Hearing Mechanisms, Internal

The ear is a highly sensitive organ that processes sounds at various frequencies between 20 Hz and 20,000 Hz. This sensory organ can also tolerate noises of extremely high intensity for a limited time of exposure. It is specifically sensitive to sounds that range from 500 to 4,000 Hz, which is considered as essential for speech processing. In order to experience average hearing, a number of mechanisms involving different components of the ear are required to function, resulting in the conversion of physical vibrations into electric signals that are recognized by the brain. The ear is subdivided into three regions, namely, the outer ear, the middle ear, and the inner ear. The outer ear and middle ear are responsible for conducting sound, whereas the inner ear is involved with transducing sound.

The outer ear functions in the transmission of sound from the environment to the tympanic membrane. The protruding portion of the ear on the side of the skull consists of cartilage and is enveloped by skin; it functions in collecting sounds and delivers these into the auditory canal. The position of the pinna is slightly slanted toward the front to allow collection of sounds from this particular region instead of those behind. The auditory canal is approximately 4 centimeters in length and is comprised of an outer region and an inner region. The surface of the outer region is covered with hairy skin that includes sweat and sebaceous glands that produce cerumen or ear wax. The hairs function as a protective barrier to the external environment, as well as a disinfectant for potentially infectious agents.

The middle ear pertains to the space connecting the back of the nose through a long, thin channel known as the Eustachian tube. This space serves as the location of three small bones, namely, the hammer, the anvil, and the stirrup. The stirrup consists of three smaller components: the malleus, incus, and stapes. The hammer, anvil, and stirrup are responsible for conducting sounds originating from the tympanic membrane and deliver these to the inner ear. The tympanic membrane serves as the outer wall of the middle ear, whereas the bony structure called the cochlea functions as the inner wall of the middle ear.

The inner ear consisting of the cochlea possesses a structure that is similar to that of a snail shell, winding for two-and-a-half turns. The cochlea encloses the hearing organ called the membranous labyrinth, which is covered by fluid known as the perilymph. Vibrations that occur at the stapes generate further vibrations that are received by the perilymph. This fluid then expands and moves into a counter-opening located in the labyrinth. This [Page 466]expansion also facilitates in the movement of the stapes while it vibrates. This continuum of structures allows the collection and transmission of external vibrations into signals that are received by the brain and are subjected to further cognitive processing.

The auditory canal functions as a resonating tube and mechanically amplifies external sounds within the range of 3,000 and 4,000 Hz. This structure also helps in increasing the sensitivity of the ear to hearing sounds within the described frequency range. Thus, the ear is highly sensitive and is capable of perceiving sounds of extremely low intensity, as well as vibrations similar to the movement of air molecules. On the other hand, the Eustachian tube functions in equalizing the pressure within the ear by opening at short intervals, usually after three or four swallows.

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