THE EAR AND HEARING
Your hearing system is active 24 hours a day. Hearing is not a sense that you can ‘turn off’; it works on several levels:
- With our hearing we perceive background sounds, such as traffic noise, or more relevant sounds, such as the ringing of an alarm clock. What is generally most important, however, is the ability to hear speech– to understand and communicate.
- Hearing allows us to distinguish among a multitude of sounds – from the soft rustle of leaves to complex sentences. When our hearing ability is reduced, we are no longer able to hear sounds optimally.
The ear is a complex sensory organ that consists of three main parts:
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The Outer Ear consists of the visible portion of the ear, called the auricle or pinna, and the ear canal. The auricle is made of cartilage and skin. Its funnel shape is important to sound localization (i.e. determining the direction and source of sound).
Sound is directed to the eardrum (tympanic membrane) via the ear canal. The outer two-thirds of the canal consists of cartilage covered by skin which has hair and glands which produce earwax (cerumen). Earwax serves to keep the ear canal moist and helps to migrate dead skin particles out of the canal. It also protects the eardrum and the middle ear from intrusion by foreign objects.The inner third of the ear canal extends from is hard and bony, and is covered only by a thin layer of skin which makes it very sensitive to touch.
The Middle Ear begins with the eardrum and as such is located between the outer and inner ear. It is normally an air filled space or cavity within the temporal bone. Middle ear air pressure is equalized to ambient air pressure by the Eustachian tube, which connects the middle ear with the back of the throat and the nose. The middle ear cavity houses three small bones -- the malleus, incus and stapes-- which are collectively known as the ossicles. (These bones are often commonly referred to as the hammer, anvil and stirrup, respectively.) The ossicles form a chain which acts as a lever mechanism to transfer the mechanical energy of sound waves from the eardrum to the fluid-filled cavities of the inner ear.
The Inner Ear like the middle ear is located in the temporal bone. It consists of the auditory (cochlea) and balance (vestibular) organs. The balance organ is comprised of the vestibule and the semicircular canals. The auditory and balance organs are connected via fluid filled cavities. The cochlea resembles a snail shell with two and one half turns.
In the lower turn of the cochlea there are two windows; the oval and the round window. The footplate of the stapes is attached to the oval window and functions as a piston moving the fluid of the inner ear. This movement of the fluid activates the cilia, or hair cells, in the inner ear. The hair cells are arranged in rows and run the entire length of the cochlea on a structure called the basilar membrane. There is one row of inner hair cells and three rows of outer hair cells. This arrangement of sensory hair cells and other supporting cells are referred to as the organ of Corti. When the haircells are stimulated, they send neural impulses via the VIII (auditory) nerve to that part of the brain (i.e. auditory cortex located behind the temporal bone)where the impulses are perceived as sound. The frequency of the sound determines which group of hair cell cilia is activated, whereby it becomes possible to distinguish between different sounds. If one’s hair cells are damaged due to age, illness or other causes, they will have problems hearing certain sounds and differentiating between sounds.