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:

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.