Anatomy & Physiology of the
Auditory System: Overview
Capabilities of the Auditory
System
What does the auditory
system do and how well does
it do it?
Hearing Sensitivity
The faintest sound that can be detected by
the human ear is so weak that it moves the
ear drum a distance that is equivalent to
1/10th the diameter of a hydrogen atom.
If the ear were slightly more sensitive we
would hear the random particle
oscillations known as Brownian motion.
We will see soon that the
Motioninner ear transforms acoustic detecting
cilia
vibrations into neural
impulses through the
operation of motion-detecting
receptor cells in the cochlear
called hair cells. Hair cells get
their name from the tiny hairs
(cilia) that project up from the
tops of these receptor cells.
The movement of these cilia
begins the process of
hair cell
converting acoustic
vibrations into impulses on
the 8th N.
How much movement of the cilia is
needed to barely detect a sound? From
auditory physiologist Peter Dallos:
If you scale the dimension of … one
cilium to the height of Chicago's Sears
Tower, the movement of the tip of the
cilium at the threshold of hearing is
equal to a two-inch displacement at the
top of the tower.
Dynamic Range
The most intense sound that can be heard
without causing pain is approximately 140
dB more intense than a barely detectable
sound.
This means that that the dynamic range of
the ear – the ratio of the most intense
sound that can be heard without pain to
the intensity of a barely audible sound – is
an astounding 100 trillion to 1.
Frequency Range and Frequency
Discrimination
The frequency range of human hearing
runs from approximately 20 Hz to 20,000
Hz, a range of 10 octaves.
For signal levels approximating
conversational speech, the ear can detect
frequency differences that are on the
order of 0.1%, or approximately 1 Hz for a
1,000 Hz test signal.
We’ll soon see that the auditory system
utilizes an elegant mechanism that delivers
sounds of different frequencies to different
physical locations along the cochlea (e.g., a
sound of one frequency will produce the
greatest neural activity at one physical
location while a sound of a different
frequency will activate a different location).
f = 300 Hz
f = 3,000 Hz
The difference in frequency that a listener
can barely detect corresponds to a
difference in physical location along the
cochlea of about 10 microns (1 micron =
one millionth of a meter, or one
thousandth of a millimeter). This
distance, in turn, is approximately the
width of a single auditory receptor cell.
Intensity Discrimination
Under good listening conditions, listeners
can detect intensity differences as small
as 0.6 dB.
Sound Localization
Listeners can locate the source of a
sound based on differences in the time of
arrival between the two ears that are as
small as 10 μs (i.e., 10 μs = 10
microseconds = 10 millionths of a
second).
Amazing Organ, Small Package
The anatomy that supports this processing is a
miracle of miniaturization. If you’re impressed
by the size of the latest IPod, consider this:
(1) The middle ear cavity is roughly the volume
of a sugar cube.
(2) The cochlea is even smaller, at
approximately 5 mm in height (~0.2 inch)
and approximately 9 mm in diameter (~1/3rd
inch) at its widest point.
(3) The hair cells, which behave like teeny
microphones, are 10 microns (1/1000 of a
millimeter) in width.
Anatomy & Physiology Overview
Three major functional subsystems:
Conductive, Sensorineural, Central
A. Conductive: pinna, ear canal, ear drum (tympanic membrane),
and middle ear (containing the middle ear bones or ossicles)
Functions: (1) transmission – mechanical vibration picked up
at the TM is transmitted to the fluid-filled cochlea by way of
the ossicles. (2) overcoming an impedance mismatch – sound
wave is generated in air, a low-impedance medium, and
transmitted to the fluid-filled cochlea, a high impedance
medium. Without two cool tricks exploited by the middle ear,
a good deal of this sound energy would be lost. We’ll see how
this works later. It’s cool and mostly pretty simple.
B. Sensorineural: Cochlea (part of the inner ear) and auditory
nerve (8th cranial N).
Functions: (1) Transduction: Transducers convert energy of one
form into energy of a different form (e.g., microphones,
loudspeakers, light bulbs, photocells). Receptors in the cochlea
(hair cells) convert mechanical energy (the energy of vibration) into
neurochemical energy (nerve impulses). (2) Spectrum analysis:
Breaking a complex sound into its individual frequency
components (like a prism or a Fourier analyzer). (3)
Transmission: The nerve impulses that are stimulated by the hair
cell transducers are transmitted to the CNS via the auditory nerve.
C. Central: Brain stem and auditory cortex.
Functions: This one is a little on the tricky side. Your text book,
like most texts, lists the primary functions of the central
system as: recognition, integration, and interpretation. Some
examples:
speech recognition, talker recognition, recognition of familiar
melodies
This part is definitely true: You can’t do any of these things
with just a conductive mechanism, a cochlea, and an 8th N. The
brain stem and cortex are heavily involved in these high-level
abilities.
The central auditory system is also heavily involved in
more basic auditory abilities as well. Some important
examples:
sound localization
pitch perception (probably)
spectrum analysis (maybe – this is very unsettled)
Bottom line: Central auditory system is definitely
involved in higher level auditory abilities (recognition,
interpretation, integration …), but it is also involved in
more primitive or basic functions such as sound
localization, pitch perception, and maybe even spectrum
analysis.
Overview of Hearing: The Big Picture
The Cochlea Unrolled
The basilar membrane snipped out and viewed from
above.
From our discussion of springs and masses, what do we
know about how stiffness affects natural vibrating
frequency?
Which end of the membrane would respond best to a high
frequency sound?
Which end of the membrane would respond best to a low
frequency sound?
So, high frequency sounds show up at the basal end of
the cochlea, low frequency sounds show up at the apical
end, and mid-frequency sounds show up at the
appropriate place in the middle.
What kind of pattern would you expect for a complex tone
consisting of 8000 Hz mixed with 200 Hz?
What we have here is a frequency analyzer – very much
like a prism, but using a mechanical resonance trick
instead of refraction.
Basilar membrane traveling wave
High Frequency
Low Frequency
Basilar membrane traveling wave for a 1,000 Hz sinusoid
Basilar membrane traveling wave for a 3,000 Hz sinusoid
Basilar membrane traveling wave for a 300 Hz sinusoid
The transduction story in a nutshell
So far all we have is mechanical vibration – the back &
forth vibration that is picked up at the TM is transmitted
through the middle ear bones and is transformed into up
& down vibration of the basilar membrane; that is, motion
is turned into motion.
The brain can’t do a thing with it until this vibratory
motion is converted into the only language that the brain
understand – nerve impulses; the simple, all-or-none
spikes that are produced by neurons. The conversion of
vibratory motion into nerve impulses is an example of
transduction.
To get a quick idea of how this works, we need to take a
closer look at the cochlea.
Again With The Unrolled Cochlea
We take a slice thru the cochlea,
which is a little like a garden
hose, then take a look to see
what’s inside the tube. There’s a
lot of stuff in there.