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 ofthe inner ear organs branch off from this central chamber. On one side is the cochlea, on theother the semicircular canals. The utricle and saccule, additional vestibular organs, are adjacentto the vestibule.The membranous labyrinth is filled with a special fluid called endolymph. Endolymph is verysimilar to intracellular fluid: it is high in potassium and low in sodium. The ionic compositionis necessary for vestibular and auditory hair cells to function optimally. The space between themembranous and bony labyrinths is filled with perilymph, which is very much like normalcerebral 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 theround window in a complementary rhythm. The membranous labyrinth is caught betweenthe two, and bounces up and down with all this sloshing. Now lets take a closer look at themembranous 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 auditoryreceptors, and outer hair cells, which help to "tune" the cochlea, as well as supporting cells. The sensitivestereocilia of the inner hair cells are embedded in a membrane called the tectorial membrane. As the basilarmembrane bounces up and down, the fine stereocilia are sheared back and forth under the tectorialmembrane. When the stereocilia are pulled in the right direction, the hair cell depolarizes. This signal istransmitted to a nerve process lying under the organ of Corti. This neuron transmits the signal back along theauditory nerve to the brainstem. As with almost all sensory neurons (the exception is in the retina), its cellbody lies outside the CNS in a ganglion. In this case, the ganglion is stretched out along the spiralling centeraxis of the cochlea, and is named the spiral ganglion. You can see most of the structures in this highermagnification of the organ of Corti; unfortunately, the inner hair cells have been artifactually pulled awayfrom 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 ofthe basilar membrane change as its shape changes; just as with guitar strings, thin things vibrateto high pitches, and thick things vibrate to low pitches. This means that the basilar membranevibrates to high frequencies at the base of the cochlea and to low frequencies at the apex. A haircell at the base of the cochlea will respond best to high frequencies, since at those frequenciesthe basilar membrane underneath it will vibrate the most. The key idea is that although the haircells 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 thehair cells.Our ability to discriminate two close frequencies is actually much better than one would predictjust from the mechanics of the basilar membrane. One theory to explain the mystery is that theouter 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, theycould 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 - its all inwhen you push.) An interesting philosophical question here is, if the outer hair cells can movethe basilar membrane, can that in turn move the oval window? And the stapes? And theeardrum? Can the ear, in fact, work in reverse and become a speaker? You may laugh, but therehas been at least one case in the history of medicine of a patient complaining of persistentwhispering in her ear. She was dismissed as crazy, until one obliging doctor finally put hisstethoscope to her ear and listened. He could hear the whispering too. You can draw your ownmoral from this story.However, most cases of tinnitus (a persistent ringing, whistling, or roaring in the ears) are notaudible to the examiner. Little is known about the phenomenon, which is unfortunate because itcan 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 thevisual pathways are split into motion and form processing. Auditory nerve fibers going to theventral 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 themicrosecond (action potentials are on the order of milliseconds, so it is no mean feat). Theventral cochlear nucleus cells then project to a collection of nuclei in the medulla called thesuperior olive. In the superior olive, the minute differences in the timing and loudness of thesound in each ear are compared, and from this you can determine the direction the sound camefrom. The superior olive then projects up to the inferior colliculus via a fiber tract called thelateral lemniscus.The second stream of information startsin the dorsal cochlear nucleus. Unlikethe exquisitely time-sensitivelocalization pathway, this streamanalyzes the quality of sound. The dorsalcochlear nucleus, with fairly complexcircuitry, picks apart the tiny frequencydifferences which make "bet" sounddifferent from "bat" and "debt". Thispathway projects directly to the inferiorcolliculus, also via the lateral lemniscus.
6. Notice that both pathways are bilateral. The consequenceof this is that lesions anywhere along the pathway usuallyhave no obvious effect on hearing. Deafness is essentiallyonly caused by damage to the middle ear, cochlea, orauditory nerve.From the inferior colliculus, both streams of informationproceed to sensory thalamus. The auditory nucleus ofthalamus is the medial geniculate nucleus. The medialgeniculate projects to primary auditory cortex, located onKeep in mind, as you try to remember this pathway, that the auditory nuclei all seem to have counterparts inthe banks of the temporal lobes.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 themedial geniculate (not the lateral). Try remembering the mnemonic, "S-L-I-M" .D. The vestibular systemThe purpose of the vestibular system is to keep tabs on the position and motion of your head in space. Thereare really two components to monitoring motion, however. You must be able to detect rotation, such as whathappens when you shake or nod your head. In physics, this is called angular acceleration. You must also beable to detect motion along a line - such as what happens when the elevator drops beneath you, or on a moresubtle note, what happens when your body begins to lean to one side. This is called linear acceleration. Thevestibular 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 threedimensions in which you move, so that each canal detects motion in a single plane. Each canal is set up asshown below, as a continuous endolymph-filled hoop. The actual hair cells sit in a small swelling at the basecalled the ampula.2. The VORAlthough the VOR works on all three muscle pairs, the medial-lateral rectus pair, coupled to the horizontalcanal, is geometrically the easiest to draw
7. The Outer EarSound energy spreads out from its sources. For a point source of sound, it spreads out according to theinverse 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 ears 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 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.
8. Auditory Canal ResonanceThe maximum sensitivity regions of human hearing can be modeled as closed tuberesonances of the auditory canal. The observed peak at about 3700 Hz at bodytemperature corresponds to a tube length of 2.4 cm. The higher frequencysensitivity peak is at about 13 kHz which is somewhat above the calculated 3rdharmonic 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 VibrationThe vibration of the eardrum istransmitted to the oval window of theinner ear by means of the ossicles,which achieve an amplification bylever action. The lever is adjustableunder muscle action and may actuallyattenuate loud sounds for protection ofthe ear. Inner Ear
10. The inner ear can be thought of as twoorgans: the semicircular canals which serveas the bodys balance organ and the cochleawhich serves as the bodys microphone,converting sound pressure impulses fromthe outer ear into electrical impulses whichare passed on to the brain via theauditory nerve.The basilar membrane of the inner ear playsa critical role in the perception of pitchaccording to the place theory.Semicircular canalsThe semicircular canals are the bodysbalance organs, detecting acceleration inthe three perpendicular planes. Theseaccelerometers make use of hair cellssimilar to those on the organ of Corti, butthese hair cells detect movements of thefluid in the canals caused byangular acceleration about an axisperpendicular to the plane of the canal.Tiny floating particles aid the process ofstimulating the hair cells as they move withthe fluid. The canals are connected to theauditory nerve.
11. Sonar detection in bats HOMING IN. The distance between pressure pulses in a bats ultrasonic chirps and echoes (represented by white and red waveoutlines, 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 aseries of sound pulses. These pulses travel outward and strike objects. The pulses arethen reflected off the objects and return back to the bats. Detected by their largeears, the sounds are quickly analyzed by the brains echolocation center. Thisanalysis is so precise that the bat can locate moving fish through a critical analysis ofthe ripples produced at the waters surface. Flying BlindMammals can hear higher frequencies than other creatures can because of the characteristic arrangementof 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 withfrequencies between 20 hertz, or cycles per second, and 20 kilohertz (kHz). But even that highest tone islow by echolocation standards: The typical bats sonar chirps exploit frequencies as high as 120 kHz, andbottlenose dolphins calls include frequencies that range up to 150 kHz or so. Birds, however, cantproduce or hear the high-pitched, short-wavelength sounds needed to track insect-size targets. The fewbirds that can echolocate use lower frequencies, and they do so only to navigate in the dark, says J. JordanPrice, a biologist at St. Marys College of Maryland in St. Marys City. Even that limited capabilityprovides a benefit, however, because it enables members of those species to nest in caves and other placesthat arent 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 theirprey by sight, says Price. Scientists have typically relied on characteristics beyond size, shape, and color todistinguish the members of one swiftlet species from those of another, simply because the birds have sofew distinguishing features, he notes. Until recently, all swiftlets known to echolocate fell within thegenus 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 inthe Philippines.
13. Evolution of the Eye
14. Evolution of the Eye:When evolution skeptics want to attack Darwins theory, they often point to the human eye. How couldsomething 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 ofthe eye -- the lens, the retina, the pupil, and so forth -- since none of these structures by themselveswould 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-likestructure, a light-sensitive pigmented spot on the skin, could have gone through changes andcomplexities 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 thehuman eye isnt even the best one, from some standpoints. Because blood vessels run across thesurface of the retina instead of beneath it, its easy for the vessels to proliferate or leak and impairvision. So, the evolution theorists say, the anti-evolution argument that life was created by an"intelligent designer" doesnt hold water: If God or some other omnipotent force was responsible for thehuman eye, it was something of a botched design.Bilogists use the range of less complex light sensitive structures that exist in living species today tohypothesize the various evolutionary stages eyes may have gone through.
15. Heres how some scientists think some eyes may have evolved: The simple light-sensitive spot on theskin of some ancestral creature gave it some tiny survival advantage, perhaps allowing it to evade apredator. Random changes then created a depression in the light-sensitive patch, a deepening pit thatmade "vision" a little sharper. At the same time, the pits opening gradually narrowed, so light enteredthrough a small aperture, like a pinhole camera.Every change had to confer a survival advantage, no matter how slight. Eventually, the light-sensitivespot evolved into a retina, the layer of cells and pigment at the back of the human eye. Over time a lensformed at the front of the eye. It could have arisen as a double-layered transparent tissue containingincreasing 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 hypothesesabout how complex eyes like ours could evolve. The first animals with anything resembling an eye livedabout 550 million years ago. And, according to one scientists calculations, only 364,000 years wouldhave been needed for a camera-like eye to evolve from a light-sensitive patch.