Considering the ear’s delicacy, it is remarkably resilient. Nevertheless, problems can occur which 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. It is helpful to understand how the ear works, and to be familiar with many of the reasons why it may cease to work properly. Such knowledge allows us to recognize and understand ear problems when they occur, and to judge whether our patients are really receiving the latest and best in medical care.
Sound creates vibrations in the air somewhat similar to the 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 by mechanical action through the middle ear over the bony bridge formed by the malleus, incus, and stapes. 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. The sensitivity of the hearing mechanism is most extraordinary. Near threshold (the softest detectable sound), the eardrum only moves approximately 1/1,000,000th of an inch. Our intensity range spans extremes from the softest sounds, to sounds of jet engine intensity, covering an intensity range of approximately 100,000,000 to 1. Over this range we are able to detect tiny changes in intensity, and in frequency. 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.
The outer ear has two parts: (a) the “trumpet-shaped” apparatus on the side of the head called the auricle or pinna, and (b) the tube leading from the auricle into the temporal bone called the external auditory canal. This opening is called the meatus.
The tympanic membrane, or “eardrum”, stretches across the inner end of the external ear to the inner ear. A bony bridge is held in place by muscles and ligaments. The middle-ear chamber is filled with air and opens into the throat through the eustachian tube. The eustachian tube helps to equalize pressure on both sides of the eardrum.
The inner ear is a fluid-filled chamber divided into two parts: (a) the vestibular labyrinth, which functions as part of the body’s balance mechanism, and (b) the cochlea, which contains the hearing-sensing nerve. Within the cochlea is the organ of Corti, which contains thousands of minute, sensory, hair-like cells. The organ of Corti functions as the switchboard of the auditory system. The eighth cranial or acoustic nerve travels from the inner ear to the brain, serving as the pathway for the impulses the brain will interpret as sound.