SPA 4321 Community Presentation Basic Ear Anatomy


Hello and welcome. To begin our discussion
on hearing loss, it is first important to understand how hearing works. An understanding
of the anatomy of the ear will provide you with the foundation to learn more about hearing
loss and the ways that the effects of hearing loss on your life can be reduced. This presentation was completed by Stephanie
Caruso, Kyla Kohagen, Jocelyn Martinez, and Juliann Hunnicutt. First, we should consider where sound comes
from. Sound is a vibration that travels through the air. When this vibration arrives at your
outer ear, the body’s process of hearing begins to occur. Sounds can vary in their volume (called amplitude)
and their pitch (called frequency). The ear’s design allows us to hear a wide variety of
sounds, from very quiet to very loud, from very high pitched (like a whistle) to very
low pitched (like a foghorn). The volume or intensity of sound is measured
in decibels (dB). We can hear sounds from 0dB to about 120 dB, when the volume becomes
painful. Normal conversation is usually about 45 to 60dB. Another important characteristic of sound
is frequency, which is measured in Hertz. Humans have the ability to hear sounds from
about 20 Hertz to 20,000 Hertz. This picture provides the frequency of several common sounds
across the spectrum. The low rumble of a truck would be a very low-pitched, low-frequency
sound, while a whistle is a high frequency sound. The human voice falls in the middle
of our frequency range. When we discuss the anatomy of the ear, we
divide the ear into 3 sections: the outer ear (shown in green), the middle ear (shown
in blue), and the inner ear (shown in purple). Each section of the ear is responsible for
different parts of the hearing process. The outer ear collects the sound waves, the middle
ear changes the air waves to a mechanical vibration, and the inner ear changes the mechanical
vibrations to electrical signals, which are then sent to the brain. Now let’s look at
each part of the ear in more detail…. Let’s take a look at the outer ear. Shown
above in the diagram you can see the pinna and the ear canal in green. The ear canal
is also referred to as the external auditory meatus. The picture of the ear at the top
of the slide is labeled with different parts of the ear including the helix, antihelix,
external auditory meatus, concha, tragus, and lobule. The ear canal is about 3 cm long
in adults and is S-shaped. The pinna is the only visible part of the ear. The pinna gathers
sound and changes the acoustics of the sound as it enters the ear canal. Ears are constantly
active as they pick up sound waves and change them to information that the brain can interpret.
Such as music or speech. After the sound enters the ear canal, it is funneled to the ear drum.
As the ear drum begins to vibrate, it sets the ossicular chain into motion. Without this
funnel, the sound waves would take a more direct route into the auditory canal. This
would be both difficult and wasteful as much of the sound would be lost making it harder
to hear and understand the sounds. Now let’s take a look at the middle ear. The
middle ear begins at the eardrum, also know as the tympanic membrane (TM). The middle
ear is highlighted in the boxed area of the first picture. Inside the middle ear are the
ossicles, three tiny bones that connect the ear drum to the inner ear. The ossicles are
nicknamed after their shape. These bones are called the malleus (hammer), incus (anvil),
and stapes (stirrup). As you can see in the bottom picture, the ossicles are smaller than
the size of a dime. The ear drum starts the process of changing the sound waves into mechanical
vibrations of the ossicles, which eventually vibrates the cochlea of the inner ear. Sound
travels down the ear canal and hits the tympanic membrane which causes it to move back and
forth. This movement makes the ossicles vibrate, causing the footplate of the stapes to vibrate
within the oval window opening of the cochlea. This vibration then transfers acoustic energy
to the fluids and tissues of the cochlea. The Eustachian Tube is also linked to the
middle ear from the nasopharynx, and is normally closed. The middle ear is filled with air
and the Eustachian tube equalizes the air pressure by opening briefly. For example,
when taking off on an airplane, we chew gum, yawn, or swallow to open the tube to release
and equalize the air pressure built up in our middle ear. The inner ear contains the sensory organs
for hearing and balance. Sensory information about motion, equilibrium, and spatial orientation
is provided by the vestibular system, which in each ear includes the utricle, saccule,
and three semicircular canals. The utricle and saccule detect gravity and linear movement.
The semicircular canals are lined with cilia (microscopic hairs) and filled with a liquid
substance, known as endolymph. Every time the head moves, the endolymph moves the cilia.
This works as a type of motion sensor, as the movements of the cilia are communicated
to the brain. As a result, the brain knows how to keep the body balanced, regardless
of the posture. In the cochlea, sound waves are transformed into electrical impulses which
are sent on to the brain. The cochlea resembles a snail shell or a wound-up hose and is filled
with a fluid called perilymph and contains two closely positioned membranes. These membranes
form a type of partition wall in the cochlea. However, in order for the fluid to move freely
in the cochlea from one side of the partition wall to the other, the wall has a little hole
in it (the helicotrema). This hole is necessary, in ensuring that the vibrations from the oval
window are transmitted to the cochlea. When the fluid moves inside the cochlea, thousands
of microscopic hair fibers inside the partition wall are put into motion. There are approximately
24,000 of these hair fibers, arranged in four long rows. The brain then translates the impulses
into sounds that we know and understand. The auditory nerve is a bundle of nerve fibers
that carry information between the cochlea in the inner ear and the brain. The function
of the auditory nerve is to transmit signals from the inner ear to the brain. The hair
fibers in the cochlea are all connected to the auditory nerve and, depending on the nature
of the movements in the cochlear fluid, different hair fibers are put into motion. When the
hair fibers move they send electrical signals to the auditory nerve which is connected to
the auditory center of the brain. In the brain the electrical impulses are translated into
sounds which we recognize and understand. This is a video to help us better understand
how all three of these parts of the ear we have discussed work together. To review the main components and steps of
hearing, take a look at this diagram. First (step 1), sound waves enter your outer ear
and travel through the ear canal to your eardrum. Then (step 2), your eardrum vibrates due to
the incoming sound and sends the vibrations to three tiny bones in your middle ear called
the ossicles. Next, (step 3), the bones in your middle ear amplify the sound vibrations
and send them to your inner ear, or cochlea. The sound vibrations activate tiny hair cells
in the inner ear, which in turn release neurochemical messengers. Finally (step 4), your auditory
nerve carries this electrical signal to the brain, which translates it into a sound you
can understand. As you can see, there are many important components
that must work properly in order to hear. It is an amazingly complex process. A problem
at any point along this path can cause hearing loss. You’ll be learning more about this in
upcoming presentations. Thank you for joining us today.

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