AUDITORY AND VESTIBULAR PATHWAYS
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.
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:
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| 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.
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.
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.
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.
E. The semicircular canals:
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.
The hair cells are arranged as a single tuft that
projects up into a gelatinous mass, the cupula. When you
turn your head in the plane of the canal, the inertia of the endolymph
causes it to slosh against the cupula, deflecting the hair cells.
Now, if you were to keep turning in circles, eventually the fluid
would catch up with the canal, and there would be no more pressure
on the cupula. If you stopped spinning, the moving fluid would
slosh up against a suddenly still cupula, and you would feel as
though you were turning in the other direction. This is the explanation
for the phenomenon you discovered when you were 5.
Naturally, you have the same arrangement (mirrored) on both sides of the head. Each tuft of hair cells is polarized - if you push it one way, it will be excited, but if you push it the other way, it will be inhibited. This means that the canals on either side of the head will generally be operating in a push-pull rhythm; when one is excited, the other is inhibited (see below). It is important that both sides agree as to what the head is doing. If there is disagreement, if both sides push at once, then you will feel debilitating vertigo and nausea. This is the reason that infections of the endolymph or damage to the inner ear can cause vertigo. However, if one vestibular nerve is cut, the brain will gradually get used to only listening to one side - this can actually be a treatment for intractable vertigo.
A large role of the semicircular canal system is
to keep your eyes still in space while your head moves around
them. If you nod and shake and swivel your head, you will find
that you have no trouble staying focused on this page. But hold
a piece of paper in front of you and shake it around, and your
eyes will not be able to keep up with the quick movements. The
reason is that the semicircular canals exert direct control over
the eyes, so they can directly compensate for head movements.
Recall that the eye is controlled by three pairs of muscles;
the medial and lateral rectus, the superior and inferior rectus,
and the inferior and superior oblique. You may also remember
that their directions of motion seemed to be at crazy diagonals.
Those same crazy diagonals are matched closely by the three planes
of the semicircular canals, so that a single canal (in general)
interacts with a single muscle pair. The entire compensatory
reflex is called the vestibulo-ocular reflex (VOR).
F. 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. Here is the setup, looking down at a person's head:
The lateral rectus muscle will pull the eye laterally, and the medial rectus will pull the eye medially, both in the horizontal plane. The horizontal canal detects rotation in the horizontal plane.
If you move your head to the left, you will excite the left horizontal canal, inhibiting the right. To keep your eyes fixed on a stationary point, you need to fire the right lateral rectus and the left medial rectus, to move the eyes to the right.
The pathway is as follows: the vestibular nerve
enters the brainstem and synapses in the vestibular nucleus.
Cells that received information from the left horizontal canal
project to the abducens nucleus on the right side, to stimulate
the lateral rectus. They also project to the oculomotor nucleus
on the left side, to stimulate the medial rectus. Although not
shown on the diagram, the same vestibular cells also inhibit the
opposing muscles (in this case, the right medial rectus, and the
left lateral rectus).
What about the other side? The right horizontal
canal is wired to the complementary set of muscles. Since it
is inhibited, it will not excite its target muscles (the right
medial rectus and the left lateral rectus), nor will it inhibit
the muscles you want to use (the right lateral rectus and the
left medial rectus). Got it? OK, then draw out what would happen
if you turned your head to the right.
A great deal of the VOR axon traffic travels via
a fiber highway called the MLF (medial longitudinal fasciculus).
The integrity of this tract is crucial for the VOR to work properly.
It is occasionally damaged by medial brainstem strokes.
G. The utricle and saccule:
The utricle and saccule detect linear acceleration.
Each organ has a sheet of hair cells (the macula) whose cilia
are embedded in a gelatinous mass, just like the semicircular
canals. Unlike the canals, however, this gel has a clump of small
crystals embedded in it, called an otolith (yes, all along you've
had rocks in your head). The otoliths provide the inertia, so
that when you move to one side, the otolith-gel mass drags on
the hair cells. Once you are moving at a constant speed, such
as in a car, the otoliths come to equilibrium and you no longer
perceive the motion.
The hair cells in the utricle and saccule are polarized,
but they are arrayed in different directions so that a single
sheet of hair cells can detect motion forward and back, side to
side. Each macula can therefore cover two dimensions of movement.
The utricle lays horizontally in the ear, and can detect any
motion in the horizontal plane. The saccule is oriented vertically,
so it can detect motion in the sagittal plane (up and down, forward
and back).
A major role of the saccule and utricle is to keep
you vertically oriented with respect to gravity. If your head
and body start to tilt, the vestibular nuclei will automatically
compensate with the correct postural adjustments. This is not
just something that happens if the floor tilts - if you watch
someone trying to stand still, you will notice constant small
wavers and rocking back and forth.
