Trace the pathways of light and sound through the eye and ear, explain balance, taste and smell, and analyze homeostatic imbalances of the special senses

Learning goal: Describe the special senses of sight, hearing, balance, smell and taste as networks of distinct structures and feedback processes; trace the pathway of light through the eye to the photoreceptors and the optic nerve and identify the eye's primary and accessory structures; identify the structures of the outer, middle and inner ear and explain the perception of sound and the maintenance of equilibrium; describe the location, structure and afferent pathways of taste and smell receptors; compare the vertebrate and molluscan eye and propose evolutionary hypotheses; and explain homeostatic imbalances such as myopia, hyperopia, astigmatism, glaucoma, cataracts, hearing loss, otitis media, vertigo, tinnitus, anosmia and olfactory auras.

What makes a sense special. The general senses, touch, pressure, pain and temperature, use receptors scattered through the skin and organs. The special senses consist of sight, hearing, balance, smell and taste, and each is served by a complex organ in the head with distinct structures that capture one kind of stimulus, convert it into electrical impulses and send them along a dedicated pathway to the brain, where they are integrated. Each also involves feedback: the pupil constricts in bright light, the lens changes shape for distance, the tiny muscles of the middle ear stiffen against loud sound, and the brain adjusts what it attends to.

The eye: primary and accessory structures. The accessory structures provide lubrication, protection and support. Eyebrows shade and divert sweat; eyelids with their lashes blink to spread fluid and block debris; the conjunctiva, a thin membrane lining the lids and covering the white of the eye, produces mucus; the lacrimal apparatus makes tears in a gland above the outer corner of the eye, washes them across the surface and drains them through small ducts at the inner corner into the nasal cavity, which is why crying makes the nose run; and six extrinsic eye muscles move the eyeball. The eyeball itself has three layers. The outer fibrous layer is the white sclera, which becomes the transparent cornea at the front. The middle vascular layer is the choroid, rich in vessels and dark pigment that absorbs stray light, continuing forward as the ciliary body, whose muscle adjusts the lens and whose processes secrete the fluid in front of the lens, and the iris, the colored ring of muscle whose opening is the pupil. The inner layer is the retina, which contains the photoreceptors, rods and cones, and the nerve cells that process their signals. The lens, a transparent flexible disc, hangs behind the pupil on suspensory ligaments attached to the ciliary body. The space in front of the lens holds watery aqueous humor, which is continuously made and drained; the large space behind holds the jelly-like vitreous humor, which keeps the eyeball's shape.

Tracing the pathway of light. Light enters through the cornea, which bends it more than any other part of the eye because of the sharp change from air to tissue; it passes through the aqueous humor and the pupil, whose diameter the iris adjusts to the brightness; the lens bends it further and fine-tunes the focus; it crosses the vitreous humor and is focused onto the retina at the visual axis, forming an image that is upside down and reversed, which the brain corrects. For a distant object the ciliary muscle is relaxed, the ligaments are taut and the lens is flat; for a near object the ciliary muscle contracts, the ligaments slacken and the lens bulges, bending light more. This adjustment is accommodation. At the retina, light must pass through the layers of nerve cells before reaching the photoreceptors, which lie at the back against the pigmented layer. Rods, roughly 120 million of them, respond to dim light and give black-and-white vision, mostly in the periphery; cones, roughly 6 million, need brighter light and come in three types most sensitive to short, medium and long wavelengths, whose combined signals the brain interprets as color. Cones are concentrated at the fovea, a small pit at the center of the visual axis that gives the sharpest vision. Stimulation of a photoreceptor changes a light-sensitive pigment and alters the signal to bipolar cells, then to ganglion cells, whose axons gather at the optic disc and leave the eye as the optic nerve. Because there are no photoreceptors at the optic disc, each eye has a blind spot, which the brain fills in. The optic nerve transmits the electrical impulses to the brain, where at the optic chiasm the fibers from the inner half of each retina cross so that each hemisphere receives the opposite half of the visual field, and the impulses are translated into an image in the occipital lobe. Binocular vision, the overlap of the two eyes' fields, lets the brain compare the slightly different images and judge depth; close one eye and try to touch the tips of two pencils together at arm's length and the loss of depth judgment is obvious.

Imbalances of sight. Myopia, nearsightedness, occurs when the eyeball is too long or the cornea too curved, so distant objects focus in front of the retina; a concave (diverging) lens corrects it. Hyperopia, farsightedness, occurs when the eyeball is too short, so near objects focus behind the retina; a convex (converging) lens corrects it. Astigmatism is an uneven curvature of the cornea or lens that blurs at every distance; a cylindrical lens corrects it. With age the lens stiffens and can no longer bulge for near vision, a change called presbyopia, corrected by reading glasses. A cataract is a clouding of the lens that scatters light; the lens can be replaced surgically. Glaucoma is usually a failure of the aqueous humor to drain, so pressure inside the eye rises and gradually damages the optic nerve, often unnoticed until peripheral vision is lost; it becomes more common with age. Conjunctivitis is inflammation of the conjunctiva. Color blindness is most often the lack or alteration of one cone pigment, usually affecting red and green discrimination; because the genes lie on the X chromosome it is far more common in males, and tinted lenses can help some people distinguish colors though there is no cure. A detached retina occurs when the neural layer of the retina separates from the pigmented layer behind it, cutting the photoreceptors off from their support. These descriptions explain the structure that has failed; they are not medical advice.

Comparing the vertebrate and the molluscan eye. The eye of a squid or octopus is remarkably like ours: a camera-type eye with a cornea, iris, lens and retina. Yet the two evolved independently from a light-sensitive patch in different ancestors, a case of convergent evolution, and the difference in their construction shows it. In the vertebrate eye the retina is inverted: the photoreceptors point away from the light, the nerve cells and blood vessels lie in front of them, and the axons must pass back through the retina at the optic disc, producing a blind spot. The neural retina is not joined to the pigmented layer by firm junctions; it is held against it by weaker forces, including fluid pumping by the pigment cells, an adhesive matrix between the layers and support from the vitreous, which is why it can detach (for example when a tear lets fluid seep beneath it or the shrinking vitreous pulls on it). In the cephalopod eye the retina is everted: the photoreceptors face the light directly, the axons leave from behind, there is no blind spot, and the retina is not a separate loosely attached layer in front of a pigment layer, so the particular mechanism that produces detachment in vertebrates is absent (the classroom claim that mollusks can never suffer retinal injury goes further than the anatomy supports). Cephalopods also focus by moving the lens back and forth rather than changing its shape. A plausible evolutionary hypothesis for the origin of any such eye starts with a patch of photosensitive cells that told an animal whether it was in light or shadow. The possible adaptive significance of this photosensitivity is that even crude light detection helps an animal survive: a passing shadow can warn of a predator, and light direction helps it find the surface, stay in shelter or time daily activity; a cup-shaped patch added information about the direction of light; narrowing the cup's opening sharpened the image; a transparent covering protected the cup and, thickened, became a lens. Each step is found in some living animal, and each offers an advantage over the previous one, so no step required the finished eye to be useful. Comparisons among vertebrates make the same point: a hawk's retina has a far higher density of cones than ours, giving sharper vision, and a squid's very large eyes gather light in the dim sea.

The ear: outer, middle and inner. The outer ear is the auricle (pinna), which collects sound, and the external acoustic meatus, the canal leading to the tympanic membrane (eardrum). The middle ear is an air-filled chamber in the temporal bone containing three tiny bones, the ossicles: the malleus attached to the eardrum, the incus, and the stapes, whose footplate sits in the oval window of the inner ear; the auditory (pharyngotympanic) tube connects the chamber to the throat and equalizes pressure, which is what pops when you swallow in an aircraft. The inner ear is a set of fluid-filled chambers and tubes in the bone: the cochlea, a coiled tube for hearing; the vestibule, with two sacs, for balance in a straight line; and three semicircular canals, at right angles to one another, for rotation.

Hearing. Sound waves funnel down the canal and vibrate the tympanic membrane. The ossicles carry the vibration across the middle ear and, because they act as levers and concentrate the force of the large eardrum onto the small oval window, they amplify the pressure many times. The stapes pushes on the oval window and sets up pressure waves in the cochlear fluid. These waves make the basilar membrane, which runs along the cochlea, vibrate; it is narrow and stiff near the base, where high frequencies make it move, and wide and flexible near the apex, where low frequencies act, so pitch is encoded by position. Resting on the basilar membrane is the spiral organ, rows of hair cells whose hair-like stereocilia bend against an overlying membrane as the basilar membrane moves; bending opens channels, the hair cell releases neurotransmitter, and fibers of the cochlear branch of the vestibulocochlear nerve carry impulses to the brainstem and on to the temporal lobe. Young adults hear roughly 20 to 20,000 vibrations per second, and the upper limit falls with age. Hearing loss is of two kinds. Conductive loss is a failure to carry sound to the inner ear: wax blocking the canal, fluid behind the eardrum, or ossicles that no longer move freely. Sensorineural loss is damage to the hair cells or the nerve: loud noise, aging and some drugs kill hair cells, which in humans do not regrow. Otitis media is infection of the middle ear, common in children because their auditory tube is shorter and more horizontal, so it drains poorly. Tinnitus, a ringing or hissing that no one else hears, often follows hair cell damage.

Balance. The vestibule's two sacs contain patches of hair cells covered by a jelly weighted with tiny calcium carbonate crystals; when the head tilts or the body accelerates in a straight line, the weighted jelly lags and bends the hairs, signaling the position of the head relative to gravity. Each semicircular canal has a swelling with a tuft of hair cells in a jelly cap; when the head rotates, the fluid in the canal lags behind and deflects the cap, signaling rotation in that plane. These signals travel by the vestibular branch of the same nerve to the brainstem and cerebellum, which combine them with vision and with signals from the muscles and joints to keep the body upright and the eyes steady. Vertigo, a false sensation of spinning, arises when the vestibular signals are wrong, as when crystals come loose or the inner ear is inflamed; motion sickness arises when the vestibular signals conflict with what the eyes report, as in a moving vehicle while reading; Meniere's disease is a disorder of the inner ear fluid that causes episodes of vertigo, tinnitus and hearing loss. The middle ear bones themselves have an evolutionary history: comparative anatomy and fossils show that the malleus and incus of mammals are derived from bones that formed the jaw joint in earlier vertebrates and that the stapes derives from a bone that braced the jaw in fish, an example of structures being repurposed over evolutionary time.

Taste and smell. Both are chemical senses that occur primarily in the oral and nasal cavities and depend on chemoreceptors, cells with receptor proteins that bind dissolved molecules. Taste buds, small onion-shaped clusters of gustatory cells, sit mainly on the sides of the papillae, the small bumps on the tongue (the mushroom-shaped fungiform papillae over the surface, the leaf-like foliate papillae on the sides and the large circumvallate papillae in a V at the back; the thread-like filiform papillae carry no buds) and also on the soft palate and throat. Each gustatory cell bears microvilli that reach the surface through a taste pore; molecules dissolved in saliva bind them and the cell signals a sensory fiber. Five basic tastes are recognized, sweet, salty, sour, bitter and umami (savory), and every region of the tongue that has buds detects all five, so the old "tongue map" is wrong. Taste impulses travel by three cranial nerves, the facial nerve from the front two thirds of the tongue, the glossopharyngeal from the back third and the vagus from the throat, to the medulla, then the thalamus, then the gustatory cortex. Smell begins in the olfactory epithelium in the roof of the nasal cavity, where olfactory receptor neurons carry cilia bearing receptor proteins into a layer of mucus; airborne molecules dissolve in the mucus, bind the receptors and trigger impulses. The neurons' axons pass through holes in the ethmoid bone to the olfactory bulbs beneath the frontal lobe, then along the olfactory tracts to the olfactory cortex in the temporal lobe and to areas handling emotion and memory, which is why a smell can bring back a memory so strongly. What we call flavor is taste, smell, texture and temperature integrated in the brain; with the nose blocked by a cold, food tastes flat because the smell component is missing. Imbalances include anosmia, loss of smell after head injury, infection or with aging; the general fall in the number of receptors with age, which changes older people's sensitivities; olfactory auras, phantom smells that can precede a seizure (and, less often, a migraine); and taste preferences that vary between people partly because of differences in their bitter receptor genes. Some chemicals act directly on the receptors: miraculin, a protein from the miracle fruit, binds the sweet receptor and, in the presence of acid, activates it, so sour foods taste sweet, while compounds in Gymnema sylvestre tea block the sweet receptor so that sugar tastes far less sweet for a while. A dog's sense of smell is far more sensitive than ours chiefly because it has many times more olfactory receptor neurons spread over a much larger olfactory epithelium, and a larger share of its brain devoted to smell.

Investigation: flavor with and without smell. With eyes closed and nose pinched, a partner places a jellybean of unknown flavor on your tongue; record what you can identify (usually only sweet or sour). Then release the nose and record the flavor identified. Repeat with five flavors in random order and pool class results. A large rise in correct identifications when the nose is opened supports the claim that flavor depends on smell as well as taste. Use only foods safe for every participant and check allergies first. The test cannot show which receptors fire or where in the brain the two inputs combine; it shows the behavior, not the mechanism.

Worked pathway of light with accommodation: A student reads a book 30 centimeters away. Light from the page passes through the cornea (most of the bending), the aqueous humor, the pupil, narrowed by the iris in the bright room, the lens, the vitreous humor and reaches the retina at the fovea. Because the object is near, the ciliary muscle contracts, the suspensory ligaments slacken and the lens bulges to bend the light more, so the image lands on the retina rather than behind it. The student looks up at a distant clock: the ciliary muscle relaxes, the ligaments tighten, the lens flattens. If the lens had stiffened with age (presbyopia), it could not bulge, and the page would blur while the clock stayed sharp.

Worked diagnosis of a focusing defect: A hypothetical person sees distant road signs blurred but reads a phone clearly. Reasoning: near objects focus correctly, distant ones do not; distant light rays need less bending, so the eye is bending too much or is too long, placing the image in front of the retina. That is myopia. A diverging (concave) lens spreads the rays before they enter the eye so that the image moves back onto the retina. The opposite pattern, clear distance but blurred near, would point to hyperopia and a converging lens; blur at all distances that changes with the orientation of lines points to astigmatism.

Worked pathway of sound and a hearing-loss classification: A 1,000-vibrations-per-second tone reaches the auricle, travels down the canal, vibrates the tympanic membrane, is passed and amplified by malleus, incus and stapes, enters the cochlear fluid at the oval window, makes the basilar membrane vibrate at a position partway along its length, bends the hair cells there, and impulses travel by the cochlear nerve to the temporal lobe. A person who hears this tone normally through a vibrator pressed to the bone behind the ear but poorly through the air has a conductive loss: the inner ear and nerve work, but the outer or middle ear is not carrying the sound. A person who hears it poorly by both routes has a sensorineural component, in the hair cells or nerve; comparing the air and bone thresholds shows whether a conductive component is also present (a mixed loss).

Worked balance scenario: A person spins on a chair and stops. During spinning, the fluid in the horizontal semicircular canals lags, then catches up and moves with the head; on stopping, the fluid keeps moving while the head is still, deflecting the hair-cell caps in the opposite direction and signaling rotation that is not happening. The brain receives a rotation signal from the ears while the eyes report a stationary room: the conflict is felt as dizziness, and the eyes make jerky movements trying to track the false rotation. The same conflict between vestibular and visual input causes motion sickness in a car.

Worked evolutionary comparison: Vertebrate and cephalopod eyes both have cornea, lens, iris and retina. Hypothesis: they are convergent, evolved independently from simpler light-sensitive patches. Evidence: the vertebrate retina is inverted (photoreceptors behind the nerve layers, a blind spot, a loosely attached neural layer that can detach) while the cephalopod retina is everted (photoreceptors facing the light, no blind spot, no separable neural layer); and the two focus differently, by changing lens shape versus moving the lens. Two designs reaching the same camera plan by different routes supports the hypothesis that the camera eye was elaborated independently in the two lineages; it does not rule out a deep common ancestor with simple light-sensitive cells, which comparative genetics suggests both groups inherited.

Representation task: Create labeled illustrations of the eye (cornea, aqueous humor, iris, pupil, lens, ciliary body, suspensory ligaments, vitreous humor, retina, fovea, optic disc, optic nerve, sclera, choroid, and three accessory structures) and of the ear (auricle, external acoustic meatus, tympanic membrane, malleus, incus, stapes, oval window, auditory tube, cochlea, vestibule, semicircular canals, vestibulocochlear nerve), with numbered arrows tracing the pathway of light in the first and of sound in the second. Add a small labeled sketch of a taste bud on a papilla and of the olfactory epithelium with its pathway to the olfactory bulb.

Independent performance: Carry out the jellybean flavor investigation and two vision tests: a binocular depth test (touching pencil tips together with one eye and with two) and a one-eye blind-spot test (cover the left eye, stare at a cross drawn on the left of a card with a dot 8 cm to its right, and slowly move the card toward your face until the dot disappears; measure that distance), and write a report. Success criteria: (a) class data for flavor identification with the nose pinched and open are tabulated and the difference is explained by the afferent pathways of taste and smell; (b) the depth-test and blind-spot results are explained by the structure of the retina and the overlap of the visual fields; (c) one imbalance of sight, one of hearing or balance and one of taste or smell are each explained by the structure or process that fails, with one common treatment named for the sight defect; (d) a comparison of the vertebrate and molluscan eye states two structural differences and proposes one evolutionary hypothesis that accounts for the similarity; (e) the report names two things each investigation cannot show about the underlying mechanism.