP%20Meeting/ponton/maturation%20evoked%20potentials. pdf Wever, E. G. (1933). The Physiology of Hearing: The Nature of Response in the Cochlea. Physiological Reviews, 13, 400. Retrieved May 31, 2010, from the Physiological Reviews website: http://physrev. physiology. org/cgi/pdf_extract/13/3/400

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