Ear Ringing And The Inner Ear
To get a better grasp of tinnitus, how it comes about
, and what can be done about it, it helps to understand something about how our sense of hearing works. This third part of a three article series explains the basics of the inner ear, how it functions, what can go wrong to precipitate tinnitus, and what can be done about it. The other two articles deal with tinnitus and the outer ear and the middle ear. Having said that, let's get right to the inner ear.
So far, we have seen how sound waves are gathered by the outer ear or pinna, and how they follow the funnel shaped auditory canal until they come into contact with the eardrum or tympanic membrane. The tympanic membrane then vibrates with the energy of that sound, and transmits it to the middle ear via the ossicles, those three tiny bones, the hammer, anvil, and stirrup, which amplify and focus the sound, leveraging the sound energy for when the stirrup or stapes strikes the cochlea, which takes us to the starting point of the inner ear.
The medium through which the sound has traveled up to this point has been air, which is much less dense than fluid, but within the inner ear the sound energy encounters the much denser medium of the fluid-filled cochlea. That is why that amplification done by the middle ear is so necessary--so that the sound energy can overcome the greater inertia or resistance posed by the denser fluid medium of the inner ear. And here in the inner ear the way that the sound continues its journey to the brain changes significantly.
The cochlea has been so named because its shape resembles a snail or spiral shell, which "cochlea" literally means. It is inside the cochlea where the mechanical vibrations are transduced or converted to electrical nerve impulses which the brain then processes as hearing.
The interior of the cochlea is composed of three tubes or canals that are filled with fluid. When the stapes of the middle ear presses its vibrations against the oval window of the cochlea, two of the canals, the vestibular and the tympanic canals, receive and transmit the energy onward. The organ of Corti, which is located in the third canal known as the cochlear canal, senses the pressure impulses and converts them into electrical impulses and sends them down the auditory nerve to the brain. Together, these three ducts curve into the snail shape of the cochlea. These canals are separated by a thin membrane called the basilar membrane.
The basilar membrane functions as a base for the sensory cells of hearing, the hair cells of which there are about 20,000. These hair cells react with the various frequencies of the sound waves that are being transferred through the cochlea, creating tiny electrical impulses. Then the organ of Corti, located within the cochlear duct on the basilar membrane, operates something like a microphone, sending the electrical impulses along the auditory nerve to the brain which interprets those impulses as the sounds that we hear.
The way our inner ear enables us to hear is amazing enough by itself, but this incredible little component of the body is also command central for the body's mechanism of balance. While other operations of the body also support balance, such as sight and muscle input, it is the vestibular system of the inner ear that serves the main role for keeping the body in balance.
Three essential components make up the vestibular system: the utricle, the saccule, and the three semi-circular canals. The utricle and saccule determine the position of the head at all times. Since the primary balance system for the body is located within the head, tracking the head's position is very important. Both the utricle and saccule operate by being sensitive to gravity and acceleration. The utricle follows horizontal movement, and the saccule follows movement up or down. Working with coordination, the utricle and saccule determine movement of the head in all three spatial dimensions, keeping the brain informed at all times, helping us to maintain proper alignment and to keep our bodies in balance.
The way these little organs actually work is yet one more marvel to keep us amazed. The utricle and saccule are filled with tiny calcium carbonate particles suspended in a thick fluid. They are also lined with tiny hair-like receptors. Whenever the head is moved, the particles suspended in the fluid are also moved by gravity or acceleration and come in contact with the sensors which respond by sending signals to the brain for processing. The brain can then determine the orientation of the head by comparing the information coming from both ears, and it can tell if the head is simply being tilted or if the entire body is moving. Of course, the brain can use input from the eyes and muscles to help make its assessment, but the inner ear is doing by far most of the work for keeping the head aligned with the body, and keeping the entire body in balance.
At the same time, the three semi-circular canals or ducts are serving much the same purpose as the utricle and saccule. But instead of focusing primarily on head position, they are providing information about the body's movement overall. These three canals have been given the names superior duct, posterior duct, and external duct. In order to keep track of movement and orientation in all three dimensions, the ducts are all perpendicular to each other. The semi-circular ducts function in much the same manner as the utricle and saccule. The semi-circular ducts are fluid filled and have hair cells that are sensitive to gravity and acceleration, and respond to motion by sending electrical impulses along nerve fibers to the brain.
Whether we are conscious of it or not, the inner ear is constantly at work performing all of these functions. To maintain our sense of balance, we depend on the vestibular system to gather motion information and send it on to the brain. The brain then serves to process the data and send out signals of its own to the other body systems, such as the muscles, to help us maintain our balance.
When balance problems and tinnitus symptoms are experienced together, Meniere's disease which accounts for nearly one percent of tinnitus cases is often indicated. However, much more common is another tinnitus issue that develops in the inner ear, namely noise-induced damage to the tiny hair receptor cells in the cochlea. Nearly all acoustic trauma can be prevented, except for freak, accidental exposure to excessively loud noise, by choosing to avoid loud sound environments, or by wearing ear muffs or ear plugs to protect our hearing.
by: David Stamon
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