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Which part of human eyes is getting tired?

Which part of human eyes is getting tired?


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It's a thing of common sense that if you read, drive or look at computer screen for too long your eyes will get tired. They burn, itch and try to close all the time.

But I was wondering which part(s) of them need rest so much… Blinking muscles? Lense muscles? Or the eyeball is drying out?


These symptoms have a name: Computer vision syndrome. Basically our eyes are made to look at longer distances from 1-6 meters without much accommodation. Typically computer screens are located at a much closer distance (30-50cm), which requires constant accommodation by the eye. This leads to high stress on the muscles in the eye which subsequently get tired. Additionally less blinking leads to drier eyes.

One of the common recommendations against this is to regularly focus further located objects to give the eye the chance to relax. This is sometimes called the "20-20-20" rule, every 20 min focus on a 20 feet (about 6 m) far object for 20 seconds. See the article in the Wikipedia and here for more information (and probably also the linked references in the articles).


Tired Eyes — What You Can Do to Wake Your Eyes Up

Written by
Dr. Mounir Bashour

We most commonly have tired eyes at bedtime, right before falling asleep. The eyelids feel heavy and they begin to droop. Your field of vision narrows as you squint and blink at the TV screen, trying to stay awake. Then your eyelids become even heavier and close when the need for sleep takes over.

It is also common to have tired eyes at the end of a long day, after many hours of concentration during which you must keep your eyes open. Staring at a computer, reading, scanning, watching — whatever your eyes do is a workout for the eye and eyelid muscles.

Most of the time, tired eyes are simply a sign of muscle fatigue. This is why rubbing tired eyes temporarily revives them. The rubbing increases the blood flow in the area, and like a massage of the calf muscles after exercise, it helps loosen the muscles, making the eyelids feel less heavy. Tired eyes from muscle fatigue may also appear red and puffy.

In some instances, the eyes may look tired but you may not be tired. In people born with thicker eyelids, the eyes appear to droop. As people age, fat can accumulate around the eyes, and the extra tissue can make the eyes look tired.

This is often described as bags under the eyes. For those with excess tissue around the eyes, eyelid surgery (blepharoplasty) can improve both cosmetic appearance and vision.

Resting tired eyes is usually all that is needed to return them to normal, and not resting tired eyes can lead to eye strain. Although it is usually harmless, eye strain can lead to other problems, such as headache, dry eyes, irritability, and eye pain.


Which part of human eyes is getting tired? - Biology

This "after-image" happens because the images we "see" are created by the brain based on signals that our eyes send. Usually what we see is pretty accurate, but our brain can be fooled too.

If you could open an eyeball, you would see that it is a hollow ball (or sphere) filled with liquid. Check out the picture here: anatomy of the eye .

Light comes in the pupil at the front and hits the retina at the back. The retina contains lots of tiny sensors that pick up light. Rods are the sensors that allow us to see black and white and work very well when there's not much light. We're interested in the cones today because they allow us to see color. (Why do you think cats are colorblind? Think about when cats do their hunting.)

When a particular color of light hits a particular spot in the retina, a particular cone sends a message to the brain. Let's say it's a cone in the center of the retina that responds only to red light. Take a look at the red dot in the green field at this site: here . That one cone sends a message to the brain saying "there's something red in the center of your vision". So do all of the many red cones around it. The chemical reactions responsible happen incredibly fast. Then it takes the cone a short time to recover. You will still see red, but some cones are recovering while other cones are sending the signal. It's like taking turns with your friends to get a big job done. You keep working, but all of you get tired. Now when you look away, the red cones are all sending weaker signals.

White light is made up of all the colors. (You can test this by using a prism to separate white light into a "rainbow.") When you look at something white, all of the cones in your eye send a signal, but now your red cones send a weaker signal. The blue and green cones send a strong signal, so the white area looks like it has a blue green dot in the center.

Think about it this way, let's say that everyone in your class were assigned a color. When your teacher held up a paper with your color on it, you would yell out that color. If your teacher held up a red paper for a long time, you would get tired of yelling. Then when the teacher held up a paper containing all of the colors, all of the blue and green kids would be able to yell their colors loudly, while you would barely be able to say "red." A person listening to your class would think the paper was blue or green.

You might be getting confused, saying, "When I mix all of my paint colors together, I get brownish black, not white." I hope you did, because that shows that you are thinking for yourself and comparing what I say to what you have seen. The reason you get brown/black when you mix paints is because you are mixing "pigments." What your eye sees is the light that is reflected off the pigments. All pigments mixed together would give you black. All of the light is absorbed by a black object and none is reflected back to your eye. (That's why black objects heat up so fast, they absorb light.) A white object reflects all of the colors back to your eye instead of absorbing them. Your class might want to experiment with this idea. All you need are lamps with colored light bulbs or flashlights with colored plastic over them.

This question has everything to do with how your eye sees color. I'm not an expert in biology but I will try to answer your question.

In the back of your eye, there are cells specifically designed to detect light of one particular color. Now, there are infinitely many colors so it would seem that you would need a lot of cells to be able to see all the colors that you see. Your eye is very tricky, though. You have cells which are designed to pick up only red, green, and blue light. If you are seeing a color in between red and green, like yellow, your eye will pick up a little red and a little green. Your brain can then take that information and combine it to make yellow.

This explains how color TVs work, by the way. There are lots of tiny red, green, and blue dots on a TV. By turning on both red and green, the red and green cells in your eye are triggered and your brain thinks that you are seeing yellow light.

If you spend a long time looking at something red, the cells in your eye that are supposed to see red get "tired". Of course, they don't really get tired the way your muscles get tired. I think what happens is that your brain is designed to recognize colors no matter what color of light you shine on them. A green leaf looks green whether you put it under a reddish light or a yellowish light. In fact, the light from the sun is not white light, it is yellow light. Your eyes are adjusted to that, though, so white objects still look white.

When you look at something red for a long time, the cells in your eye adjust by becoming less sensitive to red light. Now, when you suddenly look away from the red, your green and blue cells are more sensitive than your red cells and you end up seeing a greenish-blue spot.

Good questions! I've asked that this question get sent to a biologist. They would know more about how the brain and eyes work. Maybe they can give you a better reason why your cells become less sensitive when they see the same color too long.


Vision Basics: How Does Your Eye Work?

It’s all about light. Light reflects off an object, and if that object is in your field of vision, it enters the eye.

The first thing it touches is a thin veil of tears on the surface of the eye. Behind this is your eye’s front window, the cornea. This clear layer helps focus the light.

On the other side is liquid called the aqueous humor. It circulates throughout the front part of your eye and keeps pressure inside constant

After the aqueous humor, light passes through the pupil. This is the central round opening in your iris, the colored part of your eye. It changes size to control how much light gets in further back. Next up is the lens. It works just like a camera to focus light. It adjusts shape depending on whether the light reflects off something near to you or far away.

This light now pierces the center of the eyeball. It’s bathed in moisture from a clear jelly known as the vitreous.

Continued

Its final destination is the retina, which lines the back of your eye. It’s like the screen in a movie theater or the film in a camera. The focused light hits cells called photoreceptors.

Unlike a movie screen, the retina has many parts:

Blood vessels bring nutrients to your nerve cells.

The macula is the bull's-eye at the center of your retina. The dead center is called the fovea. Because it's the focal point of your eye, it has more special, light-sensitive nerve endings, called photoreceptors, than any other part.

Photoreceptors come in two kinds: rods and cones. They’re special nerve endings that convert the light into electrochemical signals.

Retinal pigment epithelium (RPE) is a layer of dark tissue beneath the photoreceptors. These cells absorb excess light so the photoreceptors can give a clearer signal. They also move nutrients to (and waste from) the photoreceptors to the choroid.

The choroid is separate from the RPE. It lies behind the retina and is made up of many fine blood vessels that supply nutrition to the retina and the RPE.

Continued

Sclera is the tough, white, fibrous outside wall of your eye. It’s connected to the clear cornea in front. It protects the delicate structures inside the eye.

Signals from the photoreceptors travel along nerve fibers to the optic nerve. It sends the signals to the visual center in the back of the brain.

And that’s how you see: Light, reflected from an object, enters the eye, gets focused, is converted into electrochemical signals, delivered to the brain, and is interpreted, or "seen," as an image.


Where Do Green Eyes Originate From?

Green eyes are most common in Northern and Central Europe though they can also be found in Southern Europe as well as Western Asia. As was mentioned earlier, brown hair and eyes are dominant in most regions, though there are several countries where it is actually more common to have green or blue eyes than brown eyes.

For example, in Ireland and Scotland, 86% of the population has either blue or green eyes, and in Iceland, 89% of women and 87% of men have blue or green eyes. Among European Americans, green eyes are most common in people of recent Celtic or Germanic ancestry. Green eyes also tend to be more common in women.


Human Body By Anatomical Region

The human head consists of a fleshy outer portion covering a bony substructure called the skull. The primary purpose of the head is to contain and support the brain and primary sensory organs such as the mouth, eyes, ears, and nose. The head is probably one of the more delicate parts of the human body, as the vascular and musculature structures of the head consist of a number of small interconnected parts that need to be situated very particularly to function properly. The parts of the human head include:

  • Skull
    • Cranium (holds the brain)
    • Mandibles (lower jaw)
    • Maxilla (upper jaw)
    • Nasal bone
    • Eyes
    • Nose
    • Ears
    • Mouth
      • Tongue
      • Teeth
      • Trachea
      • Esophagus
      • Cervical vertebrae

      Torso

      The torso or “trunk” is the largest section of the human body and composes the bulk of the human body. The main function of the torso is to provide shape and structure to the human body and to house its vital internal organs such as the heart, lungs, stomach, intestines, liver, and kidneys. The torso also contains the majority of blood vessels that provide oxygen to the whole body. The parts of the human torso include:

      • Shoulders
      • Chest
        • Pectoralis (upper chest)
        • Ribcage
          • Lungs
          • Heart
          • Abdominal muscles
          • Stomach
          • Kidneys
          • Liver
          • Small Intestines
          • Large Intestines
          • Colon
          • Rectum
          • Spine
          • Gluteus maximus (buttocks)

          Limbs

          The next important division of the human boy is its 4 limbs. The limbs are attached to the torso and their primary purpose is to interact with the environment via locomotion with the legs and manipulating objects with the arms. Humans are unique among tetrapods (organisms with 4 limbs) in that 2 are specialized for locomotion (legs/feet) and 2 are specialized for manipulating objects (arms/hands). Except for some parts of the face, the limbs, the hands in feet, in particular, contain the most nerve endings are so are specialized to sense touch. The main limbs of the human body are:

          • Arms
            • Brachium (upper arm)
              • Humerus
              • Biceps
              • Triceps
              • Elbow
              • Ulna
              • Radius
              • Hand
                • Carpals
                • Metacarpals
                • Thigh
                  • Quadriceps
                  • Hamstring
                  • Knee
                  • Shin
                    • Tibia
                    • Fibia
                    • Tarsals
                    • Metatarsals

                    Keeping Time

                    Body synchrony is the final and most intriguing component of the pickup. As potential lovers become comfortable, they pivot or swivel until their shoulders become aligned, their bodies face-to-face. This rotation toward each other may start before they begin to talk or hours into conversation, but after a while the man and woman begin to move in tandem. Only briefly at first. When he crosses his legs, she crosses hers as he leans left, she leans left when he smoothes his hair, she smoothes hers. They move in perfect rhythm as they gaze deeply into each other's eyes.

                    Called interactional synchrony, this human mirroring begins in infancy. By the second day of life, a newborn has begun to synchronize its body movements with the rhythmic patterns of the human voice. And it is now well established that people in many other cultures get into rhythm when they feel comfortable together. Our need to keep each other's time reflects a rhythmic mimicry common to many animals. Chimps sometimes sway from side to side as they stare into one another's eyes just prior to copulation. Cats circle. Red deer prance. Howler monkeys court with rhythmic tongue movements. Stickleback fish do a zigzag jig. From bears to beetles, courting couples perform rhythmic rituals to express their amorous intentions.


                    A New Bionic Eye Could Give Robots and the Blind 20/20 Vision

                    A bionic eye could restore sight to the blind and greatly improve robotic vision, but current visual sensors are a long way from the impressive attributes of nature’s design. Now researchers have found a way to mimic its structure and create an artificial eye that reproduces many of its capabilities.

                    A key part of what makes the eye’s design so powerful is its shape, but this is also one of the hardest things to mimic. The concave shape of the retina—the photoreceptor-laden layer of tissue at the back of the eye—makes it possible to pick up much more light as it passes through the curved lens than it would pick up if it was flat. But replicating this curved sensor array has proven difficult.

                    Most previous approaches have relied on fabricating photosensors on flat surfaces before folding them or transplanting them onto curved ones. The problem with this approach is that it limits the density of photosensors, and therefore the resolution of the bionic eye, because space needs to be left between sensors to allow the transformation from flat to curved.

                    In a paper published last week in Nature , though, researchers from Hong Kong University of Science and Technology devised a way to build photosensors directly into a h emispherical artificial retina . This enabled them to create a device that can mimic the wide field of view, responsiveness, and resolution of the human eye.

                    “ The structural mimicry of Gu and colleagues’ artificial eye is certainly impressive, but what makes it truly stand out from previously reported devices is that many of its sensory capabilities compare favorably with those of its natural counterpart,” writes Hongrui Jiang, an engineer at the University of Wisconsin Madison, in a perspective in Nature .

                    Key to the breakthrough was an ingenious way of implanting photosensors into a dome-shaped artificial retina. The team created a hemisphere of aluminum oxide peppered with densely-packed nanoscale pores. They then used vapor deposition to grow nanowires inside these pores made from perovskite, a type of photosensitive compound used in solar cells.

                    These nanowires act as the artificial equivalent of photoreceptors. When light passes over them, they transmit electrical signals that are picked up by liquid metal wires attached to the back of the retina. The researchers created another hemisphere made out of aluminum with a lens in the cente r to act as the front of the eye, and filled the space in between it and the retina with a n ionic liquid designed to mimic the fluid aqueous humor that makes up the bulk of the human eye.

                    The researchers then hooked up the bionic eye to a computer and demonstrated that it could recognize a series of letters. While the artificial eye couldn’t quite achieve the 130-degree field of view of a human eye, it managed 100 degrees, which is a considerable improvement over the roughly 70 degrees a flat sensor can achieve.

                    In other areas, though, the approach has the potential to improve on biological eyes. The researchers discovered that the nanowires’ photodetectors were actually considerably more responsive. They were activated in as little as 19.2 milliseconds and recovered to a point where they could be activated again in 23.9 milliseconds. R esponse and recovery times in human photoreceptors range from 40 to 150 milliseconds .

                    The density of nanowires in the artificial retina is also more than 10 times that of photoreceptors in the human eye , suggesting that the technology could ultimately achieve far higher resolution than nature.

                    The big limitation at the moment is wiring up these photosensors. The liquid metal connections are currently two orders of magnitude wider than the nanowires, so each one connects to many photosensors, and it’s only possible to attach 100 wires to the back of the retina. That means that despite the density of photosensors, the eye has a resolution of only 100 pixels.

                    The researchers did devise a way to use magnetic fields to connect nickel microneedles to just three nanowires at a time, but the process is a complicated manual one that would be impossible to scale up to the millions of nanowires present in the artificial retina. Still, the device represents a promising proof-of-concept that suggests that we may soon be able to replicate and even better one of nature’s most exquisite designs.

                    “ Given these advances, it seems feasible that we might witness the wide use of artificial and bionic eyes in daily life within the next decade,” writes Jiang.


                    Hypermetropia

                    Hypermetropia or long-sightedness is a defect of an eye where a person cannot see nearby objects clearly. The near-point of the hypermetropic eye is more than 25 cm away. This defect of the eye is caused due to:

                    In the case of hypermetropia, the image of an object is formed behind the retina and therefore, a person cannot see clearly nearby objects.

                    The near-point of an eye having hypermetropia is more than 25 cm. The condition of hypermetropia can be corrected by putting a convex lens in front of the eye. This is because when a convex lens of suitable power is placed in front of the hypermetropic eyes, then the convex lens first converges the diverging rays of light coming from a nearby object at the near point of the eye at which the virtual image of the nearby object is formed. Since the light rays now appear to be coming from the eye’s near point, the eye-lens can easily focus and form the image on the retina. A convex lens is used for hypermetropia so as to increase the converging power of the eye-lens.

                    Correction of Hypermetropia: The convex lens forms a virtual image of the object (lying at normal near point N) at the near point N’ of this eye.

                    The formula for calculating power of convex lens to correct hypermetropia is:

                    In this formula, object distance that is u is normal near the point of the eye (25 cm).


                    Effects of Aging on the Eyes

                    In middle age, the lens of the eye becomes less flexible and less able to thicken and thus less able to focus on nearby objects, a condition called presbyopia. Reading glasses or bifocal lenses can help compensate for this problem. For more information on the effects of age on the eye, see Changes in the Body With Aging: Eyes.

                    In old age, changes to the eye include the following:

                    Yellowing or browning caused by many years of exposure to ultraviolet light, wind, and dust

                    Random splotches of pigment (more common among people with a dark complexion)

                    Thinning of the conjunctiva

                    A bluish hue caused by increased transparency of the sclera

                    The number of mucous cells in the conjunctiva may decrease with age. Tear production may also decrease with age, so that fewer tears are available to keep the surface of the eye moist. Both of these changes explain why older people are more likely to have dry eyes. However, even though the eyes tend to be dry normally, tearing can be significant when the eyes are irritated, such as when an onion is cut or an object contacts the eye.

                    Arcus senilis (a deposit of calcium and cholesterol salts) appears as a gray-white ring at the edge of the cornea. It is common among people older than 60. Arcus senilis does not affect vision.

                    Some diseases of the retina are more likely to occur in old age, including macular degeneration, diabetic retinopathy (if people have diabetes), and detachment of the retina. Other eye diseases, such as cataracts, also become common.

                    The muscles that squeeze the eyelids shut decrease in strength with age. This decrease in strength, combined with gravity and age-related looseness of the eyelids, sometimes causes the lower eyelid to turn outward from the eyeball. This condition is called ectropion. Sometimes, because of age-related looseness affecting a different part of the eyelid, the lower eyelid turns inward, causing the eyelashes to rub against the eyeball. This condition is called entropion. When the upper eyelid is affected, the lid can droop, a condition called ptosis.

                    In some older people, the fat around the orbit shrinks, causing the eyeball to sink backward into the orbit. This condition is called enophthalmos. Because of lax tissues in the eyelids, the orbital fat can also bulge forward into the eyelids, making them appear constantly puffy. Enophthalmos, if significant, may cause a slight blockage of a person's peripheral (side) vision.

                    The muscles that work to regulate the size of the pupils weaken with age. The pupils become smaller, react more sluggishly to light, and dilate more slowly in the dark. Therefore, people older than 60 may find that objects appear dimmer, that they are dazzled initially when going outdoors (or when facing oncoming cars during night driving), and that they have difficulty going from a brightly lit environment to a darker one. These changes may be particularly bothersome when combined with the effects of a cataract.

                    Other changes in eye function also occur as people age. The sharpness of vision (acuity) is reduced despite use of the best glasses, especially in people who have a cataract, macular degeneration, or advanced glaucoma (see table Some Disorders That Affect Mainly Older People). The amount of light that reaches the back of the retina is reduced, increasing the need for brighter illumination and for greater contrast between objects and the background. Older people may also see increased numbers of floating black spots (floaters). Floaters usually do not significantly interfere with vision.


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