Sense Organs: The Eye

Anatomy by Melissa OlmanPhysiology by Layleeta Prasad

Genetics by Louis TruongEvolution by Thomas Rhodes

Biology 347Dr. Kusanda

Collaboration Project

The Anatomy of the Eye

Iris – Regulates the diameter of the pupil and also gives the eye its color

Pupil – Opening in thecenter of the iris that lets light into the interior of theEye

-The iris works in conjunction withthe pupil to control how much light enters the eye

-The iris has tiny muscles which enable it to dilate and constrict thepupil to allow more or less light intothe eye

Physiology of the eye

Process of vision

• Refraction- bending of light through cornea to the retina

• Accommodation -for objects of closer vision, ciliary muscles

contracts, making the lens more convex-for objects of further vision, ciliary muscles are relaxed and the lens is flatter.

Processing visual information to the brain

• Retinal Neurons :- photoreceptors, bipolar, ganglion, horizontal and amacrine cells.

• photoreceptors produce nerve impulses in response to the light

• photoreceptors and ganglion cells synapse with bipolar cells

• Horizontal and amacrine cells also synapse with the other neurons to assist in the integration of visual information

• nerve signals formed by these synapses, exit the eye via the optic nerve and travels to the optic chiasm bio1152.nicerweb.com

Processing visual information to the brain

• nerve signals formed by the synapses, exit the eye via the optic nerve and travels to the optic chiasm

• Visual cortex receives information

http://www.glaucoma-eye-info.com/meningioma.html

Photorecption

• Scotopic photoreception- rods- the receptors for night vision

• Photopic photoreception -cones –daytime vision and color vision

-3 cones: blue green and red • Visual pigements:

- retinal- the light absorbing molecule in both rods and cones.-It is bound to the protein opsin.

-when opsin and retinal combine in rods, it forms the visual pigment rhodopsin.

Depth perception

• Monocular vision:-visual fields do not overlap-each eye is used separately-this type of vision is rare

• Binocular vision: -the two visual fields overlap-allows for objects to be seen in three dimensions -also allows for increase depth perception.- Stereoscopic vision is within this area of overlap

Refraction disorders

• Myopia- -nearsightedness -lens is too thick causing the image to focus in front of the retina

• Hyperopia--farsightedness-lens is too thin causing the image to focus behind the retina

Age related disorders• Cataracts

- caused by hardening of the lens-involves cloudiness of the lens that blocks light from reaching the retina

• Glaucoma- caused by buildup of aqueous humor due to drainage problems- results in damage to the cells of the retina and optic nerves fibers causing blindness -For glaucoma it is common to get surgery to correct the drainage problems.

• Dry eyes- caused by a reduction of secretions causing the conjunctiva to become dry

• Presbyopia -caused by a loss of elasticity and thickening of the lens

Genes controlling eye development

Homology? Or Analogy?

Mutation of Pax6

• Drosophila= eyeless (ey)– total loss of eye facets on both sides of the head

• Mice= “small eyes” (pax6)– Homozygous “small eye” embryos are eyeless, noseless and

suffer from brain damage– Heterozygous mutation develop on adult mice had reduced eyes

• Human= Aniridia– Similar to mice– Heterozygous Aniridia patient, has a reduced or no iris – Homozygous mutant human fetus, was born with no eyes, nose

and also suffer from brain damage. Fetus dies prior to birth

Pax6 gene

• Is the master control gene in the eye development.

• Controls position of the eyes on the body plan• This gene is universal in all Bilateria• This gene has a critical affect on the eye

development but not solely to eyes. It is also involved with the formation of nervous system, brain and nose.

Pax6 Gene• Pax6 Gene in mouse placed in Drosophila

antenna

Induced Ectopic Eyes

Precursor of Pax6

• It is found that in Cnidarians have less classes of Pax genes than in bilaterian . a duplication of the Pax genes in ancestral bilateria resulted in the product Pax6

• Although they do not have Pax6 Cnidarians such as box jellyfish have complex eye with lens which has both visual and the shielding pigments

• PaxA and PaxB from Cnidarians are expressed in the eyes

• Like Pax6 they can also induce ectopic eyes in drosophila which may suggest that they were precursor of Pax6 in humans

Hox Gene

• Hox genes is regulatory gene commanding secondary genes in formation of body parts

• function controls the organization along of animal’s posterior and anterior axis

• Sets up Bilateral symmetry (giving us a pair of eyes)

Evolution of the Eye

Light Sensitive Proteins

• Also called eye spots• Used by unicellular organisms– Detection of light and dark• No specific direction• Introduced Circadian Rhythms

http://wikis.lib.ncsu.edu/index.php/Ancestry

Light Sensitive Proteins (Stigma)

• Light sensitive patch near the flagella – Stimulation of flagella in response to light– Light dependent movement • Photosynthesis • Spawning

http://www.studyblue.com/notes/note/n/lab-quiz-2-ecology--protists/deck/1199028

Multicellular “Cup Formation”

• Shallow depression where light sensitive cells – Allowed detection of light direction• Light had to be angled into cup, stimulating only a

portion of cells

– As depression became deepened, the sense of direction became finer

– Possibility formation before appearance of the brain• No need for processing

Pit and Pinhole eyes

• Formation of deeper depressions and narrow openings– Less ambient light giving finer sensitivity

Cambrian Explosion and Light switch Theory

• Introduction of Visual field with simple brains– Needed the ability to process the interaction of

light and cells to form an image• First visual field was just shadows

• Light Switch Theory – Andrew Parker• Proposed that the sudden explosion in Cambrian fossil

records was a result in vision and increased Pedation• Caused raid evolution and development

Separation from the Environment

• Transparent overgrowth of cells on the top of cup depression– Separation of light sensitive cells and external

environment • Protection • Specialization

– Higher refraction index– Color filtering– Blocks UV

Separation from the Environment

• Also allowed for the operation of the sense organ in Aquatic and Terrestrial environments– Major step in Evolution

Lens formation and diversification

• Evolved independently from multiple linages– Originally used for seeing in darker waters

• Separation into double layer with aqueous middle– Allowed for waste removal and nutrient supply– Increased protection, optical power, viewing angle

and resolution• Could not be found in fossil recoreds

Developments due to selective pressures

• Color vision– Advantage for finding food, mates and avoiding prey

• Focusing– Environmentally dependent

• Amount of light in environment

• Location– Non-predatory animals typically have eyes on the

side of the head• Increased visual range for detection of predators

Developments due to selective pressures

• Location– Predators have eyes located on the front of the

head• Increased depth perception

• Muscle Attachments– Movement of the eye

http://en.wikipedia.org/wiki/File:Diagram_of_eye_evolution.svg

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