Additional
Notes for 04/05/00
I. Inner ear physiology
(continued)
A. More on the basilar membrane
1. The movement of the stapes against the oval window
causes fluid movement in the cochlea which in turn
causes a pressure differential across the basilar
membrane which causes it to move
2. The response of the basilar membrane to a pure tone
takes the form of a traveling wave that moves along the
membrane from base to apex
3. The amplitude of this wave increases initially and then
decreases quickly producing a maximum displacement
at a particular position on the membrane
4. The position of this peak depends on the frequency of
the stimulus
a. High frequencies – near base
b. Low frequencies – vibrations all along membrane but
maximal near apex
5. In response to a pure tone stimulus, each point on the
basilar membrane vibrates in approximately a
sinusoidal manner with a frequency close to that of the
input
6. Each place on the basilar membrane can be considered
a bandpass filter with the place of maximal
displacement the center frequency
7. If there is a complex input (more than one pure tone),
the pattern of displacement becomes more complex
8. This makes up the basis of the place theory of hearing
II. 8th nerve
anatomy
A. Nerve fibers leave the cochlea and are bundled in an
orderly manner:
1. Fibers from the apical
region:
a. Group together in center of fiber bundle
b. Low
frequencies represented at core (middle) of 8th
nerve
2. Fibers from the basal
region:
a. Wrap outside over apical fibers
b. High
frequencies represented at surface of 8th nerve
C. 8th nerve fibers emerge from the internal auditory
meatus
at the brainstem preserving the same ordered arrangement
D. 8th nerve travels up to cochlear nucleus in
brainstem (first
synapse - central system) and divides to innervate
different portions of the cochlear nucleus
E. Tonotopic organization
= arrangement of fibers by
frequency
1. Low frequencies are at the core of the 8th
nerve and
high are at the surface of the 8th nerve
2. Tonotopicity is maintained in the cochlear nucleus and
up through the CANS
III. 8th nerve
physiology
A. Spontaneous firing rates and thresholds
1. Auditory nerve fibers have spontaneous firing rates
from approx. 0.5 to 250 spikes/sec
2. High spontaneous rate fibers have low thresholds
(easier to get them to fire above spontaneous rate)
3. Low spontaneous rate fibers have high thresholds
(harder to get them
to fire above spontaneous rate)
4. Some argument regarding the existence of medium
spontaneous rate fibers
B. Tuning curves
1. Frequency selectivity of a single nerve fiber can be
illustrated by a tuning curve, which plots the fiber's
threshold as a function of frequency
a. On the log frequency scale, the tuning curves
are
steeper on their high frequency side.
1) Low frequency CF fibers have broader almost
symmetrical tuning curves
2) High frequency CF fibers have sharper,
asymmetrical tuning curves
b. The frequency at which a fiber's threshold is
lowest is called its characteristic frequency
(CF)
or best frequency (BF).
c. The
frequency selectivity of a fiber is derived
from the frequency selectivity of the point
on
the basilar membrane that activates it.
d.
Sharpness of tuning on the basilar membrane
now appears to be the same as for single
neurons in the auditory nerve.
C.
Responses are also plotted on a post-stimulus
time
(PST) histogram
1. Stimulus is presented many times
2. Each firing (spike) is plotted at each point in time
relative to stimulus onset
3. Thus the PST histogram is a plot of the # of spikes
over time
4. Features of PST histograms of 8th nerve fibers
a. Initial burst of activity at onset of toneburst
b. Response drops rapidly over next 10-20 ms
c. Fiber continues to fire as long as stimulus is on
d. After stimulus is turned off there is a refractory
period before spontaneous
activity begins again
D. Neural excitation
patterns.
1. In response to low levels of sinusoidal stimulation,
there is a high level of activity in neurons with
center
frequencies close to that of the stimulus and falling
off rapidly to either side.
2. At higher levels of stimulation, saturation can
produce a high level of activity across units with a
wide range of center frequencies.
E. Phase locking.
1. Information about the stimulus is carried not only
in
the place of stimulation & the firing rate of
neurons,
but also in the temporal pattern of these firings.
2. In response to sinusoidal stimulation, nerve firings
tend to be phase locked or synchronized to the
stimulating waveform.
a. A given
fiber does not necessarily fire on every
cycle of the waveform, but
its firings occur at
roughly the same phase of the waveform.
b. Thus, the time intervals between firings are
approximately integer multiples of the period
of
the waveform.
3. This makes up the temporal theory of hearing
4. Phase locking in the human auditory system breaks
down above 4-5 kHz.
F. Two-tone suppression.
1.The activity of a single fiber in response to one
tone
can be suppressed by the presence of a second tone.
a. For a neuron responding to a tone near its
center
frequency, a second tone presented within the
excitatory area bounded by the tuning curve
for
that neuron usually increases its firing
rate.
b. When the second tone falls just outside this
area,
the firing rate is usually reduced
2. The suppression effects start and stop very
rapidly,
and are thought to occur on the basilar membrane.
3. Phase locking of the neuron may also shift from the
original tone to the suppressor tone.