Sense Organs: Maintaining Homeostasis The Eye

External/ Accessory Structures Can You Guess the Celebrity Eyes? These are organs that do not directly contribute to your sense of sight or vision, but do play a role in the health and functionality of the eye.

Eyelids= palpebrae Covers and protects the eye, thin skin Skin will not protect you from intense radiation, that’s why we use special goggles in a tanning bed

Canthus= angles where superior and inferior eyelids meet Epicanthal fold= additional fold of skin from upper lid that covers inner corner of eye common in people of Asian descent

CONJUNCTIVA is a mucus membrane that lines the eye and eyelid. CONJUNCTIVITIS: Pinkeye- inflammation of conjuctiva -bacterial: contagious -viral: contagious -allergic reaction Pink Eye Slide Show from Web MD

Extrinsic Eye Muscles: voluntary

LACRIMAL APPARATUS: -secretes and drains tears -eyelid sweeps them toward medial corner -Include glands, ducts, and sacs

GLANDS OF THE EYE LACRIMAL GLANDS: size/shape of an almond -superior lateral eyelid -produce tears, which drain into conjunctiva Function: moisten and lubricate the eye surface, with enzymes to kill bacteria (which thrive in warm, moist conditions).

Caruncle: small red body at inner canthus Lacrimal canals: -channels at inner eye that empty into lacrimal sacs Nasolacrimal ducts: -small tubes connecting lacrimal sac to nasal cavity -become plugged → crying

Figure 16.5b

3 Layers of Eye 1.Fibrous 2.Vascular 3.Inner

Fibrous Layer ●Sclera – continuation of cornea, going toward the back of the eye (white of the eye) ●Cornea - transparent, focuses light rays, lies over colored part of eye, no blood vessels Keratoconus is an eye condition in which the normally round cornea thins, causing a cone-like bulge to develop. Source: NPR

Vascular Layer: many blood vessels, pigmented 1.Choroid– middle and posterior coating, most of vascular layer 2. Ciliary Body – thickening of choroid, holds the lens in place (has muscle and ligaments) 3. Iris – colored portion of the eye, contains muscles to adjust size of pupil to control amount of light let in

4. Pupil – opening in iris for light to enter 5. Aqueous humor – liquid surrounding the lens

Inner Layer Retina - innermost coat of eyeball, only on posterior side, contains visual receptor cells Optic Nerve- exits at the optic disc and transmits visual information from the eye to the brain.

Retinal blood vessels What causes Redeye? The flash on a camera is bright enough to cause a reflection off of the retina -- you see the red color from the blood vessels.

Test: 1. Look down and to the left 2. Touch the medial side of your upper eyelid 3. Do you see anything? 4. Why do you see it there? Process of Seeing

Retina Image goes through lens and is projected onto retina at back of eye Image on retina is reversed and inverted (same as camera)

Retina Most of retina is made of nervous tissue Photoreceptors absorb light, then send electrical signals via optic nerve to occipital lobe

Fovea Centralis - region of the sharpest vision; center point on retina directly behind lens Optic Disk – where optic nerve leaves the eye, creating the blind spot Fovea Centralis Optic Disc

BLIND SPOT (optic disk) The region where the optic nerve and blood vessels enters eye; has no photoreceptors = BLIND SPOT. 1. Look at the + and 2. Cover your left eye and stare at the + 3. Bring your paper closer to your eye 4. Notice where the disappears 5. This is your blind spot On which side of your eye is the optic disk?

Four processes to form images on retina 1.Refraction 2.Accommodation 3.Constriction 4.Convergence

Light Refraction Light bends when it enters a new medium Air  cornea (filled with aqueous humor) Cornea  lens Lens  vitreous humor

Refraction At about 20 feet away, objects can be seen in focus by relaxed eye When object is closer, eye must adjust in order to focus the image on the retina

Aqueous Humor fovea centralis Vitreous Humor Cornea Lens

Accommodation Lens changes shape to focus images at different distances clearly on the retina Ciliary muscles and suspensory ligaments either cause lens to bulge or flatten Distance vision: lens is flat Near vision: lens is bulged Aging causes lenses to lose elasticity

Constriction of Pupil: makes pupil smaller Muscles of iris: - Inner circular smooth muscle fibers - Outer radial fibers

Constriction of Pupil Near reflex: lets in less light  with too much peripheral light, eye couldn’t focus on near object  occurs with accommodation to bulge lens

Constriction of Pupil Pupillary light reflex: inner circular smooth muscle fibers contract to protect retina from too much light

Convergence of Eyes Movement of two eyeballs inward so visual axes come together: you see one image instead of two Angle of eyes changes to focus on one point Try it:

Rods = monochromatic (b&w) Cones = color vision

Photoreceptors: rods and cones Rods -Photopigment= rhodopsin: made to detect light -very light sensitive -specialized to perform in low light -perceives light as intensity, NOT color …More rods next page

Rods example Bright to low light  Rhodopsin is made Rhodopsin requires vitamin A Bright light deactivates rhodopsin

Photoreceptors: rods and cones Cones - used in most situations (bright light) - responsible for acuity (sharp images) and color - 3 types: red, blue, green - highly concentrated at fovea centralis --green and red at fovea --blue=outside fovea

Ganglion cells A less dominant photoreceptor Contain melanopsin  sensitive to light in blue range that predominates at low light levels Needed to adjust biological clock

Colorblindness Abnormal function in any of the cones disrupts normal perception of color A genetic trait that affects boys more than girls. The location of the gene is on the X chromosome

Types of Color blindness 1.Protanomaly: reduced sensitivity to red light 2.Deuteranomaly: reduced sensitivity to green light 3.Tritanomaly: reduced sensitivity to blue light: RARE

Protanomaly

Deuteranomaly

Tritanomaly

Blue/Black or White/Gold?

Look at A and B. What are their colors?

Same color: your eyes assume B is light because it falls under a shadow

Pathway of vision Optic chiasma contains nerve fibers from BOTH retinas (both eyes) Optic tracts: nerve pathways from optic chiasma to brain

Right FIELD of vision goes to right brain Left FIELD of vision goes to left brain

If right optic tract were injured, what would person see?

Only left visual field from both eyes Right half of both eyes would be “blind”

We have difficulty interpreting images that are upside down Which one is the real mona lisa?

PUPILS Fun Fact: -When you are looking at someone you love, your pupils dilate, and they do the same when you are looking at someone you hate.