Auditory receptors

1. AUDITORY RECEPTORS
•A. The inner ear:
•The auditory and vestibular systems are
intimately connected. The receptors for
both are located in the temporal bone, in
a convoluted chamber called the bony
labyrinth. A delicate continuous
membrane is suspended within the bony
labyrinth, creating a second chamber
within the first. This chamber is called
the membranous labyrinth. The entire
fluid-filled structure is called the inner
ear.
•The inner ear has two membrane-covered
outlets into the air-filled middle ear - the
oval window and the round window.
The oval window is filled by the plate of
the stapes, the third middle ear bone. The
stapes vibrates in response to vibrations
of the eardrum, setting the fluid of the
inner ear sloshing back and forth. The
round window serves as a pressure
valve, bulging outward as pressure rises
in the inner ear.

2. The oval window opens into a large central area within the inner ear called the vestibule. All of
the inner ear organs branch off from this central chamber. On one side is the cochlea, on the
other the semicircular canals. The utricle and saccule, additional vestibular organs, are adjacent
to the vestibule.
The membranous labyrinth is filled with a special fluid called endolymph. Endolymph is very
similar to intracellular fluid: it is high in potassium and low in sodium. The ionic composition
is necessary for vestibular and auditory hair cells to function optimally. The space between the
membranous and bony labyrinths is filled with perilymph, which is very much like normal
cerebral spinal fluid.
B. Auditory transduction:
The transduction of sound into a neural signal occurs in the cochlea.
If we were to unroll the snail-shaped cochlea, it would look like this:
As the stapes vibrates the oval window, the perilymph sloshes back and forth, vibrating the
round window in a complementary rhythm. The membranous labyrinth is caught between
the two, and bounces up and down with all this sloshing. Now let's take a closer look at the
membranous labyrinth. If we cut the cochlea in cross section, it looks like this:

3. The membranous labyrinth of the cochlea encloses the
endolymph-filled scala media. The two compartments of
the bony labyrinth, which house the perilymph, are
called the scalae vestibuli and tympani. Within the scala
media is the receptor organ, the organ of Corti. It rests
on part of the membranous labyrinth, the basilar
membrane.
A single turn of the cochlea has been outlined in blue.
You can see the auditory nerve exiting at the base of the
cochlea; it will travel through the temporal bone to the
brainstem.
The auditory hair cells sit within the organ of Corti. There are inner hair cells, which are the auditory
receptors, and outer hair cells, which help to "tune" the cochlea, as well as supporting cells. The sensitive
stereocilia of the inner hair cells are embedded in a membrane called the tectorial membrane. As the basilar
membrane bounces up and down, the fine stereocilia are sheared back and forth under the tectorial
membrane. When the stereocilia are pulled in the right direction, the hair cell depolarizes. This signal is
transmitted to a nerve process lying under the organ of Corti. This neuron transmits the signal back along the
auditory nerve to the brainstem. As with almost all sensory neurons (the exception is in the retina), its cell
body lies outside the CNS in a ganglion. In this case, the ganglion is stretched out along the spiralling center
axis of the cochlea, and is named the spiral ganglion. You can see most of the structures in this higher
magnification of the organ of Corti; unfortunately, the inner hair cells have been artifactually pulled away
from the tectorial membrane.

4. The basilar membrane is actually thinner and narrower at the base of the cochlea than at the tip
(apex), which seems backwards given that the cochlea is widest at the base. The properties of
the basilar membrane change as its shape changes; just as with guitar strings, thin things vibrate
to high pitches, and thick things vibrate to low pitches. This means that the basilar membrane
vibrates to high frequencies at the base of the cochlea and to low frequencies at the apex. A hair
cell at the base of the cochlea will respond best to high frequencies, since at those frequencies
the basilar membrane underneath it will vibrate the most. The key idea is that although the hair
cells are arranged in order along the basilar membrane, from high-frequency to low-frequency,
it is the properties of the basilar membrane that set up this gradient, not the properties of the
hair cells.
Our ability to discriminate two close frequencies is actually much better than one would predict
just from the mechanics of the basilar membrane. One theory to explain the mystery is that the
outer hair cells help to "sharpen the tuning". Outer hair cells can actually move (change length)
in response to nerve stimulation. If they could push the basilar membrane up and down, they
could amplify or damp vibrations at will, making the inner hair cells more or less responsive.
(Just like you can push a child higher and higher on a swing or bring her to a halt - it's all in
when you push.) An interesting philosophical question here is, if the outer hair cells can move
the basilar membrane, can that in turn move the oval window? And the stapes? And the
eardrum? Can the ear, in fact, work in reverse and become a speaker? You may laugh, but there
has been at least one case in the history of medicine of a patient complaining of persistent
whispering in her ear. She was dismissed as crazy, until one obliging doctor finally put his
stethoscope to her ear and listened. He could hear the whispering too. You can draw your own
moral from this story.
However, most cases of tinnitus (a persistent ringing, whistling, or roaring in the ears) are not
audible to the examiner. Little is known about the phenomenon, which is unfortunate because it
can be very distressing to the sufferer.

5. C. Central auditory pathways:
The auditory nerve carries the signal into the brainstem and synapses in the cochlear nucleus.
From the cochlear nucleus, auditory information is split into at least two streams, much like the
visual pathways are split into motion and form processing. Auditory nerve fibers going to the
ventral cochlear nucleus synapse on their target cells with giant, hand-like terminals.
Something about this tight connection allows the timing of the signal to be preserved to the
microsecond (action potentials are on the order of milliseconds, so it is no mean feat). The
ventral cochlear nucleus cells then project to a collection of nuclei in the medulla called the
superior olive. In the superior olive, the minute differences in the timing and loudness of the
sound in each ear are compared, and from this you can determine the direction the sound came
from. The superior olive then projects up to the inferior colliculus via a fiber tract called the
lateral lemniscus.
The second stream of information starts
in the dorsal cochlear nucleus. Unlike
the exquisitely time-sensitive
localization pathway, this stream
analyzes the quality of sound. The dorsal
cochlear nucleus, with fairly complex
circuitry, picks apart the tiny frequency
differences which make "bet" sound
different from "bat" and "debt". This
pathway projects directly to the inferior
colliculus, also via the lateral lemniscus.

6. Notice that both pathways are bilateral. The consequence
of this is that lesions anywhere along the pathway usually
have no obvious effect on hearing. Deafness is essentially
only caused by damage to the middle ear, cochlea, or
auditory nerve.
From the inferior colliculus, both streams of information
proceed to sensory thalamus. The auditory nucleus of
thalamus is the medial geniculate nucleus. The medial
geniculate projects to primary auditory cortex, located on
the banks of the temporal lobes.
Keep in mind, as you try to remember this pathway, that the auditory nuclei all seem to have counterparts in
other systems, making life confusing. Fibers from the cochlear nuclei and the superior olive (not the inferior)
travel up the lateral lemniscus (not the medial) to the inferior colliculus (not the superior), and then to the
medial geniculate (not the lateral). Try remembering the mnemonic, "S-L-I-M" .
D. The vestibular system
The purpose of the vestibular system is to keep tabs on the position and motion of your head in space. There
are really two components to monitoring motion, however. You must be able to detect rotation, such as what
happens when you shake or nod your head. In physics, this is called angular acceleration. You must also be
able to detect motion along a line - such as what happens when the elevator drops beneath you, or on a more
subtle note, what happens when your body begins to lean to one side. This is called linear acceleration. The
vestibular system is divided into two receptor organs to accomplish these tasks.
1. The semicircular canals detect angular acceleration. There are 3 canals, corresponding to the three
dimensions in which you move, so that each canal detects motion in a single plane. Each canal is set up as
shown below, as a continuous endolymph-filled hoop. The actual hair cells sit in a small swelling at the base
called the ampula.
2. The VOR
Although the VOR works on all three muscle pairs, the medial-lateral rectus pair, coupled to the horizontal
canal, is geometrically the easiest to draw

7. The Outer Ear
Sound energy spreads out from its sources. For a point source of sound, it spreads out according to the
inverse square law. For a given sound intensity, a larger ear captures more of the wave and hence more sound energy.
The outer ear structures act as part of the ear's preamplifier to enhance the sensitivity of hearing.
The auditory canal acts as a closed tube resonator, enhancing sounds in the range 2-5
kiloHertz.
The tympanic membrane or "eardrum" receives
vibrations traveling up the auditory canal and
transfers them through the tiny ossicles to the
oval window, the port into the inner ear. The
eardrum is some fifteen times larger than the
oval window, giving an amplification of about
fifteen compared to the oval window alone.
The Tympanic Membrane

8. Auditory Canal Resonance
The maximum sensitivity regions of human hearing can be modeled as closed tube
resonances of the auditory canal. The observed peak at about 3700 Hz at body
temperature corresponds to a tube length of 2.4 cm. The higher frequency
sensitivity peak is at about 13 kHz which is somewhat above the calculated 3rd
harmonic of a closed cylinder
The Ossicles
The three tiniest bones in the body form the coupling between the vibration of the
eardrum and the forces exerted on the oval window of the inner ear.
With a long enough lever, you can lift a big rock with a small applied force on the
other end of the lever. The amplification of force can be changed by shifting the pivot
point.
The ossicles can be thought of as a compound lever which achieves a multiplication of
force. This lever action is thought to achieve an amplification by a factor of about
three under optimum conditions, but can be adjusted by muscle action to actually
attenuate the sound signal for protection against loud sounds.

9. Ossicle Vibration
The vibration of the eardrum is
transmitted to the oval window of the
inner ear by means of the ossicles,
which achieve an amplification by
lever action. The lever is adjustable
under muscle action and may actually
attenuate loud sounds for protection of
the ear.
Inner Ear

10. The inner ear can be thought of as two
organs: the semicircular canals which serve
as the body's balance organ and the cochlea
which serves as the body's microphone,
converting sound pressure impulses from
the outer ear into electrical impulses which
are passed on to the brain via the
auditory nerve.
The basilar membrane of the inner ear plays
a critical role in the perception of pitch
according to the place theory.
Semicircular canals
The semicircular canals are the body's
balance organs, detecting acceleration in
the three perpendicular planes. These
accelerometers make use of hair cells
similar to those on the organ of Corti, but
these hair cells detect movements of the
fluid in the canals caused by
angular acceleration about an axis
perpendicular to the plane of the canal.
Tiny floating particles aid the process of
stimulating the hair cells as they move with
the fluid. The canals are connected to the
auditory nerve.

11. Sonar detection in bats
HOMING IN. The distance between pressure pulses in a bat's
ultrasonic chirps and echoes (represented by white and red wave
outlines, respectively) determines how small a target the predator can
detect.

12. Bats navigate using reflected sound waves. This process, known as echolocation,
allows these animals to "see" in the dark. To uncover objects, bats must first emit a
series of sound pulses. These pulses travel outward and strike objects. The pulses are
then reflected off the objects and return back to the bats. Detected by their large
ears, the sounds are quickly analyzed by the brain's echolocation center. This
analysis is so precise that the bat can locate moving fish through a critical analysis of
the ripples produced at the water's surface.
Flying Blind
Mammals can hear higher frequencies than other creatures can because of the characteristic arrangement
of tiny bones in their ears as well as the structure of their cochlea, or inner ear. Even among mammals,
however, hearing ability varies widely. Among people, most young adults can detect sounds with
frequencies between 20 hertz, or cycles per second, and 20 kilohertz (kHz). But even that highest tone is
low by echolocation standards: The typical bat's sonar chirps exploit frequencies as high as 120 kHz, and
bottlenose dolphins' calls include frequencies that range up to 150 kHz or so. Birds, however, can't
produce or hear the high-pitched, short-wavelength sounds needed to track insect-size targets. The few
birds that can echolocate use lower frequencies, and they do so only to navigate in the dark, says J. Jordan
Price, a biologist at St. Mary's College of Maryland in St. Mary's City. Even that limited capability
provides a benefit, however, because it enables members of those species to nest in caves and other places
that aren't readily accessible to predators. With one exception, all birds known to echolocate are swiftlets.
Birds in this group catch insects on the fly just as a bat does, but they do so in the daytime and track their
prey by sight, says Price. Scientists have typically relied on characteristics beyond size, shape, and color to
distinguish the members of one swiftlet species from those of another, simply because the birds have so
few distinguishing features, he notes. Until recently, all swiftlets known to echolocate fell within the
genus Aerodramus. Then, Price and his colleagues found a swiftlet in another genus—the pygmy swiftlet,
Collocalia troglodytes—sitting on its nest, in complete darkness, about 30 m inside a cave on an island in
the Philippines.

13. Evolution of the Eye

14. Evolution of the Eye:
When evolution skeptics want to attack Darwin's theory, they often point to the human eye. How could
something so complex, they argue, have developed through random mutations and natural selection,
even over millions of years?
If evolution occurs through gradations, the critics say, how could it have created the separate parts of
the eye -- the lens, the retina, the pupil, and so forth -- since none of these structures by themselves
would make vision possible? In other words, what good is five percent of an eye?
Darwin acknowledged from the start that the eye would be a difficult case for his new theory to explain.
Difficult, but not impossible. Scientists have come up with scenarios through which the first eye-like
structure, a light-sensitive pigmented spot on the skin, could have gone through changes and
complexities to form the human eye, with its many parts and astounding abilities.
Through natural selection, different types of eyes have emerged in evolutionary history -- and the
human eye isn't even the best one, from some standpoints. Because blood vessels run across the
surface of the retina instead of beneath it, it's easy for the vessels to proliferate or leak and impair
vision. So, the evolution theorists say, the anti-evolution argument that life was created by an
"intelligent designer" doesn't hold water: If God or some other omnipotent force was responsible for the
human eye, it was something of a botched design.
Bilogists use the range of less complex light sensitive structures that exist in living species today to
hypothesize the various evolutionary stages eyes may have gone through.

15. Here's how some scientists think some eyes may have evolved: The simple light-sensitive spot on the
skin of some ancestral creature gave it some tiny survival advantage, perhaps allowing it to evade a
predator. Random changes then created a depression in the light-sensitive patch, a deepening pit that
made "vision" a little sharper. At the same time, the pit's opening gradually narrowed, so light entered
through a small aperture, like a pinhole camera.
Every change had to confer a survival advantage, no matter how slight. Eventually, the light-sensitive
spot evolved into a retina, the layer of cells and pigment at the back of the human eye. Over time a lens
formed at the front of the eye. It could have arisen as a double-layered transparent tissue containing
increasing amounts of liquid that gave it the convex curvature of the human eye.
In fact, eyes corresponding to every stage in this sequence have been found in existing living species.
The existence of this range of less complex light-sensitive structures supports scientists' hypotheses
about how complex eyes like ours could evolve. The first animals with anything resembling an eye lived
about 550 million years ago. And, according to one scientist's calculations, only 364,000 years would
have been needed for a camera-like eye to evolve from a light-sensitive patch.