How do we hear?

Welcome to neurally, let’s pick our brains. This video is part of a series about how our
5 senses work. Today, we’ll be covering the sense of hearing,
or audition. So, how do we hear? First, we need to understand the properties
of sound. Anything that can move air molecules can generate
sounds, since sounds are simply variations in air pressure. Specifically, areas of compressed air, known
as high pressure, or rarefied air, known as low pressure, form sound waves. The frequency of the sound wave is the number
of compressed or rarefied patches of air that pass by our ears, and this frequency is measured
in hertz (Hz)– the number of cycles between patches per second. Frequency also determines pitch, so higher
frequencies have higher pitches and lower frequencies have lower pitches. It’s also important to note that there are
some sounds which our ears cannot hear. The other important property of sound is its
intensity, which is the sound wave’s amplitude. Intensity determines the loudness we perceive,
so louder sounds have higher intensities. Now, we must understand the structure of the
ear. The ear is split into 3 parts, the outer ear,
middle ear, and inner ear. The pinna is the outside of the ear, which
collects sound. The entrance to the inside of the ear is the
ear canal, which ends at the tympanic membrane, also known as the eardrum. Sound waves vibrate the eardrum, which is
connected to a sequence of bones, called ossicles. These bones include the hammer, anvil, and
stirrup, and they transfer movements of the eardrum to the oval window. The last ossicle pushes the oval window, which
causes the fluid in the cochlea to move. This movement of fluid creates ripples in
the basilar membrane, which bends the hair cells on its surface. The hair cell movement triggers impulses in
nerve cells, sending neural messages through the auditory nerve to the brain’s auditory
cortex. The most important part of this process occurs
when the vibrations from sound waves are converted to neural signals. The process involves the movement of hair
cells on the basilar membrane located in the cochlea. The cochlea is the fluid-filled, coiled, bony
tube in the inner ear. If we uncoil the cochlea, it’s evident that
the basilar membrane vibrates differently across the length of the cochlea, making it
sensitive to different frequencies of vibration. The basilar membrane is wider at the apex
than at the base. So, at the base, which is more narrow, we
hear higher frequencies, and at the apex, which is more wide, we hear lower frequencies. On top of the basilar membrane is the Organ
of Corti, which contains hair cells. Hair cells each have about 100 stereocilia. When the basilar membrane moves, the stereocilia
are bent, which opens potassium channels and depolarizes the cell. This depolarization signal is sent to the
auditory nerve fiber, and after passing through the cochlear nuclei in the brainstem, the
signal reaches an area in the thalamus, called the medial geniculate nucleus (MGN). The signal passes from the MGN to the Auditory
Cortex, which processes auditory information. It’s also important to understand how the
sound’s intensity and frequency are encoded. A sound’s intensity, which contributes to
loudness, is coded by the firing rates of neurons and the number of active neurons. Yet a sound’s frequency is a more complicated
story. A lot of frequency sensitivity is the result
of the basilar membrane, but more importantly, in the brainstem’s cochlear nuclei, there
is a map of the basilar membrane. So, cells next to each other encode more similar
frequencies than cells far away from each other. This organization is called tonotopy, and
it allows for auditory information to be precisely transmitted to the auditory cortex. Now that we know how our brain receives sound,
we must understand how our brain interprets a sound’s location. Interaural time difference is the time it
takes for a sound to reach one ear vs. the time it takes to reach the other ear. An area in our brainstem called the medial
superior olive encodes the horizontal difference by keeping track of when sounds come from
left and right ear. As for vertical differences, the pinna’s
convolutions delay the direct path of the sound wave, so our brain knows how to analyze
these differences. When our brain processes auditory information,
we hear the world around us. But our brain processes more than just sounds. The brain helps us perceive other kinds of
information, like smells, which we will learn more about next week.

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