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A tutorial on the portion of the ear for body's balance and equilibrium is treated separately. The topic is accessible by the two buttons link above


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 Publications of this page.


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. Modified from (Stevens and Warshofsky, 1980).


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 membraneÕs initial force is amplified in a similar manner as a womanÕs 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. Modified from (Stevens and Warshofsky, 1980).


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. Modified from (Stevens and Warshofsky, 1980).


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 (Zenner et al., 1994). .


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 41 and 42.


A brief Introduction to Hearing

Auditory function is the process by which the stimulation of receptors in the inner ear is followed by transduction of the mechanical stimulus into neural energy which upon reaching the brain gives us the sensation of hearing.

Auditory function is a very difficult subject to comprehend due primarily to the diversity of cells and type of tissues that takes place in the processing of a signals reaching the ear. Such a diversity of cells and tissues is necessary in order to localize, distinguish, and process sounds from different sources and different frequencies (i.e. the sound of a violin and a bass). The auditory system is able to distinguish efficiently sound as low as 20 cycles/sec. (hertz = Hz), and 20,000 Hz. Physically, processing such a wide range of frequencies constitutes a truly miraculous perfection of engineering. For example, even today with the most sophisticated development of computers and electronic digital equipment it would be impossible to duplicate the function of the inner ear in a machine as large as a refrigerator. However, the inner ear is capable of doing all of this and is only the size of a pea.

There are very few organs in the body that can do as much as the ear can in such a little space. In order to duplicate the function of the inner ear, an engineer would have to fit into 16 cubic centimeters a sound system that is capable of equalizing (impedance) a wide range of inputs, a mechanical analyzer, a mobile relay and amplification unit, a multi-channel transducer to convert mechanical energy into neural energy, and a system to maintain a delicate hydraulic balance and an internal two-way communication system. It is impossible to cover all the physiological properties of the inner ear in only one chapter, when there are books dedicated entirely to only the function of the hair cells, which are only parts of the inner ear.

When the extreme sensitivity of the auditory system is analyzed, one has only but to wonder about its functions. In fact, if the inner ear were as protuberant as the eyes are it is likely that subjects would be more impressed and appreciative of what they can hear in the same manner that they are impressed with what they can see. The efficiency of the ear is such, that if the eye was forced to distinguish the tremendous difference between the sound of a cannon and a whisper, it is likely that the person would have to protect the eye externally. Fortunately for us, the ear protects itself with the built in mechanisms it has. For example, if the eyes were exposed to the above extremes would be the equivalent as if looking directly into the bright sun. Most likely after such an experience the individual would have to wait a few minutes before focusing the eyes completely. Nonetheless, the ear can switch between the whisper and the cannon sound with little or no effort. One example of the incredible function that the ear performs is the fact that someone can learn to live near a railroad and not be bothered by the sound of a passing train, but at the same time can wake up by the sound of an alarm clock. Similarly, during a cocktail party one can effectively ignore the noise around; and center ones attention on the conversation of the person across the table.

Before we can appreciate sound, waves in the air reach the outer ear or auricle (pinna) which in lower animals contributes to the localization of the origin of the sound. The sound waves then reach the tympanic membrane. The membrane vibrates and the vibration is transmitted to the inner ear by mean of the three small bones, malleus (hammer), anvil and stirrup. The vibration causes the stirrup to act as a piston which by displacement of a small and thin membrane on the oval window of the cochlea displaces the endolymphatic content of the cochlea duct. Displacement causes the portion of the membrane, where hair cells rest, to undulate in conjunction with another membrane on top of the hair cells, making the hair cells transduce the mechanical energy into neural stimulation. From the hair cells of the inner ear, the neural stimulus is transmitted by the afferent cochlear nerve fibers to the brain stem; and from there to the various stations along the brain center up to the cortex where speech and sound are finally decoded (Kelly, 1981; Pickles, 1982; Tsuchitani, 1983; Yost and Nielsen, 1985).

References