Auditory Anatomy - Page 2
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A hearing problem within the cochlea is called a "sensorineural loss". A sensorineural loss occurs when the mechanical energy is correctly conducted to the oval window of the cochlea, but the hair cells within the cochlea are damaged and not capable of converting the mechanical energy to a signal carried by the nerves.
The hair cells and the basilar membrane develop such that high frequencies most strongly stimulate the hair cells closest to the oval window, while low frequencies most strongly stimulate the hair cells furthest from the oval window. To accomplish this, the basilar membrane is narrow and tight near the base, and wide and loose near the apex. In addition, hair cells are slightly longer at the apex than at the base. Thus, complex sounds are divided into the frequencies of which they are composed; auditory nerve fibers at one end of the cochlea carry information about high frequencies while auditory nerve fibers from the other end of the cochlea carry information about low frequencies. The hair cells are arranged so that each octave, or doubling in frequency, covers roughly the same distance along the basilar membrane (Figure 4).
Figure 4. Responses of basilar membrance to sound.
Auditory nerve fibers synapse with neurons in the ipsilateral cochlear nucleus. From the cochlear nucleus, the pathway goes through the trapezoidal body and crosses to the contralateral side to synapse within the superior olivary nuclei. The superior olivary nuclei, including the lateral superior olivary nucleus and medial superior olivary nucleus are the first nuclei to receive substantial input from both the ipsi and the contralateral sides. Using input from both ears, it is one of the key nuclei for localizing sound. Continuing up the auditory pathway, some fibers continue up to the inferior colliculus while others synapse at the lateral lemniscal nuclei before crossing to the other side and continuing up to the inferior colliculus. From the inferior colliculus, the pathway either crosses to the contralateral inferior colliculus or continues on to the medial geniculate body (on the ventral posterior portion of the thalamus). From the medial geniculate body, signals continue up the auditory pathway to the auditory cortex (Figure 5).
Figure 5. Auditory pathway.
There are many auditory areas within the cortex. The primary auditory cortex has been the most thoroughly studied. Other areas include the secondary auditory cortex, the posterior auditory field, and the anterior auditory field. In addition, Wernicke's area (area 22) is associated with the interpretation of language. Broca's area - area 44 in the left hemisphere - is associated with the production of language in most people.
Many of the subcortical nuclei (including the cochlear nucleus and the inferior colliculus) are groups of nuclei, distinguishable by their different cell types, the different response characteristics of the cells, and the different functional organizations of the cells. Because of redundancy within the numerous pathways and crossings, injury to an individual pathway is often difficult to detect. (Compare this to the visual pathway where the location of the injury can be determined by the specific visual field that is damaged.)
In addition to the ascending pathway (from the cochlea to the cortex), there is a descending pathway (from the cortex to the cochlea). Many of these descending fibers end up synapsing to the outer hair cells as well as to afferent fibers from the inner hair cells. Little is known about this pathway other than that it aids in the detection of sounds in a noisy background.
As a finely developed system, evolved to detect small rapid changes in pressure, the auditory system is a masterpiece. Through a combination of mechanical tricks and physiological processing, the process of hearing allows humans and animals to interact with a complex environment, and to communicate with one-another.
Dr. Barbara Calhoun is the Research Manager for Scientific Learning Corporation. She has an extensive background in bioengineering and auditory neuroscience and is the recipient of several awards, including the National Research Service Award and the University of California Regent's fellowship. Dr. Calhoun holds a bachelor's degree from Brown University and a doctorate from a joint University of California, Berkeley / University of California at San Francisco Bioengineering program with additional training from the Johns Hopkins University.
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This article was created by Scientific Learning.
