Introduction to Acoustics

446 Part D Hearing and Signal Processing
Part D 12.2
The receptor potentials in an IHC responding to
tonesareshowninFig.12.10 [12.83]. The traces are
the membrane potentials recorded in an IHC, each trace
at a different stimulus frequency. These potentials are
the driving force for the Ca 2+ signal to the synapse,
and thus indirectly for the AN fibers connected to the
cell. The properties of these potentials can be predicted
from Fig. 12.9c by the thought experiment of moving
a stimulus sinusoidally back and forth on the abscissa
and following the potential traced out on the ordinate.
The potential should be a distorted sinusoid, because
25 mV
AC comp.
12.5 mV
DC comp.
0 10 20 30 40 50 60 ms
100
300
500
700
900
1000
2000
3000
4000
5000
Fig. 12.10 Receptor potentials in an IHC in the basal turn
of a guinea-pig cochlea. The stimulus level was set at 80 dB
SPL so that the cell would respond across a wide range of
frequencies. Stimulus frequency is given at right. Notice the
change in ordinate scale between the 900 Hz and 1000 Hz
traces. (After [12.83], with permission)
depolarizing responses are larger than hyperpolarizing
ones, and it should have a steady (DC) offset, also because
of the asymmetry favoring depolarization. At low
frequencies, both components can be seen in Fig. 12.10;
the response is a distorted sinusoid riding on a steady
depolarization. At high frequencies (>1000 Hz), the sinusoidal
component is reduced by low-pass filtering by
the cell’s membrane capacitance and only the steady
component is seen. The transition from sinusoidal receptor
potentials at low frequencies to rectified DC
potentials at high frequencies corresponds to the loss
of phase-locking at higher frequencies in AN fibers
(Fig. 12.8c). At low frequencies, the IHC/AN synapse
releases transmitter on the positive half cycles of the
receptor potential and the AN fiber is phase-locked; at
high frequencies, there is a steady release of neurotransmitter
and the AN fiber responds with a rate increase but
without phase-locking.
OHCs and the Cochlear Amplifier
In the discussion of the basilar membrane, the need for
a source of energy in the cochlea, a cochlear amplifier,
was discussed. At present, two possible sources
of the amplification have been suggested. The first is
OHC motility [12.86,100] and the second is hair bundle
motility [12.101]. Although hair bundle motility could
function in both IHCs andOHCs, it seems likely that
a) Treshold (dB SPL) b)
100
80
60
40
20
0
0.1
IHC
damage
Normal
1 4
OHC
damage
Normal
0.1 1 4
Frequency (kHz)
Fig. 12.11a,b Schematic summary of AN tuning curves in
normal ears and in ears with damaged cochleae. (a) In regions
of the cochlea with remaining OHCs but damaged
IHCs tuning curves mainly show a threshold elevation.
(b) In regions with missing OHCs but intact IHCs there
is an elevation of threshold at frequencies near BF and
a substantial broadening of tuning. (After [12.51])