Neuroscience 9a - Hearing
Anil Chopra
1. Explain the relation between frequency and pitch of a sound
2. Explain the relation between the intensity and loudness of a sound and explain the
usefulness of the decibel scale
3. Describe in outline the structure of the ear
4. Explain the mechanisms for amplification and safety in the middle ear
5. Label simple diagrams of the cochlea to show the 3 scala, basilar membrane,
organ of Corti, and hair cells.
6. Explain the steps by which movements of stapes brings about depolarisation of
hair cells.
7. Explain how the mechanical properties of the basilar membrane and the hair cells
result in frequency analysis of sound waves by the cochlea.
8. Distinguish between outer and inner hair cells.
9. Draw a diagram of a tuning curve of a single, cochlea nerve fibre and explain its
shape.
10. Label a diagram of the main structures and tracts forming the central projections
of the auditory nerve.
11. Define tonotopic mapping.
12. Identify the part of the auditory pathway involved in auditory reflexes.
13. List the functions of the auditory cortex.
14. List the main causes of conductive and sensorineural deafness.
15. Describe the Rinne and Weber tests and explain how they can be used to
differentiate between conductive and sensorineural hearing loss.
The Ear
The external ear: consists of
- pinna
- external acoustic meatus
The middle ear consists of
- tympanic membrane
- auditory ossicles
o malleus
o incus
o stapes
- oval window
- tensor tympani muscle on malleus
- stapedius muscle on stapes
The inner ear consists of
- cochlea
- semicircular canals
- utricle
- saccule
- vestibular and cochlear nerves
Sound Conduction
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Sound = pressure wave in air
Frequency = cycles/sec and is measured in hertz (Hz)
Pitch is how the ear perceives frequency
Amplitude = intensity which is perceived as loudness
Decibel range = log scale of loudness
The outer ear has a resonant frequency of 3 kHz. This therefore means that sounds
between 2.5-3 kHz are easier to hear because the threshold volume for hearing is
reduced by 15dB (decibels).
dB rating
sound
0
threshold of hearing
30
whisper
50-60`
normal conversation
90
shouting
120
gunshot
140
pneumatic drill
The middle ear amplifies sound by 30dB. It does this via the leverage of the ossicles
as the tympanic membrane vibrates. The tympanic membrane is also 17 times the size
of the oval window. This amplifies the sound 22-fold.
Protective Mechanisms
Vibrations of the ossicular chain are dampened down when they become too extreme
by the tensor tympani muscle (malleus) and the stapedius muscle (stapes). There is
however a slight delay between the vibration and the contraction of the muscles so it
could not protect the cochlea from a sudden loud explosion. These are also activated
before speech.
Equalisation
The tympanic membrane needs the pressure on either side of it to be equal for
maximum efficiency. The ear mucosa constantly absorbs air, and therefore the
pressure in the middle ear gradually drops below that of atmospheric pressure. The
Eustachian tube allows the pressure to equilibrate when it is opened (by swallowing
or yawning). Blockage of this tube leads to hearing defect.
Conductive hearing Loss
Sound is prevented from reaching the cochlea due to:
- Earwax
- Otitis media (inflammation of the middle ear)
- Otosclerosis of ossicles (hardening of he ossicles)
- Perforated tympanic membrane
- Congenital malformation
Sound Transduction
The cochlea is a hollow tube in bone, curled into spiral; divided longitudinally into 3
compartments separated by 2 membranes running the whole of its length:
-
Scala vestibuli - contains perilymph (like CSF)
o Reissner’s membrane – attached to tectorial membrane.
Scala media - contains endolymph (has high K+ conc.  80mV)
o Basilar membrane – has attached cochlear hair cells
Scala tympani - contains perilymph (Like CSF)
Vibration of the oval window causes the vibration of the perilymph and in turn causes
the basilar and Reissner’s membrane’s to vibrate and in turn.
Organ of Corti & Hair cells
The organ of Corti is the functional unit of sound transduction in the cochlea. As the
perilymph, endolymph, and basilar membrane vibrate, the hairs on the hair cells
(attached to the basilar membrane) are displaced. This results in oscillating changes in
the membrane potential of these cells, which causes depolarisation of the cell, a Ca2+
influx releasing glutamate when it reaches threshold (NB: very sensitive: threshold
requires only 3nm deflection!). This causes the ganglion cell to fire action potentials
to the brain via the cochlear nerve (VIII):
 Hair cells move up: hairs move away from modiolus
o K+ channels open
o Hair cells depolarise
 Hair cells move down: hairs move toward modiolus
o K+ channels close
o Hair cells hyperpolarise
Modulation of Transduced Signal
There are 2 types of hair cells:
Inner hair cells: about 3 500 cells arranged in a single row.
These provide information for the brain i.e. they detect sound.
The glutamate released by them can be inhibited by efferent
axons (presynaptic inhibition)
Outer hair cells: about 20 000 cells arranged in 3 rows. These are affected by
efferent nerves which cause them to change shape. This
results in the amplification of the response of the inner hair
cells and is used in “tuning” of the ear to important sounds.
The inner hair cells have projections to the spiral ganglion and
then to the auditory nerve and on to the brainstem, while the
outer hair cells receive input from the brainstem via the spiral
ganglion cells and auditory nerve in order to modulate the inner hair cells
Sharpening the tuning curve
The tuning curve gives the
frequency of maximum
sensitivity for each cochlear
nerve fibre
As can be seen in the
diagram the maximum
sensitivity for this nerve is
at 2.0 with reducing
sensitivity on either side. It
has a wider reach of low
frequency
than
high
frequency sounds
Single cochlea nerve fibre
Several cortical neurones
Sharpening of the tuning curve by lateral inhibition is also shown below with a single
tuning curve for a single nerve
Pitch Differentiation
Frequency determines the perceived pitch of sound.
The basilar membrane increases in width as it winds
round the cochlea (base = 100μm, apex =
500μm). The longer fibres (at the apex) have
lower resonant frequencies that then the
shorter ones, hence they will vibrate more and
send action potentials from those cells at
lower frequencies. The shorter fibres (at the
base) have higher resonant frequencies and hence will detect higher pitch sounds.
Furthermore the cells at the base are much stiffer than those at the apex and are
therefore tuned mechanically.
This increase in width, coupled to a decrease in stiffness of the basilar membrane
means that sound of high frequency maximall displaces BM at the
stapedial end, while low-frequency sounds maximally active the
apical end. i.e. high frequencies vibrate basilar membrane nearer
to base, low frequencies vibrate membrane nearer to apex.
 The form of the sterocilia mirrors their position of the basilar
membrane and so determines the hair cells response to sound
frequency
 Hair cells are embedded in a gelatinous matrix containing
calcium carbonate crystals called otoconia
 Two types of sensory hairs, 1) Stereocilia – arranged in rows
of varying heights 2)single, long kinocilium
 Two types of nerve-endings present on hairs:
 Types I; chalice like endings form ribbon synapses
 Type II; simple nerve endings
Normal Human range = 20Hz – 20KHz
Most sensitive at 1-3 KHz (human speech)
Auditory Pathways
 Complex bilateral pathways through the
brain
 Superior olivary nuclei project back to
the cochlea as well as forward to the
central pathways
 Inferior colliculi – reflexes e.g. startle,
head turn
 Collateral pathways to reticular formation
and cerebellum
 Lateral inhibition in ascending pathway
sharpens tuning curve
 Descending pathways provide feedback
at all levels
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From the cochlea, the vestibulocochlear
nerve synapses first in the dorsal
cochlear nucleus and ventral cochlear
nucleus.
From the dorsal cochlear nucleus, fibres
pass through the dorsal acoustic stria and
(contralaterally) then join the lateral
leminiscus up to the inferior colliculus.
From the ventral cochlear nucleus, the
majority of the fibres pass contralaterally
through the trapezoid body. There are
some fibres which synapse again in the
superior olivary nucleus on the same side. Both these then travel up the lateral
leminiscus on their respective sides to the inferior colliculus.
From the inferior colliculus in the midbrain, the nerves synapse once again in the
medial genticulate body of the midbrain.
From the medial genticulate body, they travel to the primary auditory complex.
Throughout all of this, the spatial organisation of the fibres in response to
frequency is preserved.
In the primary auditory complex, the cells respond to specific features of sound
and some complex patterns.
In the secondary auditory complex, the neurones respond to more complicated
sound patterns.
Localisation of Sound
Intensity and timing of the sound arrival at the two cochleas is very important. Most
neurons in the auditory pathway respond to sound from either ear
» Localisation in the horizontal plane –
Intensity and phase difference between
the two ears is computed
» Localisation in the vertical plane – The
pinna reflects sound waves from
different directions in different patterns
» Localisation from distance – high
frequencies are attenuated more than low
frequencies
» Wernicke’s area is the cortical site of language comprehension and is found in the
dominant hemisphere (usually left)
» Brocca’s area is connected to this and is responsible for the expression of speech
and language.
Rinne and Weber Tests
Use a 512Hz tuning fork.
Rinne tests. The fork held at the meatus of a normal ear will sound slightly louder
than when placed on the mastoid behind the pinna.
Conductive loss: the fork will sound louder on the mastoid.
Neurosensory loss: the fork is louder on the mastoid.
Weber test: the fork is placed on the mid-forehead above the brow level the tone is
heard ‘in the middle of the head’. It tests sensor-neural hearing
Neurosensory loss: If there is a sensory loss to one side the spatial localisation of the
tone is displaced to the good side. i.e. heard loudest in unaffected.
Conductive hearing Loss:
 Sound is prevented from reaching the cochlea:
 Caused by:
 Wax
 Otis media
 Otosclerosis of ossicles
 Perorated tympanic membrane
 Congenital malformations
Sensorineural Deafness:
 Sensory:
 Presbyacusis
 Exposure to loud noise
 Meniere’s disease
 Toxicity e.g. some antibiotics
 Hereditary disorders
 Neural:
 Acoustic neuroma
 Viral infection
 Central (rare):
 Demyelination in MS
 Injury to central auditory pathway (unlikely to cause serious deafness unless
both auditory cortices are affected