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The auditory system
Structure and function
Objective
• Recap – outer, middle and inner ear to BM
response
• To continue the hearing process from the
inner ear to the brain
• How the physical properties of sound, such
as frequency and amplitude, are represented
in the auditory system
Organ of Corti
• The organ of corti is the hearing sense organ and
lies on the BM
• It consists of supporting cells and hair cells
• 2 groups of hair cells: inner and outer hair cells
• Protruding from each hair cell are hairs called
stereocilia
• The tectorial membrane lies above the stereocilia –
shearing motion between BM and tectorial
membrane – causes stereocilia to be displaced
Inner hair cells
• Function is to convert BM mechanical movement
into neural activity – achieved in the following
way (Moore, 2004):
• The stereocilia are joined by fine links called ‘tip
links’.
• Deflection of the stereocilia leads to the opening
of “transduction channels”.
• Flow of potassium ions into the hair cell – voltage
difference between the inside and outside of the
hair cell.
Inner hair cells
• Causes the release of neurotransmitter and the
initiation of action potentials in the neurons of the
audtiory nerve.
• Action potential – ‘firing’ of a neuron.
Propagation is in one direction only down the
length of the axon
• Most of the afferent neurons make contact with the
inner hair cells
• Possibly all information about the input sound is
conveyed via the inner hair cells
Outer hair cells
• Have a role in achieving high sensitivity
and sharp tuning
• Most of the efferent neurons synapse
directly with the outer hair cells
• Efferent neurons carry information from
higher auditory system to cochlea
• Afferent neurons carry information from the
cochlea to the higher auditory system
Auditory nerve
• Consists of vestibular and cochlear nerve
• Cochlear nerve – the axon fibres of neurons
whose cell bodies are in the spiral ganglion
of the cochlea
• dendrites of these neurons synapse with the
hair cells – dendrites information receivers
Afferent auditory nerve
• Transmits hearing information from cochlea
to central nervous system
• Important findings from recording impulses
in single auditory nerve fibres.
• Spontaneous firing, frequency selectivity of
fibres, phase locking
Spontaneous firing rates
• Firing in the absence of sound stimulation
• Fibres divided into three groups based on their
spontaneous firing rates
• High, medium and low spontaneous rate fibres
• High spontaneous rate fibres generally have lower
thresholds and smaller dynamic ranges than
medium or low spontaneous rate fibres
Frequency selectivity
• Fibres show frequency selectivity – as fibres
responding to activity at restricted regions of the
BM
• Characteristic frequency (CF) – frequency at
which the threshold of a fibre is lowest
• Fibres with high CF found in periphery of
auditory nerve – orderly decrease in CF towards
centre of the auditory nerve
• Tonotopic organisation – place representation of
frequency on BM maintained in auditory nerve
Phase locking
• The firing of a neuron at one distinct point (phase)
in the period of a sound wave.
• temporal regularity in the firing pattern of a
neuron in response to a periodic stimulus
• upper limit: 4-5 kHz
• May not fire on every cycle, but will occur at
roughly the same phase of the waveform
• Time intervals between spikes are approximately
integer multiples of the period of the waveform
• an indicator of the period of the waveform
The brainstem and auditory
cortex
• Binaural perceptions, such as our ability to
localise sounds depends on the interaction of
information from both ears
• The information from both auditory nerves is first
combined in the brainstem
• Highly complex system, many of the neural
pathways within and between the nuclei have yet
to be investigated
• Brainstem nuclei: cochlear nucleus, superior
olivary nucleus, inferior colliculus, medial
geniculate body.
• Tonotopic organisation of frequency is preserved
– neurons aligned respective to the frequencies to
which they are most sensitive
• The responses of neurons at higher levels in the
auditory system – not as well studied as responses
in the auditory nerve
• Some neurons in the auditory cortex only respond
to complex stimuli, or to stimuli with time varying
characteristics
Cochlear nucleus
• The auditory nerve carries signals from the
cochlea to the cochlear nucleus in the brainstem.
• First brainstem nucleus at which afferent auditory
nerve fibres synapse
• Monaural – input from same side ear only
• 3 parts – anteroventral, posteroventral, dorsal
• Not clear what its function in auditory processing
is. – tonotopic organisation of frequency
Superior olivary complex
• Binaural - Here the inputs from both ears
converge
• Important for sound localisation
• Use of timing and intensity differences
Inferior colliculus
• Receives inputs from the olivary complex and the
cochlear nucleus
• Some cells are monaural, some are binaural
• Much (but not all) of the input to the inferior
colliculus comes from the opposite ear
• May function in sound localisation, and in
combining information from lower areas in the
brainstem
Medial geniculate nucleus
• Last stop in the auditory pathway before the
cerebral cortex.
• 2-way information flow between medial
geniculate and the auditory cortex
• Feedback from the brain is tightly
integrated with sensory information flowing
up to the brain
Auditory cortex
• Primary auditory cortex (A1) – the first area
within the temporal lobes of the brain responsible
for processing acoustic information
• Each cochlea has input to each auditory cortex
• Tonotopic organisation of frequency is maintained
in the A1
• 2-way information between cortex and brainstem
• No response to steady, unchanging tones –
pure tones
• Some cortical neurons respond to tones
increasing or decreasing in frequency
• Some respond to amplitude variations
• Next: frequency resolution (selectivity) and
pitch perception, reading material