FUNCTION OF THE EARS
Hearing is conducted over two pathways: air and bone. Sounds transmitted by air conduction travel over the air-filled external and middle ear through vibration of the tympanic membrane and ossicles.Sounds transmitted by bone conduction travel directly through bone to the inner ear, bypassing the tympanic membrane and ossicles. Normally, air conduction is the more efficient path-way. However, defects in the tympanic membrane or interrup-tion of the ossicular chain disrupt normal air conduction, which results in a loss of the sound-to-pressure ratio and subsequently in a conductive hearing loss.
Sound enters the ear through the external auditory canal and causes the tympanic membrane to vibrate. These vibrations transmit sound through the lever action of the ossicles to the oval window as mechanical energy. This mechanical energy is then transmitted through the inner ear fluids to the cochlea, stimulating the hair cells, and is subsequently converted to electrical energy. The elec-trical energy travels through the vestibulocochlear nerve to the cen-tral nervous system, where it is analyzed and interpreted in its final form as sound.
Vibrations transmitted by the tympanic membrane to the os-sicles of the middle ear are transferred to the cochlea, lodged in the labyrinth of the inner ear. The stapes rocks, causing vibrations (ie, waves) in fluids contained in the inner ear. These fluid waves cause movement of the basilar membrane to occur that then stim-ulates the hair cells of the organ of Corti in the cochlea to move in a wavelike manner. The movements of the tympanic mem-brane set up electrical currents that stimulate the various areas of the cochlea. The hair cells set up neural impulses that are encoded and then transferred to the auditory cortex in the brain, where they are decoded into a sound message.
The footplate of the stapes receives impulses transmitted by the incus and the malleus from the tympanic membrane. The round window, which opens on the opposite side of the cochlear duct, is protected from sound waves by the intact tympanic mem-brane, permitting motion of the inner ear fluids by sound wave stimulation. For example, in the normally intact tympanic mem-brane, sound waves stimulate the oval window first, and a lag oc-curs before the terminal effect of the stimulus reaches the round window. This lag phase is changed, however, when a perforation of the tympanic membrane is large enough to allow sound waves to impinge on the oval and round windows simultaneously. This effect cancels the lag and prevents the maximal effect of inner ear fluid motility and its subsequent effect in stimulating the hair cells in the organ of Corti. The result is a reduction in hearing ability (Fig. 59-3).
Body balance is maintained by the cooperation of the muscles and joints of the body (ie, proprioceptive system), the eyes (ie, visual system), and the labyrinth (ie, vestibular system). These areas send their information about equilibrium, or balance, to the brain (ie, cerebellar system) for coordination and perception in the cerebral cortex. The brain obtains its blood supply from the heart and ar-terial system. A problem in any of these areas, such as arterioscle-rosis or impaired vision, can cause a balance disturbance. The vestibular apparatus of the inner ear provides feedback regarding the movements and the position of the head and body in space.
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