It’s hard to understand hearing loss without first understanding how hearing works in the first place. When hearing loss occurs, one of the several systems that make up our auditory system is damaged. Therefore, knowing how this system functions can help us understand hearing loss and how to treat it.
The Auditory System
Although everything in our ear is connected, our auditory systems are divided into three parts: the outer ear, middle ear and inner ear. Each part serves a different function and is essential to our ability to hear properly.
When we think of hearing, we usually imagine the outer ear – the part you can see. It includes:
- The pinna, or cartilage auricle that extends outside our head;
- The ear canal;
- The tympanic membrane (the outer layer of the eardrum).
The pinna’s shape is designed to collect sound waves and pass them through the ear canal to the tympanic membrane, where sound then enters the middle ear.
The middle ear is separated from the outer ear by the eardrum. This chamber of the ear is full of air and includes the following structures:
- The ear drum
- The ossicles, which are three different bonesthe malleus (shaped like a hammer, this bone connects to the eardrum)the incus (shaped like an anvil, this bone unites the incus and stapes)the stapes (shaped like a stirrup, this bone transfers the sound to the cochlea)
- the malleus (shaped like a hammer, this bone connects to the eardrum)
- the incus (shaped like an anvil, this bone unites the incus and stapes)
- the stapes (shaped like a stirrup, this bone transfers the sound to the cochlea)
Once the sound hits the eardrum, it begins to vibrate. Because all these systems are connected, this in turn causes the ossicles to move. They turn sound waves into a mechanical vibration. The last bone, the stapes, transfers these vibrations to the inner ear.
This is the final part of the auditory system where sound is changed to an electrical signal the brain can read and interpret. The systems here include:
- The semicircular canals for balance and proprioception
- The cochlea (a spiral structure made of fluid filled tubes separated by thin membranes. One of the most important parts of the cochlea is the basilar membrane and the organ of Corti which covers it)
- The eighth cranial or vestibulocochlear nerve
The inner ear serves two functions – balance and hearing. In terms of balance, the semicircular canals transfer information about the head’s movement to the brain through the vestibulocochlear nerve.
When it comes to hearing, after the sound passes through the middle ear, the stapes vibrates against the oval window, a membrane at one end of the cochlea. This in turn causes the basilar membrane to move in wavelike motions. The basilar membrane is rigid and made of fibers which start off small but slowly increase in length as they extend throughout the cochlea. These fibers register or resonate with different frequencies. This means certain fibers will vibrate quickly when their corresponding frequency passes through the ear.
Thus, to get this information to the brain, an organ resting on top of the basilar membrane, the organ of Corti, comes into play. It contains thousands of tiny hairs or cilia. When the basilar membrane is stimulated and moves rapidly, the cilia detects the movement and converts the stimulus into an electrical impulse that can then be sent to the brain for processing via the vestibulocochlear nerve.