Download Lecture 20

Pinnae are the external part of a mammal’s ear. Their form
suggests function to improve ear sensitivity by intercepting
and reflecting sound toward the entrance to the ear canal.
In humans pinnae are immobile relative to the head, but in
other mammals dedicated muscles, along with head
turning, allow pinnae to act as effective sound collectors.
Bats echolocate using
sound pulses: ultrasonic
cries are emitted in this
leaf-nose bat through the
nostrils. The ‘leaf’ is a
sound ‘director’, a
beaming device.
Loud output sounds
create faint echoes by
which the animal steers.
Human –ear
Illustrations from
Gulick, W.L. 1971.
Hearing,
Physiology
and Psychophysics
Oxford Univ. Press,
London
Middle ear, suspensory ligaments
support 3 small bones, ossicles:
malleus, incus stapedius. There
are also 2 muscles: tensor
tympani muscle, stapedial
muscle.
Function of the middle ear
ossicles: acoustical transformer,
to match impedances between
fluid air and fluid ‘water’,
The chain of bones does this by leverage and by areal ratio. Sound
perilymph of cochlea.
collected over a (larger) eardrum area and ‘concentrated’ on the
(smaller) oval window at the stapedius footplate. Eardrum to
stapedius footplate has an area ratio of 21 to 1 [in humans].
Function of the
stapedius and tensor
tympani muscles is to
protect against sounds
damagingly intense to
the cochlea.
The contracting
stapedius pulls on the
stapes and rotates it so
that the large amplitudes
carried to it by the
malleus and incus are
“harmlessly expended”.
This protection is of
special importance to
bats because of the
necessary intensity of
their outgoing sounds
and the faintness of the
echoes. The bat needs
a very sensitive ear
while making a very
intense cry.
The cochlea is a spiral of
perilymph, getting smaller
toward its apex, coiled
upon itself within the skull.
The stapes inserts into an
oval window pushing
inward on the perilymph. A
round window is also
necessary to allow for
perilymph displacement
because of the high bulk
modulus of the cochlear
fluid. The round window
acts as a “pressure relief
point”. When the
stapedius pushes in on
the oval window the round
window membrane bulges
out.
The cochlear canals and the
basilar and tectorial
membranes within, gradually
taper – their size changes.
The mechanoreceptor
sensory cells (hair cells) sit on
the basilar membrane, their
‘cilia’ embedded in the
tectorial membrane above.
Like the sensory cells of the
locust ear, mammalian hair
cells discriminate sound
frequencies. They do this
based on where they are
located along the taper of
the cochlea. They respond
to travelling waves of the
basilar membrane that peak
at certain ‘places’.

Why is the cochlea tapered tonotopically? Why is the crista acustica of
katydids likewise tapered tonotopically?

Imagine it as it isn’t: the coiled cochlea of a constant diameter
throughout. This would prevent its function as a frequency analyser.
The taper and its relation to changing wavelengths of different sound
frequencies is the basis of frequency discrimination.

A remarkable example of convergent evolution between katydids and
mammals has been discovered recently. Katydids turn out to have a
fluid-filled acoustic vesicle as part of their ear (not previously known).
Both mammals and katydids convert sound waves from air into water
for sensory analysis – for transduction into neural impulses

Montealegre-Z F. et al. 2012. Convergent evolution between insect and
mammalian audition. Science 338: 968-
Sound is captured
by an eardrum,
impedance
matching occurs
in a ‘middle ear’
and then
tonotopic
frequency
analysis takes
place in the
cochlea of
mammals and in
the crista
acustica of
katydids
Impedance matching is required to
bring fluctuating forces into the fluid
Sensory processing: transduction of
pressure into neuron depolarizations
• Organs called ears are mechanical transducers and their essence is a
tympanum or eardrum which tracks the pressure [or displacement]
changes that associate with sound travelling through [the fluid] air.
• Insects have trachea, air-filled tubes coursing through their body to convey
gases. The tracheal system plays an important part in the workings of
ears. To ‘collect’ pressure changes requires a thin membrane of cuticle
and this must be free to move readily – it cannot be damped by body
fluids. So we have a tympanum backed by a tracheal air sac.
• The ear of insects is typically a pressure difference ear in that there are
two routes to the eardrum: internal and external. The movement of the
eardrum is thus a compounded effect of two path-lengths which may be
different.
• The moving membrane has special cells linked to it that perform the
actual transduction: converting pressure forces into depolarizing
membranes (the ‘information currency’ of the nervous system.
What is an ear and how does a locust discriminate sound
frequencies?
Ears as elaborate organs are typically bilateral, a right and a left. In the locust
each is situated within a recess in the first abdominal segment and sound has
access to the rear of both, because they touch internally via air sacs; thus they
are ‘pressure difference ears’ meaning the activity of each eardrum is a function
of sound reaching both front and back of the tympanum.
The plane of the tympanum is angled to face backward slightly.
The auditory ganglion of each ear is visible through the transparent tympanum,
its nerve running anteromedially to join the metathoracic ganglion.
Also visible on and through the tympanum are dark brown chitinous sclerites
(e.g., pyriform vesicle, folded body, styliform body, elevated process) that
lie on top of the tympanum.
At one time it was disputed
whether insects could
discriminate frequency, indeed
whether they even had airborne
hearing capacity; insect
‘baloonists’ experiments
involving interspersed calls
established otherwise.
First the anatomy using an
anchient but accurate note (it
will bear magnification) offered
as of historical interest.
The tympanum (ear drum)
is a very-much thinned
region of the cuticle with a
ganglion sitting more or
less in the middle. Behind
the tympanum, applied
overtop of ganglion and
acoustic nerve is a tracheal
sac.
Backing the membrane
with air is an important
adaptation: if the
tympanum were backed by
the haemolymph of the
circulatory system the
tympanum’s movements
would be significantly
damped by the blood and
it would not respond with
sensitivity to the airborne
sound.
Each chordotonal sensillum has at
its centre a neurosensory cell served
by accessory cell types; 60-80
sensilla occur in four groups (a-d).
The nerve cell dendrite ends in a
scolopale (‘cilium’) that transduces
the motions of e.g., the pyriform
vesicle or other cuticular eardrum
parts. Impinging sound sets the
tympanum in motion and the
sclerites and the ganglion move
relative to each other creating a
shear force at the scolopale and then
an action potential in the axon which
runs into the CNS. The cell’s position
on the eardrum, its mechanical
linkage, and the behaviour of the
eardrum itself, codes for particular
frequencies.
Listening to 3-kHz,
tympanal action
moves the whole
ganglion and the
pyriform vesicle
(pv) into the same
motion; so both
ends, K1 and pv,
fusiform body
move together.
But hit with a
pyriform
higher sound
vesicle
frequency of 10
kHz the relative
motions of
ganglion (K1) vs
pv become quite
different: so the
fusiform body is
shaken and jolted,
leading to many
firings of the
Stephen R.O., Bennet-Clark H.C. 1982. The anatomical and mechanical
chordotonal
basis of stimulation and frequency analysis in the locust ear. J. exp.
neurosensory cells Biol. 99: 279-314.
within.
James F.C. Windmill, Martin C. Gopfert and Daniel
Robert 2004. Tympanal travelling waves in migratory
locusts Journal of experimental Biology 208: 157168.
Scanning laser vibrometry used to investigate
the movements of the eardrum when
stimulated by different frequencies.
Frequency analysis in the locust involves a “travelling
wave that funnels mechanical energy to specific
tympanal locations, where distinct mechanoreceptor
neurones project”.
“For each frequency the tympanal deflections
do not stay in position, but travel across the
tympanum from posterior to anterior... At 3.3
kHz the wave travels across the thin
membrane, moving towards a focus point
located at the folded body...”
Travelling waves vs standing waves
eardrum movement when subjected to four
different frequencies;
scanning laser videos show the complex movement
of different regions; profiles: red is outward
movement of the tympanum and green is inward
movement
Summary
Windmill et al., concluded:
1.Thin and thick parts of the tympanum membrane are distinct mechanical
entities
2.The vibration patterns varied across the entire tympanal membrane in a
frequency specific manner
3. Distinct tympanal areas underwent maximal vibrations for a specific
frequency and were topographically linked to the attachment locations
of mechanoreceptor cell clusters (relay frequency specific information)
4.No support for standing wave. Rather data supports a travelling wavebecause the maximum deflections on the thin membrane moved across
the surface area (propagating through) whereas a standing wave would
not