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Week 15 · Chapter

Chapter 15 — The Special Senses

Human Anatomy and Physiology I Free sample edition

The module's primary reading · ~40–50 minutes · Objective 8 (explain the special senses — vision, hearing and equilibrium, taste, and smell — by tracing each stimulus through the structures of its sense organ to the receptor that converts it into a nerve signal, and relate the structure of each organ to its function), Objective 3 (identify the four primary tissue types and their subtypes from their structural features, name an epithelium by its layers and cell shape, and describe the layers, accessory structures, and homeostatic functions of the integumentary system, including thermoregulation as a negative-feedback loop) · Read it first this week; the lecture, the slides, the tutorial, the quiz, and the Test Your Own Senses lab all build on it.

In plain language

This week you follow the outside world into your body, one sense at a time. Every sense organ does two jobs: it captures something from outside, such as light or sound, and it converts that into a nerve signal your brain can read. The cells that do the converting are sensory receptors (cells or nerve endings that respond to one kind of stimulus, such as light, sound, or a chemical). You will trace light through the eye, sound and head motion through the ear, and chemicals through the tongue and nose, always in order. By the end, a dark room, a spinning chair, and a soup that tastes flat during a cold will each read as a short story about one structure doing its job.

Why you already care about this

Picture three made-up moments. You have a stuffy cold, and your favorite soup tastes like warm water. You switch off the bedroom light, and the colors of the room fade to gray while you can still see the shapes. A child spins on a playground merry-go-round, steps off, and the whole playground keeps turning for a few seconds after the ride has stopped.

Each moment has a structure behind it, and none of them is the one you would guess. The soup is not your tongue's fault. The gray room is not your eyes failing. The spinning playground is not happening in your hearing organ, even though that organ sits right next door. This chapter shows you which structure is behind each one.

By the end of this chapter you will be able to:

name the five receptor classes and sort a stimulus into its class;
follow light through the eye in order, from the cornea to the retina, and name each structure's job;
tell rods from cones and explain the dark-room question and the blind spot;
follow sound through the ear in order, from the pinna to the cochlea, with the three ossicles in their order;
tell the cochlea's job from the semicircular canals' job, and predict a structure from a lost ability;
name the five basic tastes, trace smell to the olfactory nerve, and explain why flavor is mostly smell.

Carry this line with you: capture, then convert — every sense, every time.

Where we are

Already covered: Week 1, Body Organization, Homeostasis & Anatomical Terminology, gave you the receptor as the part of a feedback loop that senses. Week 5, Tissues, named epithelium as sheets of cells. Week 6, The Integumentary System, showed that the skin senses touch, surface temperature, and pain. Week 9, The Skeleton & Joints, counted the three ossicles of each ear among the axial skeleton's bones. Week 12, Nervous Tissue, the Neuron & the Action Potential, turned a stimulus into an electrical signal. Week 13, The Central Nervous System, placed vision in the occipital lobe and hearing in the temporal lobe. Week 14, The Peripheral & Autonomic Nervous System, named the 12 pairs of cranial nerves.
This week: the special senses. You sort receptors into five classes, then follow light through the eye, sound and head motion through the ear, and chemicals through the tongue and nose.
How they connect: a sense organ is the front end of the nervous system. It captures a stimulus and converts it into the same kind of nerve signal you studied in Week 12, and a cranial nerve carries that signal to the brain regions you mapped in Week 13.
Where we're going next: this is the last teaching module. Week 16 brings no new material; it holds the cumulative final and its review bundle.

Here is what you lean on this week. In Week 1 you learned that a receptor senses a change and reports it. This week you meet the receptors themselves, sorted by what they detect. In Week 12 you learned that a neuron fires an all-or-none action potential once a stimulus reaches threshold. A sense organ's job is to turn light, sound, or a chemical into that kind of signal. In Week 13 you learned that every sensory signal except smell passes through the thalamus on its way to the cortex. Smell's exception returns near the end of this chapter.

1. Sort the stimulus: the five receptor classes

Sit beside a campfire on an evening hike. You smell the smoke, hear the wood crackle, 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.

Every sense organ has two jobs. First it captures the stimulus. Then it converts the stimulus into a nerve signal the brain can read. The receptors do the converting, and they are sorted by what they detect.

A receptor class is a group of sensory receptors named for the kind of stimulus they detect; this course uses five classes.
A chemoreceptor is a sensory receptor that responds to chemicals; taste and smell run on chemoreceptors.
A mechanoreceptor is a sensory receptor that responds to pressure, vibration, sound, or body position.
A photoreceptor is a sensory receptor that responds to light.
A thermoreceptor is a sensory receptor that responds to heat or cold.
A nociceptor is a sensory receptor that responds to the tissue damage that is felt as pain.

The special senses are vision, hearing, equilibrium, taste, and smell, each served by a sense organ that captures its stimulus and converts it into a nerve signal.

Worked example — the campfire, sorted. You sort each campfire stimulus into its receptor class, then count the classes.

The smell of smoke is a chemical, so a chemoreceptor catches it.
The crackle is sound, so a mechanoreceptor catches it.
The glow is light, so a photoreceptor catches it.
The warmth is heat, so a thermoreceptor catches it.
The spark's sting is tissue damage felt as pain, so a nociceptor catches it.
Count: chemoreceptor + mechanoreceptor + photoreceptor + thermoreceptor + nociceptor, so 1 + 1 + 1 + 1 + 1 = 5 receptor classes.

Now connect this to a familiar organ. In Week 6 you learned that the skin senses touch, surface temperature, and pain. Touch is pressure, so it uses mechanoreceptors; temperature uses thermoreceptors; pain uses nociceptors. The skin carries three of the five classes, even though it is not one of the special senses.

Misconception check. Students often think each receptor class belongs to one organ, so "mechanoreceptors are in the skin." It is tempting because touch is the first mechanical sense anyone meets. The cure is the definition: a class is named for its stimulus, not its place. The same class, the mechanoreceptor, catches touch in the skin and sound in the ear.

Caveat: sorting receptors into five classes is a classroom model you can hold in one hand. It is the sorting this course uses, and it is enough to place every sense this week.

2. Follow the light: cornea, pupil, lens, retina

Walk out of a dark movie theater into a sunny parking lot. For a moment everything is too bright, and then your eyes adjust. Part of that adjustment happens at the front of your eye, where light first gets in.

Picture the eye as a labeled figure, a ball seen from the side with light arriving from the front. Light passes four structures in a fixed order: cornea → pupil → lens → 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 iris is the ring of muscle in the eye that opens or narrows the pupil to match the amount of light.
The pupil is the opening at the center of the iris through which light passes from the cornea to the lens.
The lens is the structure behind the pupil that does the fine focusing of light onto the retina and changes shape for near and far objects.

Worked example — one beam of light, in order. You follow light from a reading lamp into the eye and name each structure's job.

The light first meets the cornea, the transparent window, which admits it and starts the focusing.
It passes through the pupil, the opening whose size the iris sets.
It passes through the lens, which does the fine focus.
It lands on the retina, where it is converted into a nerve signal.
Count: cornea → pupil → lens → retina, so 1 + 1 + 1 + 1 = 4 structures, with the conversion at the last one, the retina.

In the sunny parking lot, the iris narrows the pupil and lets in less light. In the dark theater, it opens the pupil wide. That is the eye's capture job, tuned to the light.

Misconception check. Students often say "the lens does most of the focusing," because the word "lens" makes them think of a camera. It is tempting, but the cornea and the lens focus together, and the cornea is where focusing begins. The lens has a special job of its own: it changes shape to fine-tune the focus.

3. Focus near and far: accommodation and the retina

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 so that the near thing and the far thing could each land sharply.

That something is the lens. It changes shape for near and far objects. Accommodation is the change in the lens's shape that keeps near and far objects in focus on the retina.

The retina is the photoreceptor layer of the eye, where the rods and cones convert light into nerve signals. The retina is where the eye's second job, converting, happens. Everything in front of it, the cornea, pupil, and lens, serves the first job, capturing and focusing.

Worked example — the phone and the tree. You read the near-to-far switch as structure to function.

Structure: the lens sits behind the pupil, in front of the retina.
Function: it changes shape for near and far objects, which is accommodation, so the phone in your hand and the tree across the street can each come into focus on the retina.
Where the image lands: on the retina, the photoreceptor layer, where light becomes a nerve signal.

Now the clinical connection, read with the three-step read from Week 8. Structure: the cornea and the lens. Function: together they focus light onto the retina. Clinical connection, in standard terms: when light comes to a focus in front of or behind the retina, the image blurs. You name the focusing error and stop; this course gives no advice about any condition.

Misconception check. Students often think the retina "focuses" the image, because the image ends up there. The cure is to split the two jobs. The cornea and the lens focus the light, the capture job; the retina converts it, the convert job. A blurry image is a focusing problem in front of the retina, not a retina that forgot how to focus.

Pause and think, then choose one. (a) Describe a moment from your own day when your eyes switched focus from something near to something far, and name the structure that did the switching. (b) Explain the cornea's and the lens's shared job to a younger relative using an everyday object you have at home, and say where your comparison stops working.

4. See in the dark: rods versus 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 the same shade of gray. Your eyes did not fail. A different set of photoreceptors took over.

The retina has two kinds of photoreceptors.

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, packed most densely at the fovea.

The retina's two photoreceptors compared by the light they work in and whether they detect color
Photoreceptor	Works best in	Detects color?
Rods	low light	no
Cones	bright light	yes

Worked example — the dark-room question. You explain why a dark room looks gray, step by step.

The light is dim.
In dim light, only the rods are firing.
Rods detect no color.
So you see gray shapes and no color.

The chain is dim light → only the rods fire → rods detect no color → gray shapes and no color. Switch the light back on and the cones join in, so the red cup and the blue cup separate again.

Misconception check. Students often swap the pair and say "rods see color, and cones work in the dark." It is tempting because both names are short and both cells sit in the same layer. The cure is one sentence: cones for color, rods for the dark. If the room is dim and the colors are gone, the rods are on duty.

5. Find the sharp spot and the blind spot: the fovea and the optic disc

Read one word in the middle of this line without moving your eyes. The word is crisp, but the words at the edge of the line are fuzzy. Your retina is not equally sharp everywhere, and it even has one spot that sees nothing at all.

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 there.
The optic disc is the spot on the retina where the optic nerve begins; it has no photoreceptors and forms the blind spot.
The blind spot is the part of each eye's field of view that falls on the optic disc, where there are no photoreceptors to detect light.

You can find your own blind spot at home. The dot-and-cross card does it: you 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, and then it comes back.

Worked example — the vanishing dot. You predict which structure explains a dot that disappears on the card.

The dot vanished while your eye stayed fixed on the cross, so the dot's image landed on one spot of the retina.
That spot cannot detect light, so it has no photoreceptors.
The one spot of the retina with no photoreceptors is the optic disc, where the optic nerve begins.
In one line: a dot that vanishes on the card → the optic disc.

The distance at which your dot vanished is your own value. It depends on your card and your eye, and it is never a norm to meet.

Misconception check. Students often think the blind spot is a damaged part of the eye, because "blind" sounds like a problem. The cure is the structure. Every eye has an optic disc, where the optic nerve begins, and that spot has no photoreceptors by design. The blind spot is normal anatomy, not an injury.

6. Follow the sound: from the pinna to the oval window

Stand in a hallway when the doorbell rings. The sound travels through the air, into your ear, and across a chain of tiny bones before it ever reaches the cells that hear it. The ear's capture job is long, and its order is a gradable fact.

Read the ear from the outside in, as a labeled figure seen in a cross-section from the side of the head.

The pinna is the visible outer ear, also called the auricle, which with the auditory canal leads sound to the eardrum.
The auditory canal is the passage that carries sound from the pinna inward to the tympanic membrane.
The tympanic membrane is the eardrum, the thin membrane at the inner end of the auditory canal that vibrates when sound reaches it.
The malleus is the first of the middle ear's three ossicles, fixed to the eardrum, which passes the eardrum's vibration to the incus.
The incus is the middle of the three ossicles, which passes the vibration from the malleus to the stapes.
The stapes is the last of the three ossicles, whose footplate sits in the oval window of the inner ear.
The oval window is the opening into the inner ear where the footplate of the stapes sits and passes the vibration into the fluid of the cochlea.

Worked example — the doorbell, in order. You follow the sound of a doorbell from the eardrum to the cochlea and count the structures.

The pinna and the auditory canal lead the sound to the tympanic membrane, which vibrates.
The malleus, fixed to the eardrum, passes the vibration to the incus.
The incus passes it to the stapes.
The stapes's footplate, sitting in the oval window, passes it into the cochlea.
Count from the eardrum: tympanic membrane → malleus → incus → stapes → oval window → cochlea, so 1 + 3 + 1 + 1 = 6 structures, with the 3 ossicles in the middle.

The ossicles are bones, and you have met them before. In Week 9 the axial skeleton included seven associated bones: the hyoid and the ossicles of both ears.

Worked example — the ossicles in the skeleton. You check the Week 9 count with this week's ossicles.

Each ear has 3 ossicles: the malleus, the incus, and the stapes.
Two ears: 3 ossicles in each ear × 2 ears = 6 ossicles.
Add the hyoid: 1 + 6 = 7 associated bones of the axial skeleton.

Misconception check. Students often scramble the ossicles, because three short Latin names are easy to shuffle. The cure is the order of contact. The malleus is fixed to the eardrum, so it comes first; the stapes sits in the oval window, so it comes last; the incus is between them. Malleus, incus, stapes, from the eardrum inward.

7. Convert the vibration: the cochlea and its hair cells

Put your hand flat on a table while someone taps the far end. You feel the tap arrive as a shake. In your inner ear, a shake arrives too, and special cells turn it into a signal.

The cochlea is the fluid-filled part of the inner ear whose hair cells convert vibration into nerve signals for hearing.
Hair cells are mechanoreceptors of the inner ear that convert the movement of fluid into nerve signals, for hearing in the cochlea and for balance in the semicircular canals and vestibule.

The vibration from the stapes moves the fluid of the cochlea, and the hair cells convert that movement into nerve signals for hearing. That is the ear's convert job.

Worked example — capture versus convert in the ear. You split the doorbell's path into the ear's two jobs.

Capture: the pinna, the auditory canal, the tympanic membrane, and the three ossicles bring the vibration in and pass it to the oval window.
Convert: the hair cells of the cochlea turn the moving fluid into nerve signals.
Receptor class: hair cells respond to movement, so they are mechanoreceptors.

Misconception check. Students often think the eardrum "hears," because it is the part everyone has heard of. The cure is the two jobs. The eardrum vibrates and passes the vibration on; it captures. The hair cells of the cochlea convert. No signal leaves the ear until the hair cells fire.

8. Keep your balance: the semicircular canals and the vestibule

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, and nothing in your hearing organ caused it. The cause is in the part of the inner ear that senses motion.

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 is the part of the inner ear that senses the position of the head and works with the semicircular canals for equilibrium.
Equilibrium is the sense of balance, served by the semicircular canals and the vestibule and carried on the vestibulocochlear nerve.

Hearing and equilibrium share one nerve. Both travel on the vestibulocochlear nerve, cranial nerve VIII, written CN VIII.

Worked example — the stopped chair. You predict which structure explains a room that keeps spinning.

While you spun, the fluid in the semicircular canals moved with your head.
When the chair stopped, the fluid kept moving for a moment.
The hair cells at the base of each canal still sensed turning.
So a room that keeps spinning → the semicircular canals.

Worked example — split the inner ear by job. You sort the inner ear's parts by what each one senses.

The cochlea hears: 1 structure for hearing.
The semicircular canals sense the head turning, and the vestibule senses head position: 1 + 1 = 2 structures for equilibrium.
All of them send their signals on the vestibulocochlear nerve.

Misconception check. Students often say "the cochlea handles balance," because the cochlea and the canals sit side by side and share one nerve. The cure is one sentence: the cochlea hears, and the canals balance. If the problem is a spinning room, look to the canals and the vestibule, not the cochlea.

9. Taste the five: taste buds and the basic tastes

Set out five small spoons in a kitchen: one of honey, one of salted water, one of lemon juice, one of strong black coffee, and one of savory broth. Each spoon wakes up a different part of your sense of taste, and this course names five basic tastes.

Taste buds are clusters of taste receptors set mainly in the papillae of the tongue, which respond to the five basic tastes.
Papillae are the small bumps on the surface of the tongue in which most taste buds sit.
The basic tastes are the five taste qualities taste buds respond to: sweet, salty, sour, bitter, and umami.
Umami is the savory taste of glutamate, one of the five basic tastes.

Taste buds respond to chemicals, so they are chemoreceptors.

Worked example — the five spoons. You list the five basic tastes and count them.

Sweet, salty, sour, and bitter are four of the basic tastes.
Umami, the savory taste of glutamate, is the fifth.
Count: sweet + salty + sour + bitter + umami, so 1 + 1 + 1 + 1 + 1 = 5 basic tastes.
Spicy is not on the list.

Honesty note: whether fat is a sixth taste is still under study. This course teaches five basic tastes and says so plainly; a quiz never asks you to settle that open question.

Misconception check. Students often say "spicy is a taste," because hot sauce is something you put in your mouth. The cure is the receptor. Spicy is a pain-and-temperature signal, not one of the five basic tastes.

10. Smell the air: the olfactory epithelium and chemoreception

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 and met receptors waiting for them.

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.
Chemoreception is the detection of chemicals by chemoreceptors, the way both taste and smell work.

The olfactory epithelium's bipolar receptor neurons are chemoreceptors, and their axons form the olfactory nerve, cranial nerve I, written CN I. In Week 5 you learned that an epithelium is a sheet of cells that covers or lines a surface; this one lines the top of the nasal cavity and carries the receptors for smell.

Smell is also the one sense whose signal does not relay through the thalamus on its way to the cortex. In Week 13 you learned that the thalamus relays every sensory signal except smell. Here is that exception, in place.

Worked example — three sensory nerves. You count the purely sensory cranial nerves this week uses.

Smell travels on the olfactory nerve, CN I.
Vision travels on the optic nerve, CN II, which begins at the optic disc.
Hearing and equilibrium travel on the vestibulocochlear nerve, CN VIII.
Count: olfactory (CN I) + optic (CN II) + vestibulocochlear (CN VIII), so 1 + 1 + 1 = 3 of the 12 pairs.

Misconception check. Students often think taste and smell are unrelated senses, because one lives on the tongue and the other in the nose. The cure is the receptor class. Both run on chemoreceptors, so both are chemoreception, and that shared class is why they blend so easily, as the next section shows.

11. Answer the hook: flavor is mostly smell

Go back to the stuffy cold and the soup that tasted like warm water. 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. Most people who think they have a taste disorder have a problem with smell.

Worked example — the flat-tasting meal. You predict which structure explains a meal that tastes flat during a cold.

The tongue's taste buds still respond to sweet, salty, sour, bitter, and umami.
A blocked nose keeps the food's aroma from reaching the olfactory epithelium.
Flavor is mostly smell, so most of the flavor is lost.
In one line: a flat-tasting meal during a cold → smell, not the taste buds.

You can test this yourself in this week's lab. You taste a food with your nose pinched, then taste it again with your nose open, and you write down what changed. You 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.

Misconception check. Students often say "my tongue stopped working during a cold," because the tongue is where taste seems to happen. The cure is the definition of flavor. The five tastes still arrive, but the aroma does not, and flavor is mostly smell. Your cold blocked the aroma, not your tongue.

Key terms

Receptor class — A group of sensory receptors named for the kind of stimulus they detect; this course uses five classes: chemoreceptors, mechanoreceptors, photoreceptors, thermoreceptors, and nociceptors. (first used: 1. Sort the stimulus: the five receptor classes)
Chemoreceptor — A sensory receptor that responds to chemicals; taste and smell run on chemoreceptors. (first used: 1. Sort the stimulus: the five receptor classes)
Mechanoreceptor — A sensory receptor that responds to pressure, vibration, sound, or body position; the hair cells of the inner ear are mechanoreceptors. (first used: 1. Sort the stimulus: the five receptor classes)
Photoreceptor — A sensory receptor that responds to light; the rods and cones of the retina are photoreceptors. (first used: 1. Sort the stimulus: the five receptor classes)
Thermoreceptor — A sensory receptor that responds to heat or cold, such as those that let the skin sense surface temperature. (first used: 1. Sort the stimulus: the five receptor classes)
Nociceptor — A sensory receptor that responds to the tissue damage that is felt as pain. (first used: 1. Sort the stimulus: the five receptor classes)
Special senses — Vision, hearing, equilibrium, taste, and smell, each served by a sense organ that captures its stimulus and converts it into a nerve signal. (first used: 1. Sort the stimulus: the five receptor classes)
Cornea — The transparent front window of the eye, which admits light and, together with the lens, focuses it onto the retina. (first used: 2. Follow the light: cornea, pupil, lens, retina)
Iris — The ring of muscle in the eye that opens or narrows the pupil to match the amount of light. (first used: 2. Follow the light: cornea, pupil, lens, retina)
Pupil — The opening at the center of the iris through which light passes from the cornea to the lens. (first used: 2. Follow the light: cornea, pupil, lens, retina)
Lens — The structure behind the pupil that does the fine focusing of light onto the retina and changes shape for near and far objects. (first used: 2. Follow the light: cornea, pupil, lens, retina)
Accommodation — The change in the lens's shape that keeps near and far objects in focus on the retina. (first used: 3. Focus near and far: accommodation and the retina)
Retina — The photoreceptor layer of the eye, where the rods and cones convert light into nerve signals. (first used: 3. Focus near and far: accommodation and the retina)
Rods — Photoreceptors of the retina that are sensitive in low light and detect no color. (first used: 4. See in the dark: rods versus cones)
Cones — Photoreceptors of the retina that work in bright light and detect color, packed most densely at the fovea. (first used: 4. See in the dark: rods versus cones)
Fovea — 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. (first used: 5. Find the sharp spot and the blind spot: the fovea and the optic disc)
Optic disc — The spot on the retina where the optic nerve begins; it has no photoreceptors and forms the blind spot. (first used: 5. Find the sharp spot and the blind spot: the fovea and the optic disc)
Blind spot — The part of each eye's field of view that falls on the optic disc, where there are no photoreceptors to detect light. (first used: 5. Find the sharp spot and the blind spot: the fovea and the optic disc)
Pinna — The visible outer ear, also called the auricle, which with the auditory canal leads sound to the eardrum. (first used: 6. Follow the sound: from the pinna to the oval window)
Auditory canal — The passage that carries sound from the pinna inward to the tympanic membrane. (first used: 6. Follow the sound: from the pinna to the oval window)
Tympanic membrane — The eardrum, the thin membrane at the inner end of the auditory canal that vibrates when sound reaches it. (first used: 6. Follow the sound: from the pinna to the oval window)
Malleus — The first of the middle ear's three ossicles, fixed to the eardrum, which passes the eardrum's vibration to the incus. (first used: 6. Follow the sound: from the pinna to the oval window)
Incus — The middle of the three ossicles, which passes the vibration from the malleus to the stapes. (first used: 6. Follow the sound: from the pinna to the oval window)
Stapes — The last of the three ossicles, whose footplate sits in the oval window of the inner ear. (first used: 6. Follow the sound: from the pinna to the oval window)
Oval window — The opening into the inner ear where the footplate of the stapes sits and passes the vibration into the fluid of the cochlea. (first used: 6. Follow the sound: from the pinna to the oval window)
Cochlea — The fluid-filled part of the inner ear whose hair cells convert vibration into nerve signals for hearing. (first used: 7. Convert the vibration: the cochlea and its hair cells)
Hair cells — Mechanoreceptors of the inner ear that convert the movement of fluid into nerve signals, for hearing in the cochlea and for balance in the semicircular canals and vestibule. (first used: 7. Convert the vibration: the cochlea and its hair cells)
Semicircular canals — The fluid-filled canals of the inner ear whose hair cells, at the base of each canal, sense the head turning. (first used: 8. Keep your balance: the semicircular canals and the vestibule)
Vestibule — The part of the inner ear that senses the position of the head and works with the semicircular canals for equilibrium. (first used: 8. Keep your balance: the semicircular canals and the vestibule)
Equilibrium — The sense of balance, served by the semicircular canals and the vestibule and carried on the vestibulocochlear nerve. (first used: 8. Keep your balance: the semicircular canals and the vestibule)
Taste buds — Clusters of taste receptors set mainly in the papillae of the tongue, which respond to the five basic tastes. (first used: 9. Taste the five: taste buds and the basic tastes)
Papillae — The small bumps on the surface of the tongue in which most taste buds sit. (first used: 9. Taste the five: taste buds and the basic tastes)
Basic tastes — The five taste qualities taste buds respond to: sweet, salty, sour, bitter, and umami. (first used: 9. Taste the five: taste buds and the basic tastes)
Umami — The savory taste of glutamate, one of the five basic tastes. (first used: 9. Taste the five: taste buds and the basic tastes)
Olfactory epithelium — The sheet of tissue high in the nasal cavity whose bipolar receptor neurons detect odors and send their axons into the olfactory nerve. (first used: 10. Smell the air: the olfactory epithelium and chemoreception)
Chemoreception — The detection of chemicals by chemoreceptors, the way both taste and smell work. (first used: 10. Smell the air: the olfactory epithelium and chemoreception)
Flavor — The combined perception of a food's tastes and its aroma, most of which comes from smell. (first used: 11. Answer the hook: flavor is mostly smell)

Summary

Every sense organ captures a stimulus, then converts it into a nerve signal.
Five receptor classes: chemo-, mechano-, photo-, thermo-, and nociceptors.
Light path: cornea, pupil, lens, retina.
The cornea and lens focus together; the lens accommodates.
Rods work in the dark without color; cones see color in bright light.
The fovea is the sharpest spot; the optic disc is the blind spot.
Sound path: eardrum, malleus, incus, stapes, oval window, cochlea.
The cochlea hears; the semicircular canals and vestibule balance.
Five basic tastes: sweet, salty, sour, bitter, umami. Spicy is not one.
Taste and smell are both chemoreception.
Flavor is mostly smell.

Check your understanding

Work these before looking at the answers — they are ungraded, and the answers (with reasoning) are at the bottom of the page.

A made-up study card lists the light path as "lens, cornea, retina, pupil." Put the four structures in the correct order and name the one structure where light becomes a nerve signal.
On a made-up winter hike, a hiker notices four things: cold air on the cheeks, the crunch of snow underfoot, the smell of pine, and a blister that stings. Sort each stimulus into its receptor class, and say how many of the five classes appear.
In a made-up case, a person steps off a spinning carnival ride, and the ground seems to keep turning for a few seconds. Which part of the inner ear would you point to, and why not the cochlea?
A made-up friend says, "When I ate chili with a stuffy nose, I could still taste the spice, so my sense of taste was fine and flavor has nothing to do with smell." Find the two errors and correct them.

Answers (worked reasoning — read after attempting)

The order is cornea → pupil → lens → retina. Light enters through the transparent cornea, passes through the pupil that the iris sizes, is fine-focused by the lens, and lands on the retina. The retina is the photoreceptor layer, so that is where light becomes a nerve signal. Count: 1 + 1 + 1 + 1 = 4 structures.
Cold air is a temperature stimulus, so a thermoreceptor. The crunch of snow is sound, so a mechanoreceptor. Pine scent is a chemical, so a chemoreceptor. The blister's sting is tissue damage felt as pain, so a nociceptor. That is 1 + 1 + 1 + 1 = 4 classes of the five; the photoreceptor is the one not on this list.
Point to the semicircular canals, whose hair cells sense the head turning; with the vestibule they serve equilibrium. The fluid in the canals kept moving after the ride stopped, so the hair cells still sensed turning. The cochlea hears; it does not balance, so a turning world points to the canals.
First error: spice is not a taste. It is a pain-and-temperature signal, so feeling it says nothing about the five basic tastes. Second error: flavor is mostly smell. The taste buds may work fine, but with a stuffy nose the aroma does not reach the olfactory epithelium, so the food's flavor goes flat.

Next up in this module: the lecture, the slides, the Readings & Resources page, the tutorial, the practice set, the quiz, and the Test Your Own Senses lab. After this week comes no new mechanism or rule, only the cumulative final, where every ordered path you learned this term returns.

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