EYE

CONJUNCTIVA• Transparent mucous membrane

lining the eyelids and covering anterior surface of eyeball except cornea

• Richly innervated • Vascular

Lacrimal Apparatus

• Tears with bactericidal enzyme flow across the eyeball, wash away foreign particles, and help with diffusion of O2 & CO2.

Extrinsic Eyes Muscles

• Innervated by cranial nerves III, IV and VI• 4 rectus muscles move eye up, down, left & right• superior & inferior oblique are complicated

trochlea

The Tunics of the Eyeball

• Fibrous layer (tunica fibrosa) = sclera and cornea • Vascular layer (tunica vasculosa) = choroid, ciliary body &

iris• Internal layer (tunica interna) = retina and optic nerve

OPTICAL COMPONENTS

• Transparent structures that refract (bend) light rays to focus them on the retina1. Cornea - covers anterior surface of eyeball2. Aqueous humor - clear serous fluid located

between lens and cornea3. Lens - suspended by ring of suspensory

ligaments4. Vitreous humor - jelly-like located between

the lens and retina

NEURAL COMPONENTS

1. Retina - out growth of diencephalon - pressed against rear of eyeball by vitreous body

2. Optic nerve - attached to retina at the optic disc

IMAGE FORMATION1. Light passes from an object through the

lens2. A tiny inverted image forms on the retina• Pupillary constrictor (smooth muscle) in iris

narrows the pupil in bright light• Pupillary dilator (radial & myoepithelial)

widens the pupil in dim light

Refraction• Bending of light rays occurs when light passes through

substance with different refractive index at any angle other than 90 degrees– refractive index of air

is arbitrarily set to n = 1– refractive index of

cornea is n = 1.38– refractive index of lens

is n = 1.40• Cornea refracts light more than lens does

– lens fine-tunes the image as shift focus between near and distant objects

ACCOMMODATION• Allows the lens to focus on close

objects 1. Contraction of ciliary muscle relaxes suspensory ligaments

2. Lens becomes more convex in shape

3. Light is refracted more strongly & focused onto retina

Emmetropia (Distant Vision) & Accommodation (Near Vision)

Accommodation of Lens

EYE DEFECTS

• Hyperopia - farsightedness - due to short eyeball - correct with convex lenses• Myopia - nearsightedness due to

long eyeball - correct with concave lenses

Effects of Corrective Lenses

Retinal Cells• Posterior layer of retina is pigmented

epithelium - absorbs excess light & prevents reflections• Photoreceptors are anterior to epithelium1. Rod cells - allow night vision - outer segment has a stack of membranous discs containing rhodopsin (pigment)2. Cone cells - allow color vision in bright light - outer segment tapers to a point

Non-receptor Retinal Cells• Bipolar cells (1st order neurons)

– synapse on ganglion cells– moderate convergence occurs

• Ganglion cells (2nd order neurons)– axons form optic nerve– great convergence occurs

• Horizontal & amacrine cells form connections between other cells

VISUAL PIGMENTS1. Rhodopsin (visual purple) - in rod cells - consists of opsin (protein) & retinal

(vitamin A derivative)2. Photopsin (iodopsin) - in cones- 3 kinds of cones absorb different

wavelengths of light to produce color vision

PHOTOCHEMICAL REACTION IN RODS

• Rhodopsin absorbs light & converts from a bent shape (cis-retinal) to a straight (trans-retinal) form that dissociates from opsin (bleaching)

• 50% of rhodopsin is regenerated 5 minutes after bleaching occurs

Rods and Impulse Transmission

Color Vision• Cones permit color vision• Cones are named for absorption

peaks of photopsins– blue cones peak sensitivity at 420 nm– green cones peak at 531 nm– red cones peak at 558 nm (orange-yellow)

• Perception of color is based on mixture of nerve signals• Color blindness is hereditary lack of one photopsin• Red-green color blindness (sex-linked recessive trait found

in 8% of males) occurs if an individual lacks either red or green cones

Test for Red-Green Color Blindness

Rods vs. Cones RODS CONES