Normal ear functions
Hearing is arguably the most important sense in our everyday lives! From localization of various environmental cues to development of spoken language, our sense of hearing is essential to both survival and communication. The auditory system is incredibly complex and requires a number of functions to work properly. This process is as follows:
- Sound is transmitted through the air as sound/acoustic waves from the environment. The sound waves are gathered by the outer ear and sent down the ear canal to the eardrum.
- The sound waves cause the eardrum to vibrate, which sets the three tiny bones in the middle ear into motion.
- The vibratory motion of the three bones cause the fluid in the inner ear, or cochlea, to move like waves.
- The movement of the fluid in the inner ear causes the hair cells in the cochlea to bend. The hair cells change the movement into electrical impulses.
- These electrical impulses are transmitted to the hearing (auditory) nerve and up to the central auditory system (brain), where they are interpreted as sound.
The outer ear
The visible part of the outer ear is called the pinna, or auricle. The pinna, with its grooves and ridges, along with the ear canal aid us in localization and provide a natural volume boost for sounds in the 2000-3000 Hz frequency range, where we perceive many consonant sounds of speech.
The ear canal is the other important component of the outer ear. The ear canal is lined with only a few layers of skin and fine hair, and is a highly vascularized area. This means that there is an abundant flow of blood to the ear canal. Wax (cerumen) accumulates near the ear drum and migrates outward to serve as a protective barrier to the skin from bacteria and moisture. Earwax is normal and varies in amount based on the person. It only becomes problematic if it completely blocks the ear canal, which often occurs over time when q-tips are used – you are pushing in wax against its natural flow. Think, “Nothing smaller than your elbow” when it comes to sticking anything in your ear canal!
The middle ear
The eardrum, or tympanic membrane (TM), is the dividing structure between the outer and middle ear. Although it is an extremely thin membrane, the eardrum is made up of three layers to increase its strength.
The ossicles are the three tiny bones of the middle ear located directly behind the tympanic membrane. These three bones form a connected chain in the middle ear. The ossicles take mechanical vibrations received at the tympanic membrane, increase the strength of these vibrations and transmit them into the inner ear.
The Eustachian tube is the middle ear’s air pressure equalizing system. The middle ear is encased in bone and does not associate with outside air except through the Eustachian tube the connects the middle ear to the back of the throat. This tubular structure is normally closed, but it can be involuntarily opened by swallowing, yawning or chewing. It can also be intentionally opened to equalize pressure in the ears, like when flying in an airplane. When this happens, you might hear a soft popping sound.
The inner ear
The inner ear is an organ located deep within the temporal bone, which is the bone of the skull on both sides of the head above and to the sides of the outer ear. The inner ear has two main structures: the semicircular canals/vestibular organs and the cochlea.
The semicircular canals do not contribute to hearing, but assist in maintaining vestibular function/balance as we move. The cochlea is the hearing organ of the inner ear, which is a fluid-filled structure that looks like a snail. The cochlea changes the mechanical vibrations from the tympanic membrane and the ossicles into a sequence of electrical impulses. Sensory cells, called hair cells, bend in the cochlea as the fluid is disrupted by the mechanical vibrations. This bending of the hair cells causes electrical signals to be sent to the brain by way of the auditory nerve. The cochlea is arranged by frequency, much like a piano, and encodes sounds from 20Hz (low pitch) to 20,000Hz (high pitch) in humans.
THE EAR IS JUST THE BEGINNING – The central auditory system
Many patients say, “I can hear, but I can’t understand” when presenting for a hearing evaluation. We also get the question, “How can I hear just fine in quiet, but not in noise?” In addition, there are also many questions revolving around when someone should start to have their hearing evaluated or how to better notice hearing loss before it is too late. Hearing loss is truly an invisible disability, but why? This is because the sense of hearing isn’t just about sound awareness provided by the outer, middle, and inner ear. The meaningfulness of sound to us humans results from interpretation of the central auditory system using localization, spacial orientation, timing cues, and auditory memory. All of this occurs in the auditory brainstem and cortex (temporal lobe).
New research is focusing more and more on the central auditory system as they improve testing strategies to look beyond the outer, middle, and inner ear to develop better hearing loss treatment technology (i.e. hearing aids and cochlear implants). We are beginning to understand what happens to the brain permanently the longer hearing loss goes untreated, including brain reorganization (i.e. one reason why we favor vision to help us understand communication), memory loss, and long-term communication deficits resulting in mental illness and isolation.
Moral of the story? Hearing is complex and still being understood. It’s never too early in life to establish a baseline evaluation so that you are better prepared when your hearing health begins to decline as we will all experience at one point or another.