Download Other examples of complex waves

Tuning in the basilar membrane

Vibration of basilar
membrane can be
described by a ‘tuning
curve’

Amplitude required for
the basilar membrane to
vibrate at a certain
constant displacement, as
a function of the
frequency of the input
sound
Neural connections in the cochlea

Afferent and efferent fibers of the VIIIth cranial nerve (auditory
nerve)
 Afferent:
From organ of Corti to brain
 Efferent:
From brain to organ of Corti

Peripheral processes of auditory nerve neurons enter the cochlea
through small openings on the edge of the osseous spiral lamina

These openings are called ‘habenulae perforata’

These fibers are then gathered in the modiolus
Organization of the auditory nerve bundle in the modiolus

Fibers from the apex in
the middle

Fibers from the base on
the outside

This nerve bundle then
goes to the cochlear
nucleus in the brain
Afferent fibers

Around 30,000 neurons in man

Only 5-15% of these innervate
the OHC


These are called Type II or
outer spiral fibers

One neuron connects to one
OHC (one-to-many)
Rest innervate the IHC

These are called Type I or
radial fibers

Many neurons connect to
one IHC (many-to-one)
Efferent fibers

Originate from the olivocochlear bundle in the
auditory brainstem

Efferent fibers synapse on the afferent nerves
innervating the IHC

Efferent fibers synapse directly on the OHC
Discharge pattern of a neuron

Neural spike has an initial large
potential shift

Following this, refractory or
rest period of around 1 msec
Other terms

Spontaneous discharge rate: Neuron’s discharge
rate without a stimulus

Threshold: Minimum stimulus level needed to
cause an increase in the discharge rate above
the spontaneous discharge rate
Spontaneous rate and thresholds

Neurons with high
spontaneous rate: Low
threshold

Neurons with low
spontaneous rate: High
threshold
Rate-Level function

Also called input-output
or intensity function

Increase the level of the
acoustic stimulus and
measuring changes in the
discharge rate of a single
neuron
Response area

Also called isolevel or
isointensity curve

Iso: “Same”

Plotting how a neuron
fires in response to
sounds of different
frequencies at a fixed
intensity
Tuning curve

Define a certain
threshold for a neuron

Plot the level of the
tone required for this
neuron to discharge at
this threshold, as a
function of the
frequency of the tone
Encoding of frequency
Two theories:
1.
Place theory
Tonotopic organization
2.
Temporal theory
Based on the periodic nature of nerve firing
Place theory
Different neurons in the nerve respond to different
frequencies
Frequency of input determined by which neuron(s) in
the auditory nerve fires at the greatest rate
Temporal theory

For lower frequencies (< 5000 Hz), discharge
rates of neurons are proportional to the period
of the input stimulus.

So for lower frequencies, rate of discharge of
the neuron also provides information about the
frequency of the stimulus.
Encoding of intensity

Increase in discharge rate with increase in stimulus
intensity

However, for most neurons, increase in discharge rate
only occurs for a limited range of input intensity

Possible that discharge rate of many neurons may be
combined to account for the 140 dB dynamic range of
the ear