HEARING TESTS
H
HEARING
The ears are the organs ofhearing. Each ear has three
regions: the outer ear, middle ear, and inner ear. The outer
ear channels sound vibrations through the eardrum to
the middle ear. The eardrum has to have equal air pressure
on each side so that it can vibrate freely; the pressure is
equalized via the eustachian tube, which runs from the back
ofthe throat to the middle ear. Inside the middle ear,
a complex system of membranes and tiny bones conveys
the vibrations to the inner ear via a membrane called the
oval window. In the inner ear, the vibrations are converted
to nerve impulses and sent to the brain via the
vestibulocochlear nerve.
Structure and function ofcochlea
Running the length ofthe cochlea is a fluid-filled tube called the
cochlear duct. It contains several membranes and a structure
called the organ of Corti. Tiny hairs on the organ of Corti brush
against a membrane called the tectorial membrane, producing
electrical signals. These signals are detected bynerve fibres.
M a l l e u s ( h a m m e r )
a n d i n c u s ( a n v il)
T h e m a l l e u s , a t t a c h e d t o
t h e e a r d r u m , t r a n s m it s
v ib r a t io n t o t h e in c u s .
S t a p e s ( s t ir r u p )
a n d o v a l w i n d o w
T h e s t a p e s t r a n s m it s
v i b r a t i o n s f r o m t h e in c u s
t o t h e o v a l w in d o w , t h e
m e m b r a n e b e t w e e n t h e
m i d d l e a n d i n n e r e a r .
.
V e s t i b u l o c o c h l e a r n e r v e
T in y n e r v e f i b r e s f r o m t h e
c o c h l e a j o i n u p t o f o r m
t h e v e s t i b u l o c o c h l e a r
n e r v e , w h ic h c a r r i e s
i m p u l s e s f r o m t h e
c o c h l e a t o t h e b r a in .
ROUTE TO THE BRAIN
Electrical signals are picked up by
nerve fibres in the cochlea and
pass along the vestibulocochlear
nerve to the medulla. From there,
they pass via the thalamus to the
superior temporal gyrus - the part
ofthe cerebral cortex (see
b r a i n )
involved in receiving and
perceiving sound.
T h a l a m u s
M e d u ll a
S u p e r io r
t e m p o r a l
V e s t i b u l o c o c h l e a r
g y r u s
n e r v e
O u t e r e a r
___________
T h e p i n n a ( t h e v i s i b l e
p a r t o f t h e e a r ) c h a n n e ls '
s o u n d w a v e s in t o t h e
e a r c a n a l t o w a r d s t h e
e a r d r u m .
E a r d r u m
_
T h e e a r d r u m s e p a r a t e s
t h e o u t e r a n d m i d d l e e a r .
S o u n d w a v e s o f d i f f e r e n t
f r e q u e n c i e s c a u s e t h e
e a r d r u m t o v ib r a t e a t
d i f f e r e n t s p e e d s .
O v a l w in d o w
C o c h l e a
T h e c o c h l e a c o n s i s t s o f a
h o l l o w s p i r a l p a s s a g e . It
p i c k s u p s o u n d v ib r a t io n s
t r a n s m i t t e d t h r o u g h t h e
o v a l w i n d o w a n d c o n v e r t s
t h e m i n t o n e r v e i m p u l s e s
N o r m a l c o i l e d
s h a p e o f
c o c h l e a
E u s t a c h i a n t u b e
T h is t u b e a l l o w s
a i r t o p a s s in t o
a n d o u t o f t h e
m i d d l e e a r .
C o c h l e a r d u c t
N e r v e f i b r e s
Cochlea
(shown uncoiled)
COMPARISON OF FREQUENCY RANGES
The frequency of a sound
(how high or low it is) is
measured in Hertz (Hz).
Different animals can
hear different ranges of
sound frequencies. The
diagram shows the
normal ranges that can
be heard by a human, a
bat, a dolphin, and a dog.
Humans can hear a range
between about 20 and
20,000 Hz.
10 Hz
100 Hz
1,000 Hz
10,000 Hz 100,000 Hz
\ B o t t o m o f
Frequency
\
t o p o f
h u m a n s ’ r a n g e
h u m a n s ’ r a n g e
Electron micrograph ofinner ear
This image shows four rows of hair cells (centre)
in the organ of Corti. On their surfaces are
sensory hairs (top right), which convert sound
waves into electrical impulses; these are picked
up by the vestibulocochlear nerve (bottom right).
364
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