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AOHS Foundations of Anatomy and Physiology I

Lesson 10The Senses

Student Resources

Resource Description

Student Resource 10.1 Notes: The Senses

Student Resource 10.2 Reading: The Senses

Student Resource 10.3 Reading: Taste and Smell

Student Resource 10.4 Observations: Taste and Smell Stations

Student Resource 10.5 Demonstration: Touch Sensors

Student Resource 10.6 Diagram: Hearing and Balance

Student Resource 10.7 Reading: Hearing and Balance

Student Resource 10.8 Investigations: Hearing and Balance

Student Resource 10.9 Reading: Hearing Loss

Student Resource 10.10 Notes: The Eye and Vision

Student Resource 10.11 Reading: The Eye and Vision

Student Resource 10.12 Glossary: The Senses (separate Word file)

Student Resource 10.13 Lab: Cow Eye Dissection

Student Resource 10.14 Reading: Optical Illusions

Copyright © 2014‒2016 NAF. All rights reserved.

AOHS Foundations of Anatomy and Physiology ILesson 10 The Senses

Student Resource 10.1

Notes: The SensesStudent Name:_______________________________________________________ Date:___________

Directions: Complete the notes as you view the PowerPoint presentation The Senses.

1. Describe what is special about the cells involved in each of the senses.

2. Complete the chart.

Sense organ Sense Type of cell



3. Which tactile sensations does your skin detect?

4. The skin on what parts of the body has the most sensory receptors?

5. Why don’t you notice the feel of your clothes on your body after you’ve been wearing them for a few minutes?




Reading: The Senses



One of the defining characteristics of our senses is that they each involve receptor cells that are specially designed for the task they carry out. Special cells called photoreceptors in your eyes detect light, letting us see. Photoreceptors we call rods enable us to see black and white; others we call cones enable us to see color.

Cells in your nose and tongue are sensitive to specific molecules and chemicals that give you the experiences of taste and smell. Your ears have cells that can detect motion, whether it’s caused by sound waves or by movements of your head. You skin contains many kinds of receptors that can sense temperature, pressure, touch, and pain.



The most developed sense in humans is vision. In fact, 70% of all your sense receptors are located in your eyes. In addition to our eyes enabling us to take in the scenery around us, they enable us to read, recognize faces, and see movement that may be too far away to hear. Our vision also plays a role in balance.

We can see because light bounces off objects around us and enters our eyes, hitting the photoreceptor cells—the rods and cones—in our retinas, which are at the back of our eyes.



You probably think of your ears as organs to hear with. But structures in your inner ears also play the important role of telling your brain what position your head is in, whether you’re standing up or lying down, and then sending signals to your motor cortex and your cerebellum to keep you steady on your feet or let you relax on your back. Your ears can sense your position using special cells called mechanoreceptors that detect how the fluid within the semicircular canals of your ear has moved.

Similar mechanoreceptor cells are also involved in your ability to hear: sound waves passing through the air hit tiny structures inside your ear. When these structures move, they stir the fluid in the cochlea of your ears, which the mechanoreceptors sense. The mechanoreceptors then send signals to the sound and hearing areas in the temporal lobe of your brain.



Taste can be considered your most intimate sense: it’s called into action when you’re going to put something into your body. Its main role is really to judge whether that something is good, and should be swallowed, or not so good, and should be spit out.

But the sense of taste is also one of our great pleasures. When you put something tasty into your mouth, special cells in your tongue send the message “tasty” to your thalamus. In response, your brain tells your digestive system to get ready for some food being sent its way, and it tells your digestive system to pump the necessary substances into your digestive tract to help it break down your food.



Much of what we think of as taste is really a mix of taste and smell. In fact, smell makes up a lot more of your experience of flavor. Your tongue can only detect a few types of molecules, but receptors in your nose can detect thousands. And they don’t need much to work with: for some odors, sometimes just a few molecules of a scent will trigger your sense of smell. Because your taste and smell receptors respond to specific molecules, you can think of them as detecting particular chemicals. This will help you to remember that they’re called chemoreceptors.



Maybe you’ve noticed how the scent of your favorite meal can make you feel cozy, or the smell of a friend’s perfume can make you almost feel as if that person is there. One very special feature about our sense of smell is that the part of the brain that processes smells closely communicates with the brain regions that deal with memory and emotion. Quite often, our brains will remember and associate a scent with something from the past for much longer than we’ll remember and associate a sound or an image. That’s why scents can sometimes feel like they affect much more than our senses and give us an almost lifelike feel of something or someone from the past.



Our skin contains receptors for many different tactile senses. Touch can be thought of as a combination of different kinds of touch, such as light touch, deep touch, vibration, light pressure and deep pressure. Some of these sensations use different receptors. Your skin has separate receptors for heat and cold (and you can actually trick these receptors into both firing at the same time). Pain receptors are yet another type found in your skin.

At the base of the hairs on your skin are structures that can detect whether the hair has moved, relaying messages that something has moved lightly across your skin.

Each of these types of sensors is connected to the nervous system. The many types of sensors work together to let you feel the difference between porcelain and wood, cotton and silk, fur and feathers.

Blind people use their tactile senses to help them discern shapes, textures, and many other qualities that we normally use vision for.



Although there are several kinds of receptor cells in the skin, you don’t have the same number of each. You have, by far, more pain receptors than you have other types of receptors. Pain receptors are just bare nerve endings. But if any of the other types of receptors—heat, cold, and pressure—are met with a really strong stimulus, that stimulus will stimulate pain receptors as well as temperature or pressure receptors. The touch receptors in the skin are mechanoreceptors. Mechanoreceptors are stimulated by some kind of physical interaction of the receptor itself with the environment.



Although you have tactile sensors in virtually every part of your body, some areas are much more sensitive than others. In 1950, two neuroscientists named Wilder Penfield and Theodore Rassmussen mapped parts of the body on to the areas of the sensory cortex that served those parts. The size of the body parts shows how many sensory neurons are devoted to that part. For example, the hand part of this map is much bigger than the arm; in real terms, your arm is actually bigger than your hand, but your hand has many more sensory neurons going to it. You probably have an intuitive feeling for this map, if you stop to think about it: biting your tongue hurts a lot more than biting your forearm. If you find a soft, fuzzy stuffed animal, you’re likely to rub it against your hands or cheeks, not against your wrist.



Every moment, our skin and all of our other organs are subjected to enormous numbers of stimuli. All around us, sounds, sights, smells, and sensations are bombarding us. Some neurons sustain their firing for as long as the stimulus remains. Other neurons fire only when the conditions change. For example, you might hear the hum of the car engine when it starts up and notice when it stops, but while the car is running the sound becomes part of the background. This is a necessary skill for our senses to develop: if we couldn’t stop some of the sensations coming at us, we would be overwhelmed. This filtering is one of the many ways in which our brains and sensory organs work together to create our perception of the reality around us.




Reading: Taste and SmellStudent Name:_______________________________________________________ Date:___________

Directions: Go through the reading with a partner and answer the questions in the spaces provided.

The Experience of FlavorImagine what your favorite food tastes like. Now imagine what it smells like. They’re probably similar experiences, but not quite the same. If you try to describe them and the difference between them, you may find yourself at a loss for words.

When we eat, a very complex interaction between taste and smell takes place, giving us the sensation we call flavor. While these two senses work together, taste and smell are quite different from each other—they work in different ways and go to different parts of your brain.

Your Sense of TasteLike all your senses, taste relies on special receptor cells, which are located in clusters we call taste buds. The receptor cells and taste buds have a lifespan of about a week. You have about 10,000 taste buds, most of which are on your tongue, but you also have some at the back of your throat. Each taste bud contains about 40 chemoreceptor cells whose job it is to detect certain types of molecules. Once these chemoreceptors have sensed something, they stimulate a nearby neuron to send a signal to the brain.

Each taste cell recognizes only one kind of taste, and most of your taste buds contain cells for two or more tastes. While some areas of your tongue may be more sensitive to one taste or another, you can sense all the tastes in all parts of your tongue.

Your taste cells recognize five distinct tastes:

Salty

Sour

Sweet

Bitter

Umami

Umami is a taste that is often described as savoriness or meatiness. Soy sauce, parmesan cheese, and tomatoes all have a lot of umami flavor. Each taste is characterized by a certain kind of compound or ion that triggers it. In each case, the compound or ion interacts with the cell membrane, initiating a process that results in the release of neurotransmitters.


A taste bud: Molecules from food affect proteins on the membranes of taste cells and tell them to send signals to the brain.


Type of taste receptor

What triggers the taste? Interaction with cell membrane

Salty Na+ ions Opens ion channels

Sour H+ ions Opens ion channels

Sweet Sugars Binds with receptor protein

Bitter A variety of compounds Binds with receptor protein

Umami Molecules related to the building blocks of proteins

Binds with receptor protein

Why do you think your taste receptor cells only live for a week or so?

Your Sense of SmellSmell is perhaps the most elusive sense, and it has even evaded researchers’ ability to really understand the fine details of how it works. What they do know is that you have a patch of very special neurons called the olfactory bulb located at the top of your nasal cavity, behind your eyes. These cells have chemoreceptors. But unlike taste buds, they can trigger their own action potential, and they lead directly to the brain.


Question: What region of the brain is responsible for processing smell?

Your olfactory bulb contains the special cells that are responsible for your sense of smell.

Olfactory bulb


Your smell cells, technically known as olfactory cells, are stimulated by odor molecules floating in the air that end up in the nasal cavity. These cells are very sensitive to aromas—sometimes they can detect just a few molecules. At the end of the cell that is embedded in the mucous membrane lining your nose are tiny hair-like projections called cilia. The receptor proteins that bind with the odor molecules are on these cilia. While your olfactory bulb is only about the size of a quarter, you have somewhere between 10 and 20 million receptors packed into that little space. And there are so many cilia with receptor cells that if you could measure their total surface area, it would be about the same as the surface area of your skin.

Like the sense receptors in your skin, your olfactory cells adjust to most odors; you might walk into the kitchen and smell chocolate chip cookies baking, but after a few minutes, you don’t notice it anymore. Unlike your senses of touch or sight, though, it’s tough to fool your sense of smell.

Your olfactory cells are connected directly to a part of your brain that processes emotions and memory. Because of that connection, we tend to have strong reactions to odors, and often really love a smell or really don’t. Scents often evoke intense, emotional memories, at times more vividly than even a photo or the sound of someone’s voice.

Questions:a) What scents can you think of that remind you of a certain person, place, or event?

b) What scents do you have a particularly strong reaction to, either positive or negative?

c) What memories do you have associated with any of these scents?




Observations: Taste and Smell StationsStudent Name:_______________________________________________________ Date:___________

Directions: With your group, complete the activities at the three stations. Fill out the charts below with your observations at each station as you go along and answer the questions following each chart.

Jelly Bean StationTake turns so that each person in the group gets to do the activity with two different jelly beans.

Choose one person to start as the jelly bean taster. Tell this person to close his or her eyes.

Select one jelly bean from the bag. Record its color on the chart. Then hand the jelly bean to the taster. Remind the taster not to look at the jelly bean.

Tell the taster to hold his or her nose and chew the jelly bean, describing what he or she tastes. Record the taster’s comments in your chart.

Ask the taster to identify the flavor of the jelly bean and record the answer.

Have the taster repeat the tasting with another jelly bean but this time holding his or her nose for a count of 10 and then letting go. Record their flavor guess for this jelly bean and any differences the taster noticed between the two trials.

Taste trial Color/flavor of jelly bean

Taster’s description of flavor

Flavor identification correct/incorrect?

Name of Taster:

1 – nose held to end

2 – nose held 10 sec

Name of Taster:



Name of Taster:





1) Why was there a difference between tasting the jelly bean with your nose held and with your nose open?

2) How do you think this experiment might have been different if you could have seen the jelly bean?

Color of Taste StationDo this experiment independently, and discuss your results afterward. Do NOT share your observations until everyone in your group has finished filling out his or her chart.

Each group member should take a cup of each color of Jell-O and a spoon to taste it with. You should also each take one of the sealed envelopes, which you will open after you complete your charts.

Try each color of Jell-O and record your observations on the chart.

When everyone in your group is finished, compare your answers.

Once you’ve compared answers, open the envelope.

Jell-O sample # Color of Jell-O Flavor prediction Actual flavor Correct?

1

2

3

1) Why do you think we don’t eat many things that are gray?

2) What would you think if you saw green meat?

3) Which of your senses do you think had the most to do with your prediction of the Jell-O flavor?



Scent Identification StationCollect one set of the five containers containing aromatic ingredients for your group. Make sure you have a set with each number, 1 through 5. Also take one of the sealed envelopes for your group.

Pass the first container around so that each person can smell what’s inside. On the chart, write the container number and the name of what you think you are smelling for each container.

When your group has guessed all the scents, open the envelope. Record which guesses were correct and which were incorrect.

Container # Your scent identification Correct/incorrect?

1) What memories or emotions did the scents provoke?




Demonstration: Touch SensorsStudent Names:_______________________________________________________ Date:___________

Instructions: Working in pairs, test the sensitivity of the skin on different parts of your partner’s body and record the results below.

You and your partner should have four strips of cardboard of different lengths, each with two paper clips taped to it a certain width apart.

For each part of the body, start with the strip that has the paper clips farthest apart. Tell your partner to close his or her eyes. As you hold the strip with the tips of the clips pointing toward your partner, press the tips lightly into your partner’s skin (do not press hard!). Ask your partner how many paper clips he or she feels, and record the answer.

Forearm

Width of paper clips How many points are felt?

40 mm

30 mm

20 mm

5 mm

Forehead


40 mm

30 mm

20 mm

5 mm



Cheek


40 mm

30 mm

20 mm

5 mm

Palm of hand


40 mm

30 mm

20 mm

5 mm

Fingertips


40 mm

30 mm

20 mm

5 mm

What conclusions can you draw about the sensitivity of skin on different parts of your body?




Diagram: Hearing and BalanceStudent Name:_______________________________________________________ Date:___________

Directions: Label the diagram and make notes as you watch the PowerPoint presentation Hearing and Balance.



Fill in the chart below.

Structure Outer, middle, or inner ear? Function

Auricle

Auditory canal

Eardrum

Auditory ossicles

Stirrup Anvil Hammer

Cochlea

Vestibule

Semicircular canals

What part of the brain are sound signals sent to?

What part of the brain are equilibrium signals sent to?

What is the difference between dynamic and static equilibrium?




Reading: Hearing and Balance



A baby’s hearing is developed before it’s born, when it is about 20 weeks old. In the womb, a baby can hear its mother’s voice, heartbeat, and breathing. As newborns, babies seem to pay more attention to high-pitched voices of women than lower pitched voices of men. Many people believe that hearing is the last sense to go when a person is dying, and there is some evidence to support this. Some researchers have found that the brain waves of people who are in a coma are affected when people speak to them, even though they show no signs of response to other senses.



When we’re talking about someone’s ears, we’re usually referring to their outer ears, the part we can see. The main visible part of the ear is called the auricle. Your ears would actually function just fine without this part, but in many other mammals, the auricle is shaped in a way that helps gather sound and direct it into the ear. Your auricle surrounds the auditory canal, a small tunnel that sound waves travel through to the other parts of your ear. The auditory canal is about an inch long and contains glands that secrete the waxy yellow substance we know as earwax.



At the end of the auditory canal—and the end of the outer ear—is your eardrum, which is also sometimes called the tympanic membrane. The eardrum is a small window of membrane that stretches across the end of the auditory canal, like a tiny skin stretched over a drum, creating a flat end to the auditory canal. This is where all the action involved in hearing begins. When sound waves hit the eardrum, they set it in motion, making tiny vibrations that reflect the volume, pitch, and other qualities of the sound.



Just behind your eardrum are the three smallest bones in your body. Together, the three bones—the hammer, anvil, and stirrup—would fit onto a dime. Note that the medical terms for these bones are the incus, malleus, and stapes, respectively. The eardrum vibrates against the hammer, which moves the anvil, which in turn moves the stirrup. These three bones are called the auditory ossicles. The middle ear is also the place where you feel your ears “fill up” or “pop” when you go up or down in an airplane. This happens because the pressure outside of your ear changes when you change altitude, but the pressure on the middle ear doesn’t. When your ears fill up, voices and other sounds might be muted, because the eardrum vibrates best when the pressure on both sides of it is the same. The secret to fixing this? In your middle ear is a tube that leads from the ear to your throat. Usually this tube is flattened and closed, but when you yawn or swallow, it opens a bit, allowing the pressure to even out. Whatever you do, don’t hold your nose and blow to try to solve the problem: it means you’re putting a dangerous amount of pressure on your eardrum.



The stirrup bone sits right up against a spiraling, bony structure in the ear called the cochlea. The cochlea contains fluid that picks up the vibrations of the stirrup as they are passed along, like ripples in a pond. The inside walls of the cochlea are lined with cilia, which serve as mechanoreceptors, detecting these tiny ripples as they move the cilia. These signals then go to the cochlear nerve, which sends the information to the temporal lobe of the brain.



Sound usually hits each ear at slightly different times. Your brain processes this stereo sound as a way to figure out where the sound is coming from.



You’ve probably heard the phrase “the five senses,” but we really have six. The sense of equilibrium is harder to characterize, because it is more of an intuitive feeling than a sensation brought on by outside stimuli. Your sense of equilibrium is at work all the time, allowing you to orient yourself in space and to keep from falling over. Two parts of your inner ear, the vestibule and the semicircular canals, attend to this sense. These two parts contain gelatinous fluids and, like the cochlea they are connected to, their inner surfaces contain cilia that act as mechanoreceptors. But while the receptor cells in the cochlea respond to tiny vibrations within the fluid, the receptors responsible for equilibrium detect a change in position of the fluid container—your ear—as you move your head or move your body in space. These two structures send signals to your cerebellum via the vestibular nerve, making equilibrium an absolutely separate sense from hearing.



Your sense of static equilibrium is how you know which way is up. Inside the vestibule, there are tiny structures that move with the force of gravity when you shift your head. As they move, they slide over the cilia in the vestibule, which triggers a message sent to your brain. Even when you stop moving your head, the vestibule continues to send information so that you know where your head is positioned. It’s the sense of static equilibrium that directs you to keep your head upright unless you intend to look down.



You do a lot more during the day than just tilt your head in a straight line, and your body has to be able to adjust to all your movements. The three semicircular canals in your inner ear detect movements in any direction, because each of these half circles is oriented in one of the three planes of space. As in your vestibule, inside the canals there is a gelatinous fluid that gets pulled across cilia mechanoreceptors at the base of the canals.




Investigations: Hearing and BalanceStudent Name:_______________________________________________________ Date:___________

Instructions: Working in pairs, complete the two experiments described below and record your results as answers to the questions that follow.

Rinne Hearing TestThe Rinne test is a common test used by audiologists to compare how well someone can hear sound that is conducted through their bone with how well they hear sound conducted through the air. Often, people with hearing loss can hear a sound if it is caused by something vibrating against a bone rather than traveling through the air.

Before doing this test, you may want to locate the bone behind your ear or behind your partner’s ear.

1. Hold the base of the tuning fork with one hand and tap the tines gently against the heel of the other hand. This sets the tuning fork into vibration.

2. Place the base of the tuning fork against the bone behind your partner’s ear. Tell your partner to let you know when he or she no longer hears the sound.

3. Immediately move the tuning fork to about one-half inch away from the auditory canal. Ask your partner whether the sound of the tuning fork is still audible.

4. Have your partner repeat the exercise on you.

What differences did you notice in the sound when the tuning fork was against your bone versus next to your ear?

Spin TestWhen you spin and suddenly stop, the fluid in your ears keeps moving for a little bit. This sends the signal to your brain that you’re still spinning even though you aren’t. While your brain thinks that you’re spinning, it will tell your eyes to keep expecting what’s in front of them to unfold in the same direction as the spin.

5. Choose one person to be the spinner and the other to be the observer. The spinner closes his or her eyes.

6. The observer should spin the spinner around 5–10 times.

7. The observer should then stop the spinner and tell the spinner to open his or her eyes.

8. The observer should note what the spinner’s eyes are doing immediately after the spinner has stopped.

What did you see happening to the spinner’s eyes?




Reading: Hearing LossStudent Name:_______________________________________________________ Date:___________

Instructions: First, read about the different kinds of hearing loss below. Then, using the information you have learned, answer the questions that follow about the kind of hearing loss you think is involved in each case described.

Types of Hearing LossConduction (conductive) hearing loss arises when something interferes with the passage of sound waves from outside the ear through the auditory ossicles and into the cochlea. It can be caused by something as simple as a buildup of wax in the ears, or it can be a sign of infection in the inner ear or an injury to the eardrum. Patients with conduction hearing loss can hear a tuning fork held against the bone even if they can’t hear the sound when the tuning fork is held next to the ear, because the structures responsible for the sense of hearing are still intact. This type of hearing loss is often treatable by removing the blockage or treating the infection.

Sensorineural hearing loss results from damage to or degradation of the mechanoreceptors in the cochlea. Some sensorineural hearing loss can be inherited and come with aging. This type of hearing loss is also common among younger people who listen to loud music for extended periods, particularly using headphones or earbuds. In patients with sensorineural hearing loss, there is no difference in what they hear in a Rinne test regardless of whether the tuning fork is on the bone or next to the ear, because the damage is to the receptors responsible for hearing. This type of hearing loss is not treatable, but hearing aids may help.

CasesAs an audiologist, Anders asks each patient a few questions and performs a few tests to help him understand what has gone wrong. Here are notes from two of his cases:

1. The patient explained that she could hear fairly well in her right ear but sounds are faint or muffled in the left. She was tested for sensitivity to volume and tone, and this test showed that she was not hearing fainter sounds in her left ear. When asked if her ears were regularly exposed to loud noises or loud music, she said no. A Rinne test showed that she could hear through bone conduction better than through air conduction. An examination of the ear canal revealed a waxy blockage in the left ear.

What type of hearing loss does this patient have, and why do you think so?

What do you think is causing the hearing loss?



2. The patient explained that people were telling him that it seemed he didn’t hear things that he once used to hear. A test for sensitivity to volume and tone showed that both ears were less sensitive to faint sounds than is normal. What asked if his ears were regularly exposed to loud noises or music, the patient said that he frequently uses earbuds while riding the bus, where he needs to turn them up loud to hear over the other noise. A Rinne test showed that bone and air conduction were the same. Upon examination, each ear canal looked clear.

What type of hearing loss does this patient have, and why do you think so?

What do you think is causing the hearing loss?




Notes: The Eye and VisionStudent Name:_______________________________________________________ Date:___________

Directions: Label the diagrams and make notes as you watch the PowerPoint presentation The Eye and Vision.

1. Label the diagram

2. Fill in the chart below for each part of the eye that you have labeled.

Structure in the eye Function



Structure in the eye Function

3. What do the aqueous and vitreous humors do?

4. What nerve carries the message from your eyes to your brain?

5. What are the two types of photoreceptors and what kind of light is each sensitive to?

6. What causes colorblindness?

7. Draw a diagram of where light is focused in a normal eye.



8. Draw diagrams of where light is focused in:

a) A nearsighted eye

b) A farsighted eye




Reading: The Eye and Vision



Seeing probably isn’t something you feel that you have to work at, but you did when you were a newborn. Unlike the other senses, vision takes months to fully develop after birth. A newborn baby has to develop visual skills that we take for granted: being able to focus on an object as it’s moving, being able to tell one object from another, and having a sense of how far away something is when you see it. In fact, babies often have a hard time even getting both eyes to move together (try now, as an adult, to get them to move separately…not so easy!), and researchers believe that babies don’t see color fully until they are four or five months old. So during the first year of your life, a lot of development was taking place in your eyes.



Your eyeballs measure about an inch across, though you can only see about a sixth of their surface area. Unlike your other sense organs, the eyes have a lot of protective casing: they’re protected by the eye sockets of your skull, a set of muscles, your eyelids, a cushion of fat around your eye sockets, and a salty solution we know as tears, that keeps the eye clean and prevents infection. There are six muscles attached to each eyeball, and the movements they make are the fastest and most precise movements of any body part.



The superficial layer of your eyeball, the part that contains the “whites of your eyes,” is covered by a tough, fibrous protective layer called the sclera. Lining the inside of your eyeball is the retina. The retina is the sensory layer of the eye, where the photoreceptor cells are located. Part of retina also contains cells that destroy and remove dead or damaged photoreceptor cells. Between these two layers is a layer that provides a rich blood supply and that keeps light from scattering once it has entered the eye.



The cornea is a region in the sclera that is clear and allows light to pass through. The cornea covers the colored part of your eye and your pupil, and is a very effective protector. The cornea is the most exposed and vulnerable part of your eye and can easily be scratched. But it is remarkably quick at healing itself. It can patch up a scratch in just a couple of days. Because it doesn’t have any blood vessels of its own, it is not connected to the immune system, making it the only tissue in the body that doctors can transplant without having to be concerned about it being rejected by the patient.



The iris is the colored part of your eye. The pupil isn’t actually a part at all, but a hole in the iris. By controlling the size of the pupil, the iris allows you to focus on an object as best you can given the light in your environment. When there is less light, or if you’re looking at something that’s close to your eyes, the muscles in the iris contract, making the pupil larger and letting in more light. When you’re in brighter circumstances or looking at something far away, the iris muscles relax and the pupil gets smaller.



The lens is a flattish, flexible structure connected to the eye by ligaments. It changes shape to focus light coming from different distances onto the retina at the back of the eye. Between the lens and the cornea is a small space filled with a fluid called aqueous humor. Aqueous humor helps maintain the internal pressure in the area anterior to the iris, keeping it separate from the cornea. The aqueous humor also supplies nutrients and oxygen to the cornea and lens. A similar but more gelatinous fluid called vitreous humor fills the posterior part of the eyeball. This fluid also serves the purpose of maintaining shape and pressure inside the eyeball.



On the retina are two different photoreceptor cells named to roughly reflect their shapes. Rods detect dim light and gradations of gray and shadows. Rods allow us to see at night and also to detect when objects are moving, because we can see the movement of dark and shadowed parts of those objects. Rods are also important to peripheral vision; you have the most rods on the edges of your retina, with fewer toward the middle. Rods contain vitamin A, which is important in their function of being stimulated by light.

Cones give us color vision. There are three types of cones, and each one is sensitive to a different range of color. All are sensitive to at least a bit of green, which means that you can perceive more shades of green than of any other color. When light hits any of the photoreceptors on your retina, these special cells stimulate interneurons, which then send signals to sensory neurons that are part of the optic nerve, where they are carried to the brain, ultimately ending up in the occipital lobe.



If one type of a person’s cones is missing or not functional, that person will have a form of color blindness. Most often, it will either be the cone that detects mostly reddish hues or the cone that detects mostly greenish hues. In those cases, the person will see these two colors as the same: if they’re missing the red cone, they’ll see red as green, and if they’re missing the green cone, they’ll see green as red. In other words, they won’t see the number in this image. There are also cases where none of a person’s cones work, and these people see no color at all. Color blindness is more common in men than women.



Usually your rods and cones are working together somewhat equally. But unlike your rods, your cones require a significant amount of light to be able to relay color information. When you step into a dark movie theater, it’s hard to see, because your cones aren’t getting enough light. After 5 or 10 minutes, though, your rods start to take over the task of seeing. When your eyes are adjusted to the dark, you can’t see colors as well, but you can see light and shadow better than before. When you step back out into the light, the world can appear very bright, because your rods are a bit “blown out” by the amount of light and need a few minutes to readjust to the conditions.



All the neurons leading from near your retina to your occipital lobe leave through the posterior of your eye, wrapped as a bundle called the optic nerve. The optic nerve is so complex that doctors can’t reconstruct it, which is why they can’t transplant eyes.

At the place where this nerve bundle exits your eye you don’t have any photoreceptor cells. We call this the blind spot, because any light or image that falls on this part of the retina isn’t conveyed to the brain. Your eyes and brain have ways to make up for your blind spots, and you’re usually not aware of them. But you can test yourself by looking at two spots on the wall, for example, that are a short distance apart, and seeing that, as you move closer to them, one of them will disappear when the image of it is falling on your blind spot.



When light passes through the lens of your eye, the light rays get bent. When you’re focusing your sight on an object, the place where all these bent rays come together is a small distance in front of the retina. Because the light rays cross each other slightly after having been bent, the image that reaches your retina is actually upside down. When your brain gets the information from your eyes, it turns the image right side up. The lens in your eye has the ability to change shape ever so slightly to make sure the light rays are bent at the right angle for you to see an object, no matter whether it’s very close up or very far away.



If the lens in your eye bends the light too little or too much, the point of focus won’t land directly on your retina. If the light is bent too far (or the distance of the eyeball is too long), the point of focus will fall too far in front of the eyeball, and you won’t be able to see things that are far away. You may, however, be able to see things close up; in other words, you will be nearsighted. If the lens doesn’t bend the light rays enough, the point of focus will be somewhere beyond the eyeball, which causes farsightedness.



To make up for nearsightedness and farsightedness, we use eyeglasses and contacts to adjust how much the light is bent before it enters our eyes. If you wear glasses, take them off and see what shape the lenses are. If they’re thinner in the middle than at the edges, they are concave lenses, which means you’re nearsighted. These lenses spread the light out just a bit so that it can travel further through your eye and take the point of focus a little further back. If your lenses are thicker in the middle, they are convex lenses. Convex lenses will bring the point of focus forward, which addresses farsightedness. If you wear glasses, the lenses in them are made especially for your eyes, to bend the light in the right way so that the point of focus falls on your retina.



Anyone who has looked into someone else’s eyes knows that this famous line from St. Augustine is true: we can get hints about what someone is feeling by looking at their eyes. How this happens is something of a mystery, but researchers have found that our pupils get bigger when we’re experiencing strong emotions, and also when we’re focused on a challenging task. In fact, some scientists have said that the amount our pupils respond to emotion and concentration is the most precise and predictable muscle movement we make in our whole bodies. So, we may use our eyes mostly to see the world around us, but people in the world around us can also look at our eyes and learn something about us.




Lab: Cow Eye DissectionStudent Name:_______________________________________________________ Date:___________

Instructions: With a partner, follow the steps below to dissect a cow eye. Answer the questions that follow and identify the parts of the eye according to the instructions.

1. Examine the outside of the eye. What observations can you make about the external features of the eye?

a)

b)

c)

2. What parts of the eye can you see from the outside?

3. Cut away as much fat and muscle as you can. Locate the optic nerve. You may be able to pinch the optic nerve and see the fibers that make it up. You may see a bit of white pasty substance come out when you squeeze it; this is myelin.

4. Using the scalpel in your dissection kit, make an incision across the cornea. Be careful not to cut yourself!

What is the clear liquid that comes out, and what is its function?

5. Now you are going to cut off the back half of the eyeball. Again, use the scalpel to make an incision through the sclera, and use the scalpel or a scissors to cut all the way around the eyeball, into a front half and a back half.

What is the substance that’s inside the back part of the eyeball, and what does it do?

6. Now look at the inside of the front half of the eye. You should see the round lens and the iris. The iris may be stuck to the cornea or it may be loose. Pull it out and try to keep it in one piece.

a) What type of tissue is the iris?

b) What is the hole in the middle of it?



c) Draw a picture of the iris.

7. Pull out the lens of the eye. a) Hold it up and look through it at your partner. What do you notice?

b) Put the lens on the word in the box on the right (put it directly on the paper).

What do you notice?

8. Now look at the back half of the eye.

a) What is the thin filmy piece that is attached to the back of the eye?

b) What is its function?

c) What is the area where this part connects to the back of the eye?




Reading: Optical IllusionsStudent Name:_______________________________________________________ Date:___________

Instructions: Complete the reading and answer the questions below.

If an unreliable friend promises to do something for you, you may tell him, “I’ll believe it when I see it.” And if you see it happen, you’re likely to believe it’s true. But as you’ve learned, your senses sometimes perceive things differently from what they really are. By studying our perception of magic tricks and other sensory illusions, scientists have learned a lot about how the brain works, and some of its limitations, too.

It’s possible to trick any of our senses, but the most popular illusions are optical illusions. Try out a few of these, and then look at the explanations that follow.

A. What is this a picture of?

B. Are there circles in these figures?



C. Is it possible to make one of these?

D. Are the horizontal lines in this image parallel or slanted? Hold a straight edge up to them to find out.



A. This illusion is called a Rubin vase. The image changes from faces into a vase because our brains translate what we see based on the other objects around it, and we can only focus on one main object while the others become the background objects. So if you focus on the white, the faces come into view. If you focus on the black, the faces fall to the background and you perceive a face.

B. There aren’t any circles, but your brain fills in the contrasting spaces of alternate black and white, making you think there is a shape there. This type of image is called a contour figure illusion.

C. No. This is an illusion called an impossible object. When you first see this kind of image, you initially think that it’s an ordinary shape. But your brain tells you something isn’t quite right. When you look more closely, you can see that the object has an impossible outline. Because your brain wants to explain the object, it continues to translate the image as being something three dimensional.

D. This is a very famous effect called the Café Wall Illusion, and it’s a form of distortion illusion. The horizontal lines are parallel even though they look tilted. There are many things at work on your brain in this image; one important aspect is that when black and white shapes are placed in particular patterns relative to each other, your brain may translate the pattern in a way that makes some of the lines appear distorted.



Now, see if you can figure out which of these illusions demonstrates which of the principles above:

What type of illusion is this?

What type of illusion is this?

What type of illusion is this?What type of illusion is this?


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