Special Senses II: The Ear

Objectives for This Exercise

The ear is an extraordinarily complex organ with two functions: sound reception and maintenance of positional equilibrium.  Although at first it might seem odd that both of these functions should be combined in one organ, as we'll see in this exercise, the logic of doing so becomes obvious upon closer examination.  In both cases, nervous impulses are generated by the physical displacement of "hairs" inside the ear, and the mechanics of the auditory and balance sense are very similar.

Regions of The Ear

The ear has three distinct parts: the outer ear (the pinna and external auditory meatus); the middle ear (the tympanic cavity, the auditory ossicles, and the Eustachian tube); and the inner ear (the sealed, fluid filled chambers responsible for transduction of sound into nervous impulses and for providing information about body position).

In this exercise we'll look mainly at the inner ear, with a view to understanding the relationship between its microscopic structure and its twin functions.  We'll not deal with the middle ear at all, and only briefly with the external ear.  Those parts are best studied using gross anatomical techniques.

It's exceedingly difficult—heck, no, it's flat out impossible—to visualize the three dimensional structure of the inner ear solely from two dimensional microscope slides.  You are urged to compare what you see on the slides with the model provided.

The Labyrinths of the Ear

The classic description of the ear includes a discussion of the two "labyrinths" of which it's composed.

The bony labyrinth consists of the convoluted channels in the actual bone of the skull.  Inside these channels in bone sits the membranous labyrinth, components of which are the actual functional structures for balance and hearing.

It's necessary to remember that the one labyrinth is inside the other.  Their relationship is like that between an automobile tire and an inner tube;  the tire is the bony labyrinth and the inner tube the membranous labyrinth.

The bony labyrinth is divisible into several regions:  the vestibule, the semicircular canals and the cochlea.  The first two contain those parts of the membranous labyrinth that are involved in the balance sense.  The last part contains the portion of the membranous labyrinth that is involved in hearing perception.

There is considerable confusion about the cochlea, by the way.  It's NOT the place in which transduction of sound occurs.  That happens in the cochlear duct, which is that part of the membranous labyrinth inside the bony cochlea.

Similarly, the semicircular canals, contrary to what you may have been told, aren't the actual site of balance sensation.  That occurs in semicircular ducts, elements of the membranous labyrinth which run through those canals.  This may seem to be hairsplitting, perhaps, but it's not.  There are important functional and anatomic distinctions between the two labyrinths.

Perilymphatic Spaces

The vestibule, cochlea, and semicircular ducts all are connected.  The vestibule is a fairly large space, and coming off it are the bony cochlea and the semicircular canals.  The cochlea is a corkscrew shaped passageway through the bone of the skull, and the semicircular ducts are simple loops that come off and return to the vestibule.  The bony labyrinth contains a small amount of fluid, the perilymph that fills it and in which the structures of the membranous labyrinth are bathed.  Perilymph somewhat resembles extracellular fluid.  To return to the tire/tube analogy, it's as if there were a small amount of water in the space between the inside surface of the tire and the outside surface of the tube.  It acts as a shock buffer and a means of transmitting vibrations to parts of the membranous labyrinth.

The perilymphatic spaces are functionally connected to the subarachnoid space in the meningeal covering of the brain, and this may be the route by which excess perilymph is removed.  The actual site of perilymph production is a matter of debate.

Endolymphatic Spaces

The actual perception of balance, acceleration, and sound transduction all take place inside the spaces of the membranous labyrinth.

The membranous labyrinth, too, is filled with fluid.  This fluid is endolymph, and it's very different in composition from perilymph.  Endolymph is secreted by the cells making up the lining of the membranous labyrinth, and (unlike perilymph or tissue fluid) it's high in potassium and low in sodium and soluble proteins.  The functional significance of the differences between these two fluids is not clear.

The Apparatus of Sound Transduction

Let's first examine the apparatus for sound transduction.  Begin with slide 1206, which shows most of the structures of the inner ear. At low magnification, you will easily recognize that most of this section consists of a chunk of bone, with various shaped cavities in it.  These cavities are the bony labyrinth, and within them are the membranous labyrinth structures.

To see this region, click here.

Cochlear Spiral:  Scala Vestibuli, Scala Tympani, Scala Media

Look first at the cochlea and the cochlear duct which fills it.  The cochlea is a spiral channel through the bone, and in the middle of it is a large bony projection which looks something like a wood screw, cut longitudinally.  This is the modiolus. Resting inside the cochlea is the cochlear duct, in which the events related to sound transduction occur.  The duct follows the spiral of the cochlear lumen.

To see this in some more detail, click here.

The cochlear duct is filled with endolymph, but the cochlea which surrounds is filled with perilymph.  (The former is membranous labyrinth, the latter is bony labyrinth.)  In cross section, each turn of the spiral shows three chambers; the first is the scala media, which is the cochlear duct itself, and in which are located the parts of the sound transduction apparatus.

Above the scala media, and separated from it by the very thin vestibular membrane is the scala vestibuli. This is continuous with the vestibule, another perilymphatic space, and so of course the fluids in the two areas are in continuity.

Below the scala media, and separated from it by the thick basilar membrane, is the scala tympani.  The scala tympani is also a perilymphatic space, is also in communication with the vestibule, and is also in communication with the scala vestibuli.

The best analogy I can think of here is the double spiral ramp of a vertical parking garage.  Think of the scala vestibuli as the "UP" ramp, corkscrewing its way to the top of the garage, and of the scala tympani as the "DOWN" ramp, doing exactly the reverse.  If you were to enter the garage at the bottom you could drive to the top, where the two ramps meet, and come back down again into the lobby of the garage (i.e., the vestibule).  To complete the analogy, think of the scala media as a water filled pipe lying between the two ramps.  Remember this picture, as we will come back to it later in considering the mechanism of sound transduction.

Scala Media & Organ of Corti

Inside the cochlear duct (scala media) itself is the actual organ of sound transduction.  This is the organ of Corti (Marquis Alphonso Corti, 1822-1888, an Italian anatomist).  The organ of Corti is a long, flat, spiral structure which follows the turns of the cochlear duct as it winds up to the top of the cochlear spiral. It is inside an endolymphatic space.

When the organ of Corti is viewed in cross section (as you see it in this slide) you'll see two groups of cells sitting on the basilar membrane; these are surmounted by a thin, transparent, and acellular tectorial membrane.  The tectorial membrane is made of a keratin-like material, and in H&E sections rather resembles a piece of fingernail.  It's anchored at one end to the wall of the cochlear duct, and it's quite stiff.  It, too, is spiraled and it covers the two groups of cells all the way up to the top of the spiraling scala media.

To see an example of this, click here.

The cells in question are hair cells and they are the actual transducers of mechanical movement into neural impulses.  Each is an epithelioid cell with projecting "hairs" that contact the bottom of the overlying tectorial membrane.  It's important to realize that these aren't "hairs" at all, of course; they are microvilli and modified cilia.  It should be fairly obvious that any movement of the flexible basilar membrane will deform the hair cell projections against the tectorial membrane.  The arrangement is what you would have if you laid a playing card against the bristles of a toothbrush.  Any movement of the brush will cause its bristles to scrape along the bottom of the fixed, stiff card.

The hair cells are transducers, specialized detector elements, and they're surrounded by sensory fibers of neurons whose cell bodies are in the spiral ganglion.  The spiral ganglion is sensory and the neurons in it are of the pseudounipolar type, just as with any other craniospinal ganglion.  Deformation of the hair cell projections causes changes in their membrane potentials, which are detected by the cells of the spiral ganglion.  Thus, mechanical movement of the basilar membrane relative to the fixed tectorial membrane results in the generation of a nervous signal.

Mechanics of Sound Detection

Let's at this point go back to the parking garage model.  Imagine you are driving up the UP ramp (the scala vestibuli) to the top, and the floor of the ramp is flexing beneath the weight of your car.  The movement is mechanically transmitted to that drainpipe underneath the ramp.  Then you reach the top of the spiral, cross over to the DOWN ramp, and as your car passes along on the way back to the lobby, the air it displaces impinges on the drainpipe from the underside.  This is a crude depiction of the method by which mechanical movement of fluids is transduced into sound perception.

Sound waves coming in via the external auditory meatus cause the eardrum to move back and forth.  In the middle ear, a series of small bones constituting a lever system magnifies this initial signal and mechanically transmits it to an opening in the side of the vestibule.  The pulsations of this movement are then sent into the perilymph as actual fluid waves.

The waves induced in the perilymph of the vestibule travel UP the scala vestibuli to the top of the cochlea.  Since the scala tympani and the scala vestibuli are in communication at the top of the spiral (the term for this area is the helicotrema), the fluid pressure wave in the perilymph "crosses over" and starts back DOWN again, via the scala tympani.  Hence, there is perilymphatic fluid moving on both sides of the scala media.

All this sloshing back and forth in the perilymph causes the suspended cochlear duct (which is filled with endolymph) to vibrate as well.  This inevitably moves the basilar membrane up and down, causing scraping of the hair cells along the bottom of the tectorial membrane.  The hair cells send the resultant signal out via the cells of the spiral ganglion.

The cochlear nerve consists of the axonal projections of the cell bodies in the spiral ganglion and, like any other sensory nerve, runs back into the brain.  In this case, it becomes one of the components of cranial nerve VIII, the vestibulocochlear nerve.

Balance Sensation

So far, nothing has been said about the organs involved in detection of balance and acceleration.  The mechanics of the balance sense, while different in detail, are similar in many ways to those of sound detection.  Both involve hair cells as transducers and both require fluid movement.  Both occur in endolymphatic spaces.  Both send their output to the central nervous system via pseudounipolar neurons of the spiral ganglion.

The Vestibular Apparatus

We have repeatedly referred to the vestibule, that portion of the bony labyrinth from which the cochlea and the semicircular canals arise.  As with other bony labyrinth, the vestibule and the canals are filled with perilymph, and in them are suspended parts of the membranous labyrinth, constituting the vestibular apparatus of balance sensation.

As is the case with the cochlear duct, the vestibular apparatus is filled with endolymph and sealed off from the surrounding perilymphatic spaces.  The vestibular apparatus consists of three large dilated areas and three semicircular ducts which run through channels of the bony labyrinth.

Semicircular Ducts

The semicircular ducts (and the semicircular canals through which they run) are oriented in three planes of movement:  vertical, horizontal, and anterior-posterior.  Inside the semicircular ducts are elevated areas of epithelium, covered with hair cells similar to those of the organ of Corti.  These are the cristae ampullares, and they can be thought of as analogous to speed bumps in a roadway, because they project out into the lumen of the semicircular duct.  You will see one of these structures on slide 1206.

To see an example of a semicircular duct inside a semicircular canal, click here.

Now look at the crista itself.  It's covered with a pseudostratified epithelium composed of hair cells and some supporting cell types.  The hair cells, like those of the sound detection system, are transducer cells with deformable elements on the free surface.  At their bases, they are surrounded by the ends of nerve fibers from the vestibular branch of the vestibulocochlear nerve.  The hairs in this organ are polarized—that is, they are of different lengths, so that there is a directionality in their arrangement with respect to the axis of the semicircular duct.

Movement of the fluid in the semicircular duct will obviously cause deformation of the hairs, and this in turn will cause depolarization of the hair cells and a change in their membrane potential.  This change is transmitted via the sensory fibers and the vestibulocochlear nerve to the brain.  As you can see, this is the same sort of mechanism at work in hearing; mechanical movement of fluid is translated into neural impulses.  Movement of the head in any direction causes the fluid in the semicircular ducts to flow relative to the hairs, and sends the sensation of movement to the brain.

To see another example of a crista ampullaris, click here.

If you examine slide 1206 carefully, you'll be able to see the vestibular branch of the nerve where it emerges from the bone, and joins the cochlear branch to form the vestibulocochlear nerve coming from the spiral ganglion.

Function of the System

This system is designed to detect acceleration—i.e., changes in speed or direction over a given time.  Any movement of the head relative to the three planes in which the semicircular ducts are fixed produces a corresponding movement of the endolymph inside them.  This deforms the hairs and provides information to the brain about the direction of movement.  However, if the motion becomes constant, (that is, if acceleration stops) movement becomes constant, the fluid "catches up" to the movement of the head, and the stimulus ends.  You experience this every time you drive your car away from a stoplight:  when your motion settles down to a steady speed, you no longer perceive the acceleration because it has ceased.

This can have unexpected consequences.  Many other clues to position and acceleration exist, principally visual input.  It's necessary for the brain to sort these inputs correctly and decide what the body is really doing, and how it's moving.  Pilots frequently have problems with conflicts between the information coming from visual and vestibular inputs, especially if they swivel their heads in the cockpit during some aerobatic maneuver.  This introduces odd forces on the vestibular apparatus, and the pilot gets disoriented.  The brain can't sort out a reasonable solution to the questions, "Where am I? Which way am I moving?"  You get the same effect by spinning rapidly on a carousel or ice skates and then stopping; the visual input becomes stable, but the vestibular sensation tells you are still moving.  This results from a persistence of the vestibular signal even after motion has ceased.  Since the integration of the signals is done in the CNS, the system is (as you might expect) affected by alcohol and many other drugs.

Which Way Is Down?

There is one other portion of the vestibular system not on your slides, but a demonstration has been provided.  This is the maculae.  The maculae are located in the large bulbous expansions of the vestibular system (also filled with endolymph), and their function is to provide a reference to the pull of gravity.

The expanded portions of the membranous labyrinth are the saccule and the utricle.  Both these regions contain areas with hair cells similar to those in the cristae.  Gravitational detection is clearly an important consideration for any terrestrial animal (especially arboreal ones).

Unlike the semicircular duct system, the sensory areas of the maculae don't depend on fluid movement.  The maculae have within them small calcareous secretions, the otoliths, or "ear stones," which lie on top of the hairs of the hair cells.  Otoliths are formed in the embryo as part of normal development.

To see examples of the maculae and the otoliths, click here.

When an animal is stable with respect to gravity, the force the otoliths exert on the hairs is more or less constant, and the brain interprets this as "I am not moving downward, I'm sitting still on the ground."

If, however, the animal falls, the otoliths exert different forces on the hairs, causing a different input to the CNS.  If you've ever been in an airplane that's in a stall you will have experienced the peculiarly unpleasant feeling of having your otoliths exert no force on the hairs at all—the sensation of being in free fall.  This often causes disorientation problems for pilots, too, and for astronauts, who are constantly "falling" around the earth while in orbit.

Otoliths in many non-mammals and invertebrates often contain iron and are affected by magnets.  If you tape a magnet to the shell of a crab, for example, his otoliths are attracted to it, and he'll swim upside down.  Some migratory birds make use of the magnetic field of the earth for navigation, and have a built in compass in their otoliths.  Reports indicate that taping magnets to the heads of pigeons and migratory waterfowl causes them to lose their way.

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