Lecture: The Special Senses
Lecture script · about 15–20 minutes spoken · Week 15
Opening
Think about the last time you had a stuffy cold. Your favorite soup came to the table, and it tasted like warm water. It's easy to blame the tongue. In this lecture, I'll show you that the tongue was fine the whole time, and that the real story sits inside your nose.
Here's where we are. In Week 1, you met the receptor as the part of a feedback loop that senses a change. In Week 6, you learned that the skin senses touch, surface temperature, and pain. In Week 12, you watched a neuron turn a stimulus into an action potential. In Week 13, you placed vision in the occipital lobe and hearing in the temporal lobe. And in Week 14, you met the cranial nerves. In this lecture, we follow the outside world into the body, one sense at a time.
Here's the plan, and it rests on one idea. Every sense organ has two jobs. First it captures the stimulus. Then it converts the stimulus into a nerve signal the brain can read. Hold on to that pair, capture then convert, because we'll use it on light, on sound, on head motion, and on chemicals. This is the last teaching module of the course, so we'll also tie a few earlier weeks together on the way.
1. Five kinds of receptor
Picture yourself beside a campfire on an evening hike. You smell the smoke. You hear the wood crackle. You see the orange glow, feel the warmth on your face, and flinch when a spark stings your hand. Five different stimuli reached you at once, and a different kind of receptor caught each one.
A receptor class is a group of sensory receptors named for the kind of stimulus they detect. This course uses five. A chemoreceptor responds to chemicals, so taste and smell run on chemoreceptors. A mechanoreceptor responds to pressure, vibration, sound, or body position. A photoreceptor responds to light. A thermoreceptor responds to heat or cold. And a nociceptor responds to the tissue damage that is felt as pain.
Now remember the skin from Week 6. It senses touch, surface temperature, and pain. Touch is pressure, temperature is heat or cold, and pain is tissue damage. So the skin carries three of the five classes.
Pause and try this. At the campfire, which receptor class caught the crackle of the wood?
Say the stimulus first, then the class.
Here's the answer. A mechanoreceptor. The crackle is sound, and sound is a mechanical stimulus, a vibration.
2. The light path
Walk out of a dark movie theater into a sunny parking lot. For a moment, everything is too bright. Then your eyes adjust. Part of that happens right at the front of your eye.
Picture the eye from the side, a ball with light arriving at the front. Light passes four structures in a fixed order: the cornea, the pupil, the lens, and the retina. The cornea is the transparent front window of the eye, which admits light and, together with the lens, focuses it onto the retina. The pupil is the opening at the center of the iris. The iris is the ring of muscle that opens or narrows the pupil to match the amount of light. And the lens does the fine focusing.
Here's a trap I want you to see coming. It's tempting to think the lens does most of the focusing, because the word lens makes them think of a camera. The cornea and the lens focus together, and the cornea is where focusing begins.
Pause and try this. Put these four in the order light meets them: lens, retina, cornea, pupil.
Picture the light arriving at the front.
Here's the answer. Cornea, then pupil, then lens, then retina. The cornea is the front window, and the retina is the last stop.
3. Near and far
Look at your phone in your hand, then look up at a tree across the street. Both come into focus within a moment. Something inside your eye changed shape so that each one could land sharply.
That something is the lens. It changes shape for near and far objects, and that change has a name. Accommodation is the change in the lens's shape that keeps near and far objects in focus on the retina.
And the retina is the photoreceptor layer of the eye, where the rods and cones convert light into nerve signals. So here's our pair again. The cornea, pupil, and lens capture and focus. The retina converts.
Now the clinical connection, read the way you learned in Week 8: name the structure, state its function, name the connection, and stop. The cornea and the lens focus light onto the retina. When light comes to a focus in front of or behind the retina, the image blurs. That's the focusing error, and that's where we stop.
Pause and try this. Which job does the retina do, capture or convert?
Think about where light becomes a signal.
Here's the answer. Convert. The retina is the photoreceptor layer, where light becomes a nerve signal; the structures in front of it capture and focus.
4. Rods and cones
Get up at night in a dark kitchen for a glass of water. You can find the table and the doorway, but a red cup and a blue cup look like the same gray. Your eyes didn't fail. A different set of photoreceptors took over.
Rods are photoreceptors of the retina that are sensitive in low light and detect no color. Cones are photoreceptors of the retina that work in bright light and detect color. So in a dark room, only your rods are firing. Rods detect no color. You see gray shapes and no color.
This pair gets swapped all the time, so here's a line to keep: cones for color, rods for the dark.
Pause and try this. In a dim room, a striped towel looks gray. Which photoreceptors are doing the work?
Ask yourself which cells work in low light.
Here's the answer. The rods. They work in low light and detect no color, so the stripes lose their colors and show up as shades of gray.
5. The sharp spot and the blind spot
Read one word in the middle of a line without moving your eyes. That word is crisp, but the words at the edge are fuzzy. Your retina isn't equally sharp everywhere.
The fovea is a small pit in the retina where photoreceptors sit alone, with the other retinal layers and the blood vessels pushed aside, so vision is sharpest there. Cones are packed at the fovea.
Then there's a spot that sees nothing at all. The optic disc is the spot on the retina where the optic nerve begins. It has no photoreceptors, so it forms the blind spot. You can find yours at home. Hold a card with a dot and a cross at arm's length, close one eye, focus on the cross, and bring the card slowly toward your face. At one distance, the dot disappears. Then it comes back.
That distance is your own value, by the way. It isn't a norm to meet.
Pause and try this. The dot vanished while your eye stayed on the cross. Which structure explains it?
Find the one spot with no photoreceptors.
Here's the answer. The optic disc. The dot's image landed where the optic nerve begins, and there are no photoreceptors there to detect it.
6. The sound path
Now let's switch organs. Stand in a hallway when the doorbell rings. That sound travels through the air, into your ear, and across a chain of tiny bones before any cell hears it.
We read the ear from the outside in. The pinna, also called the auricle, is the visible outer ear. With the auditory canal, it leads sound to the tympanic membrane, the eardrum, which vibrates. Then come the middle ear's three ossicles, in order. The malleus, fixed to the eardrum, passes the vibration to the incus. The incus passes it to the stapes. And the stapes has a footplate that sits in the oval window of the inner ear.
Remember Week 9? The axial skeleton had seven associated bones: the hyoid and the ossicles of both ears. Each ear has three ossicles, so two ears give six, and the hyoid makes seven.
Pause and try this. A study card says "incus, stapes, malleus." Fix the order, starting at the eardrum.
Start with the bone touching the eardrum.
Here's the answer. Malleus, incus, stapes. The malleus is fixed to the eardrum, so it's first, and the stapes sits in the oval window, so it's last.
7. Hair cells hear
Put your hand flat on a table while someone taps the far end. You feel the tap arrive as a shake. Inside your ear, a shake arrives too.
The cochlea is the fluid-filled part of the inner ear whose hair cells convert vibration into nerve signals for hearing. The vibration from the stapes moves that fluid. And hair cells are mechanoreceptors that convert the movement of fluid into nerve signals.
So here's our pair one more time. The pinna, the canal, the eardrum, and the three ossicles capture the sound and pass it inward. The hair cells of the cochlea convert it. No signal leaves the ear until the hair cells fire.
Pause and try this. Which receptor class are the hair cells of the cochlea?
Ask what they respond to.
Here's the answer. Mechanoreceptors. They respond to the movement of fluid, which is a mechanical stimulus.
8. Keeping your balance
Spin slowly on an office chair, then stop it with your feet. For a few seconds, the room seems to keep turning. Nothing in the room moved. The cause sits right next to your hearing organ.
The semicircular canals are the fluid-filled canals of the inner ear whose hair cells, at the base of each canal, sense the head turning. The vestibule senses the position of the head. Together they serve equilibrium, the sense of balance. While you spun, the fluid in the canals moved with your head. When the chair stopped, the fluid kept moving for a moment, so the hair cells still sensed turning.
Hearing and equilibrium share one nerve, the vestibulocochlear nerve, cranial nerve eight. But they don't share a job. The cochlea hears, and the canals balance.
Pause and try this. Someone steps off a playground merry-go-round and the playground keeps turning. Cochlea or semicircular canals?
Think about which part senses the head turning.
Here's the answer. The semicircular canals. Their fluid kept moving after the ride stopped, so their hair cells still signaled turning; the cochlea hears.
9. Five tastes
Set out five small spoons in a kitchen: honey, salted water, lemon juice, strong black coffee, and a savory broth. Each one wakes up a different part of your sense of taste.
Taste buds are clusters of taste receptors set mainly in the papillae, the small bumps on the tongue. They respond to five basic tastes: sweet, salty, sour, bitter, and umami. Umami is the savory taste of glutamate. Taste buds respond to chemicals, so they're chemoreceptors.
An honest note here: whether fat is a sixth taste is still under study. This course teaches five.
And here's the trap. Spicy is not a taste. It's a pain-and-temperature signal, not one of the five basic tastes.
Pause and try this. Is spicy one of the five basic tastes?
Name the five, then check.
Here's the answer. No. The five are sweet, salty, sour, bitter, and umami. Spicy is a pain-and-temperature signal.
10. Smell
Walk past a bakery, or open a jar of coffee beans. The smell reaches you before you see anything. Molecules in the air have traveled up into your nose.
The olfactory epithelium is the sheet of tissue high in the nasal cavity whose bipolar receptor neurons detect odors and send their axons into the olfactory nerve, cranial nerve one. Those neurons are chemoreceptors. So taste and smell are both chemoreception, the detection of chemicals by chemoreceptors.
Here's a callback to Week 13. You learned that the thalamus relays every sensory signal except smell. Smell is that exception. It's the one sense whose signal doesn't relay through the thalamus on its way to the cortex.
Pause and try this. Taste and smell live in different places. What do they share?
Think about the receptor class.
Here's the answer. They share chemoreception. Both run on chemoreceptors, receptors that respond to chemicals.
11. Flavor is mostly smell
Now back to that soup. Your taste buds were working the whole time. What changed was the air moving through your nose.
Flavor is the combined perception of a food's tastes and its aroma, most of which comes from smell. The five tastes combine with a food's aroma, and without smell, food tastes bland. In fact, most people who think they have a taste disorder have a problem with smell. Your cold blocked the aroma, not your tongue.
You'll test this yourself in the Test Your Own Senses lab. You'll taste a food with your nose pinched, then with your nose open, and write down what changed. You'll also find your blind spot with the dot-and-cross card and locate the eye's and ear's structures on a free virtual atlas.
Pause and try this. A friend says their tongue stopped working during a cold. Which sense would you point to instead?
Think about what flavor is mostly made of.
Here's the answer. Smell. Flavor is mostly smell, and a blocked nose keeps the aroma from reaching the olfactory epithelium.
Closing
Let's pull it together. Every sense organ captures a stimulus and converts it into a nerve signal. Receptors come in five classes: chemoreceptors, mechanoreceptors, photoreceptors, thermoreceptors, and nociceptors. Light travels cornea, pupil, lens, retina, and the cornea and lens focus together while the lens accommodates. Rods work in the dark without color, and cones see color. The fovea is the sharpest spot, and the optic disc is the blind spot. Sound travels from the eardrum through the malleus, incus, and stapes to the oval window and the cochlea, where hair cells hear. The semicircular canals and the vestibule keep your balance. Taste buds sense five basic tastes, smell runs on the olfactory epithelium, and flavor is mostly smell.
Next in this module come the slides, the Readings & Resources page, the tutorial, the practice set, the quiz, and the Test Your Own Senses lab. After that, the course has no new module. The cumulative final is where every ordered path you learned returns, so carry capture, then convert, with you.
Built with The Course Maker · samples.thecoursemaker.com