Hearing Loss
Hearing loss, or deafness, is the partial or total inability to hear sound in one or both ears. The human ear is amazing. It is one of the smallest and most complex organs in the body, capable of turning the tiniest disturbances in air molecules into a form the brain can understand - and doing so instantaneously, over an enormous range of pitch and loudness. Considering the ear's delicacy, it is remarkably resilient. Nevertheless, illness or injury can impair our ability to hear properly.
In recent years, substantial advances have made it possible to determine the cause of hearing impairment in nearly all cases, and to treat the hearing loss in many ears.
How Does Hearing Function?
Hearing involves a complex chain reaction within the ear:
- Sound creates vibrations in the air somewhat similar to the rippling waves created when a stone is thrown into a pond.
- The outer-ear "trumpet" collects these sound waves, and they are funneled down the external ear canal to the eardrum.
- As the sound waves strike the eardrum, they cause it to vibrate.
- The vibrations are transmitted through the middle ear over the bony bridge formed by the hammer, anvil, and stirrup.
- These vibrations, in turn, cause the membranes over the openings to the inner ear to vibrate, causing the fluid in the inner ear to be set in motion.
- The motion of the fluid in the inner ear excites the nerve cells in the organ of Corti, producing electrochemical impulses that are gathered together and transmitted to the brain along the acoustic nerve.
- As the impulses reach the brain, we experience the sensation of hearing.
What we hear are sound waves provided by vibrations of air molecules. The size and energy of these waves determine the loudness, which is measured in decibels (db). The number of vibrations or cycles per second makes up frequency - the more vibrations, the higher the pitch of the sound. Sound frequency is expressed in cycles per second, or hertz (Hz).
The sensitivity of the hearing mechanism is most extraordinary. With the softest detectable sound, the eardrum only moves approximately one-millionth of an inch. Our ability to detect sounds from the softest to the loudest covers an intensity range of starting at approximately 100,000,000.
Many young, healthy humans (through teens and early twenties) can hear frequencies from about 20 Hz to 20,000 Hz, and can detect frequency differences as small as 0.2%. That is, we can tell the difference between a sound of 1000 Hz, and one of 1002 Hz.
Consequently, it is no surprise that such a remarkably complex system can be damaged by various illnesses and injuries.
Ear Structure
The ear is divided into three major parts:
- The outer ear
- The middle ear
- The inner ear
The outer ear has two parts:
- The trumpet-shaped, protruding part of the ear on each side of the head, called the auricle or pinna
- The tube leading into the ear, called the auditory canal; the opening is called the meatus
In the middle ear :
The eardrum (tympanic membrane) stretches across the inner end of the auditory
canal.
The middle-ear chamber is filled with air. There is an opening from the middle ear to the throat through a tube called the eustachian tube. The eustachian tube helps to equalize pressure on both sides of the eardrum.
A chain of three small bones, called ossicles, connects the eardrum to the inner ear. These bones are named for their shapes: the hammer, anvil, and stirrup.
The inner ear is a fluid-filled chamber divided into two parts:
-
The vestibular labyrinth, which is the portion of the inner ear that functions as part of the
- body's balance mechanism.
- The cochlea, which contains the hearing-sensing nerve.
The cochlea is a hollow tube inside the inner ear that is coiled to resemble a snail's shell. It contains thin fluid and a highly specialized structure called the organ of Corti, which contains thousands of minute, sensory, hair-like cells. The organ of Corti functions as the switchboard of the hearing system. It is to the cochlea that sound vibrations picked up by the middle ear are carried.
The acoustic nerve (also called the eighth cranial or auditory nerve) leads from the inner ear to the brain, serving as the pathway for the nerve impulses that the brain will interpret as sound.
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