Vision Boom

Refraction part 2

*The lens is an avascular structure formed from about 1,000 layers of cells aligned in parallel that are tightly joined together. 1. Gaps don't form between the layers of cells as the lens changes shape. 2. The cells have lost their organelles; the cytoplasm is packed with crystallin proteins. b. The image on the retina is inverted (upside down) and reversed from left to right. i. The brain must do the work of adjusting the inverted image so that we can correctly see the world.

Cones

*require bright light; sensitive to different wavelengths of light (confer the ability to distinguish colors)

Rods

*very sensitive to light, but do not see color; thus, at night or in a darkened room, we see only shades of gray.

presbyopia

1. The impaired ability of the lens to thicken, and the consequent increase in the near point of vision, is known as presbyopia. -Bifocal lenses may be necessary for those with corrective lenses

Color vision

Cones are located primarily in the fovea and are activated by bright light. i. Allow us to detect the fine detail and color of an object -Color vision depends on three types of cones *Cones with a blue (S) pigment 1. absorbs light at short wavelengths *Cones with a green (M) pigment 1. absorbs light at medium wavelengths *Cones with a red (L) pigment 1. absorbs light at long wavelengths 2. Each pigment = retinal and opsin, but there is a slight difference in the structure of each opsin protein, which accounts for the individual absorption patterns of the different cones. -The stimulation of various combinations of these three types of cones (which represent the three primary colors) allows us to perceive any color. *The eventual perception of color is determined by our brain.

vision

Light is refracted as it passes through the eyeball, enabling images to be focused onto the retina

Farsightedness (hyperopia)

can see distant objects better than close objects i. Such people have a shortened eyeball. -When they try to see close objects, the image is focused behind the retina. -can be corrected with convex lenses to increase the bending of light rays so that the image of a close object can be focused on the retina

Visual accomodation part 2

ii. When we view a distant object, the ciliary muscle is relaxed (the suspensory ligaments attached to the ciliary body are pulled taut, widening the circle); thus, the lens becomes thinner and flatter. iii. When we view a nearby object, the ciliary muscle contracts (releasing the tension on the suspensory ligaments and causing the circle to narrow); thus, the lens rounds up (becomes thicker and rounder) due to its natural elasticity.

Photoreceptors

sensory receptor cells that transduce light stimuli into action potentials.

Effects of Belladonna

("deadly nightshade," in Italian = beautiful lady) 1. The plant produces atropine and scopolamine, drugs with anticholinergic effects. 2. Inhibitory effect is greatest on the parasympathetic nerves that normally cause the iris to constrict (i.e., smaller pupil) → causes the pupils to dilate

Rods

Rod cells are plentiful throughout the entire retina, except at the fovea. i. Therefore, rods provide us with peripheral vision and perception of motion. ii. The visual pigment in rods is a deep purple pigment called rhodopsin. -Rhodopsin iv.

photobleaching

Rods are very sensitive to light; in bright light, rhodopsin is destroyed faster than it can be manufactured (photobleaching). -As a consequence, rods are of little use in daylight, but very useful at night (night vision). - There is a "dark-adaptation period" needed for rhodopsin to regenerate after you leave the sunlight and enter a darkened room. *Complete recovery may take 20-30 minutes. *Sudden exposure to bright white light will destroy rhodopsin and the adaptation process will need to occur again. 1. Interestingly, red light used in a darkened environment will not break down rhodopsin.

Visual pathway to the Brain

The pathway for vision begins once light has been focused on the photoreceptors in the retina. i. Our photoreceptors are buried beneath a few layers of neurons; thus, incoming light must pass through these layers to reach our rods and cones. -Rods and cones have an outer segment joined to an inner segment by a stalk.

Color Blindness

a genetic defect a. In most instances, only one type of cone is defective or deficient in number. i. Extremely rare; most common is the inability to see the colors red and green (affects L and M cones). -affects about 8% of the male population compared to 0.5% of females *such individuals are referred to as dichromats *the affected gene is found on the X chromosome (males only inherit one X chromosome)

Nearsightedness (myopia)

can see close objects better than distant objects i. Such people have an elongated eyeball. -When they attempt to look at a distant object, the image is brought to focus in front of the retina. *They can see close objects because their lens can compensate for the long eyeball. -can be corrected with concave lenses, which diverge the light rays so that the image focuses on the retina

LASIK surgery

i. A cut is made in the cornea, allowing it to be folded back. ii. A laser is then used to reshape the cornea. -reduces the curve of the cornea for myopia (reduces refraction, moving the image back to the retina) -increases the curve for hyperopia (increases refraction) -creates a more spherical cornea for astigmatism (allows even refraction) iii. The cornea is allowed to heal on its own.

Glaucoma

drainage ducts are blocked, and aqueous humor builds up (increased intraocular pressure). -If untreated, the resulting pressure compresses the arteries that serve the nerve fibers of the retina, where photoreceptors are located. -The nerve fibers begin to die due to a lack of nutrients → total blindness can result -Detection = a puff of air is applied to the cornea, to measure its tightness

Eyestrain

fatigue of the ciliary muscle, caused by prolonged periods of work involving close objects. -The closest you can put an object to your face and still focus is called the near point of vision. *The near point of vision becomes greater with age because the lens begins to lose some of its elasticity (usually after age 40).

Pigment Molecules

*Pigment molecules, which harvest incoming light energy, are embedded in the membrane of the many discs present in the outer segment. *Synaptic vesicles are located at the synaptic endings of the inner segment. 1. After rods and cones are activated by light, they stimulate neurons called bipolar cells, which in turn stimulate neurons called ganglion cells. 2. The axons of the ganglion cells produce action potentials that are conducted along the optic nerve to the thalamus, then to the primary visual area of the occipital lobe of the cerebral cortex.

Aqueous Humor

*The anterior chamber is filled with a clear, watery fluid called aqueous humor. 1. Aqueous humor is secreted by the ciliary body into the posterior chamber and passes through the pupil into the anterior chamber. 2. Normally, aqueous humor leaves the anterior chamber and rejoins the venous blood by way of tiny ducts (canal of Schlemm). In glaucoma, these drainage ducts are blocked, and aqueous humor builds up (increased intraocular pressure).

Iris

-The iris is a pigmented muscle that regulates the diameter of the pupil, a hole in the center of the iris through which light enters the eyeball. *The iris contracts and dilates according to changes in light intensity (how many photons). 1. contracts in bright light; dilates to let in more light under low-light conditions -The color of the iris (i.e., the color of your eyes) correlates with its pigmentation. *Heavily pigmented = brown; lightly pigmented = green or blue

Optic disc

-They exit the retina, forming the optic nerve, which takes nerve impulses to the visual cortex (occipital lobe). -The optic disc does not contain photoreceptors. *blind spot 2. The two eyes together provide complete vision because the blind spot for the right eye is not the same as the blind spot for the left eye. -The blind spot for the right eye is right of center while the blind spot for the left eye is left of center.

ciliary body

-contains the ciliary muscle, which controls the shape of the lens for near and far vision -The lens, which is involved in focusing incoming light onto the retina, is attached to the ciliary body by suspensory ligaments. *The lens divides the eye into two compartments: anterior cavity in front of the lens and posterior cavity behind it. -The anterior cavity is in turn divided into the anterior chamber (the space between the cornea and lens) and the posterior chamber (the space between the iris and lens).

Rhodopsin

= the protein opsin and a light-absorbing molecule called retinal, which is a derivative of vitamin A *Thousands of rhodopsin molecules are present in the discs of the outer segment in each rod. 1. Carrots, which are rich in vitamin A, can contribute to the proper functioning of our rods. iii. When a rod absorbs light, rhodopsin splits into opsin and retinal, leading to a cascade of reactions and the closure of ion channels in the rod cell membrane. -Stops the release of inhibitory transmitter molecules from the rod's synaptic vesicles -As a result, signals are sent to the other neuron layers in the retina.

Fovea centralis

a pinhead-sized region of the retina densely packed with cones but with few or no rods -fovea = "pit" -located within a larger area of the retina known as the macula lutea 2. Light is normally focused on the fovea when we look directly at an object. -As you read this sentence, your eyes move to keep each successive word focused on your fovea. 3. Vision is most acute (sharpest) in the fovea centralis. -Birds have multiple foveae.

Light = radient energy

a. Visible (or white) light is one type of radiant energy; others include X-rays, ultraviolet light, and microwaves. i. The "electromagnetic spectrum" b. Light is often described as a wave, but it can also be thought of as discrete bundles, or packets, of energy called photons. i. Our photoreceptors are sensitive to light energy of particular wavelengths. *Interestingly, among mammals, only humans have expansive color vision. 1. This may be because of the limited number of mammals with a diurnal lifestyle (active during the day).

Cataract Formation

a. lens of the eye becomes "cloudy" and impairs vision i. affects >50% of people over the age of 65 ii. caused by clumping (aggregation) or changes in shape of the crystallin proteins in the lens b. can be delayed or prevented by the use of UV filter sunglasses and proper nutrition i. treatment = surgical replacement of the lens with an artificial one

Processing of visual information

begins in the retina i. There are many more rods and cones than ganglion cells. -About 150 million rods, 6 million cones, 1 million ganglion cells ii. The sensitivity of cones versus rods is mirrored by how directly they connect to ganglion cells. -As many as 150 rods may activate the same ganglion cell. *Thus, the stimulation of our rods results in vision that is relatively blurred and indistinct. -In contrast, some cone cells in the fovea centralis activate only one ganglion cell. *Thus, cones, especially in the fovea centralis, provide us with a sharper, more detailed image of an object.

Refraction part 1

bending of light as it passes from a medium of one density to a medium of a different density. i. Light is refracted as it passes from air into the cornea, from the cornea to the aqueous humor, from the aqueous humor into the lens, and from the lens into the vitreous humor. ii. The refraction of light as it passes through the eye is affected by the shape of the eyeball. -The cornea does not change shape, so the degree of refraction at this point is constant. -The shape and thickness of the lens are adjustable.

Visual accomodation part 1

changes in lens shape to maintain the focus of an image on the retina i. If you've ever used a camera with a manual focus, think back to what would happen if you focused on the object you wanted to photograph at 10 feet away then changed your distance so that you were 20 feet away. -The image would become blurry and you would need to refocus. b. The shape of the lens is controlled by the ciliary muscle within the ciliary body. i. Imagine that the ciliary muscle surrounds the lens like a trampoline frame surrounds a trampoline mat. -If the frame is widened, the mat is tightened. -If the frame is narrowed, the mat is under less tension and sags. -

Light perception among animals

i. Flatworms known as planarians have simple eyespots that enable them to determine the direction of light only. ii. In contrast, image-forming eyes (in which the photoreceptors form actual images) provide information about an object, including how far away it is. -Among invertebrates, arthropods (e.g., shellfish, arachnids, and insects) have compound eyes, which are composed of visual units, each of which includes a lens to focus light rays on photoreceptors. *The images produced are crude, because of the small size of compound eyes (limit of about 28,000 visual units). -Vertebrates (us) and some mollusks (e.g., squids and octopuses) have camera-type eyes, in which a single lens focuses light on the photoreceptors, which number in the millions and are tightly packed together within a retina.

Snellen eye chart

i. When a person with "normal" vision stands 20 feet away from a Snellen eye chart, he/she will be able to read the line of letters marked "20/20." ii. If you have 20/40 vision, it means that you must be as close as 20 feet to read what a person with normal vision can see at 40 feet. -What you can do is indicated by the first number; what a normal person can do is indicated by the second number.

sclera

i. called the "white of the eye" -white and fibrous except for the cornea *cornea = the "window" of the eye; made of transparent collagen fibers

Retina

i. located in the posterior cavity, which is filled with a clear, viscous material called vitreous humor -"Floaters" [muscae volitantes] are translucent specks that float across the visual field *They are, in fact, small bits of protein or cells floating in the vitreous humor [i.e., they are inside the eye]; thus, blinking does not make them go away. ii. The retina contains photoreceptors called rods and cones, which lie at the back of the retina beneath several layers of supportive cells. -Rods -Cones iii. Neurons in the retina contribute axons that gather at a region of the retina called the optic disc.

coroid layer

i. vascular ii. absorbs stray light rays that the photopigments in the retina have not absorbed iii. In some animals (e.g., cats) there is a layer of reflective tissue within the choroid (tapetum lucidum) that allows greater sensitivity in dark environments. -This is why cats' eyes seem to glow in the dark when illuminated by a flashlight. v. The pattern of blood vessels in the choroid, and the overlying retina, is as unique as your fingerprints and can be scanned as a "biometric identifier".

Astigmatism

i. when the cornea and/or lens is asymmetrically curved -light rays cannot be evenly focused on the retina because the cornea and/or lens are refracting the light to different degrees *This condition can be corrected by an unevenly ground lens to compensate for the uneven cornea/lens. e. Many people today opt to have LASIK surgery instead of wearing lenses.