Kategorie: Angličtina (celkem: 879 referátů a seminárek)

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  • Přidal/a: anonymous
  • Datum přidání: 23. září 2006
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Příbuzná témata


The temporal bone and the ear
The pinna or auricle is the most external portion of the ear. Itcommunicats via the external auditory canal to the middle ear. The middle ear contains the tympanic membrane, the middle ear bones, a muscle, and ligaments which are not shown in this diagram. The internal cochlea (auditory portion ) as well as the labyrinth (for balance and equilibrium) constitutes the entire inner ear .
Resonant Chamber
The bones of the middle ear form a suspension bridge-like structure between the tympanic membrane and the oval window of the cochlea. The pressure in middle ear cavity is controlled by a tube to the larynx reason why when the body goes into high altitude there is the need to release the pressure. Without the support of the middle ear muscle and ligaments, the bones will move freely and without control. The hammer and the anvil are anchored to the temporal bone by ligaments, and stirrup is attached by a muscle to the temporal bone. Contractions of the muscle and ligament causes rigidity and support to the entire bridge-like structure restricting movement in various directions.
Reduction & Amplification.
The tympanic membrane is 15 to 30 times larger than the oval window where the stirrup is connected to the cochlea itself. Such reduction concentrates force from the initial mechanical displacement of the middle ear in such a way that the tympanic membraneąs initial force is amplified in a similar manner as a womanąs weight is condensed on a heel of a shoe. In other words, if a person weighs 100 lbs. and sat on the floor the mark left would be unnoticed, however if the same weight is place on the heel of one shoe, the mark would be noticeable once the person moves.
Frequency Selectivity
Different sound makes the basilar membrane move with different amplitudes and at different locations. The thickness of the basilar membrane (BM) in the basal segment is 3X thicker than the apical segment. For this reason, sounds of high frequency move the basal segment very lightly. The basilar membrane is 5x wider in the apical segment than in the basal segment. This diagram illustrates that the thickness and width of the basilar membrane allows sound of different frequencies to travel from the base to the apex at different speeds.

The maximal deformation of the BM occurs in different places.
Cochlear Duct
Transversal view of the internal auditory canal in which the organ of Corti and associated structures are visible. Since the concentration of potassium and sodium is different in the perilymph and the endolymph, it is supposed that mixing the two can induce an electrical potential in the auditory hair cell, which in some instances may contribute to certain pathological conditions .

Organ of Corti
The organ of Corti rests on the basilar membrane. The organ of Corti is formed by supporting cells, hair cells (which are specialized paraneurons) and they are classified into internal hair cells and external hair cells. The organ of Corti contains a tunnel with special ionic mixture. It was hypothesized that the stereocilia of both the outer and inner hair cells are in embedded in the tectorial membrane.
Role of Cations
The electrical resonance of the auditory hair cells results from the deformation of the stereocilia, which by opening and closing of the pores allow potassium to enter into the cytoplasm. When this occurs, a positive charge is added to the positive charge calcium. High concentration of calcium then recruits other pores that are sensitive to potassium and in this manner the membrane is polarized. At the right of the figure it can be seen how the pores in the stereocilia can open or close depending on the direction that the cilia are bent.
The Microphones
Hair cells act like microphone to pick up vibrations and are divided into two types: Outer hair cells and inner hair cells. The inner hair cells receive 95% of the afferent terminals, whereas the outer hair cells receive 95% of the efferent terminals. The inner hair cells are located in the portion of the organ of Corti that is restricted in its movement, whereas the outer hair cells are located in the portion of the basilar membrane that can move maximally.
The CPU of Hearing
The peripheral nerve fibers of the VIIIth cranial nerve end in the so called cochlear or olive complex of the brain stem. Fibers from both sides (bilateral) project through the brain center and such bilateral projection is essential for the proper localization of sound. From the olive, the fibers reach the lateral lemniscus of the mesencephalon still in a bilateral manner. From there the fibers continue to the medial geniculate complex of the thalamus and from where they reach the primary auditory memory center of the auditory cortex in the area known as Broadmann.

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