Special Senses Lecture (Day 1:Eye anatomy & Vision)

The eye…..

•Works somewhat like a camera. It takes in light but for imageto be “understood/seen”, it must be processed in the brain.

•Eyes are actually extensions of the brain •(i.e. the retina, which is on the back of the eye is all nervous tissue)

Anatomy of the Eye:

The eye is round, like a ball or a balloon.

It is made of many separate components and tunics (layers).

Only 1/6 of the eye is exposed/visible (the rest is enclosed in fat and bone).

Diagram of the Eye (frontal view)

sclera

lateral canthus

irispupil

meibomiumglands (oily)

medial canthus

conjunctiva (membrane)

. . . . .

ciliaryglands(lubricate eye)

Interior apparatus associated with the eye:

Tears contain antibodies and lysozymes (enzymes that kill bacteria).

lacrimal glands

nasolacrimalduct

Six external eye muscles control eye movement:

4 rectus muscles (up, down, left and right)

2 oblique muscles Inferior oblique (elevates eye & moves it laterally)

Superior oblique (depresses eye & moves it laterally)

Now let’s learn the parts of the eye itself….

Tunic Layers :a) Sclera (outermost layer): thick, tough,

whites of the eye.

b) Choroid (middle layer of eye): blood rich tunic, that contains

a dark pigment that prevents light from scattering in the eye.

c) Retina (inner back of the eye): innermost

delicate tunic, that contains millions of receptor cells (rods & cones) that receive and respond to light. sclera

choroid

retina

Internal Anatomy of the Eye:

Two hard lenses (cornea & lens) help bend lightappropriately in order to focus light onto the retinaso image can be clearly seen.

cornea

lens

Attached to the lens are ciliary bodies (smooth muscles) and the iris (pigmented “color” part of the eye).

The ciliary bodies pull on the iris which dilates and constricts to regulate light input.

Two “liquids” in eye:

a) Aqueous humor: watery liquid just behind cornea. Helps bathe cornea and give it oxygen & nutrients.

b) Vitreous humor: thick, jelly-like substance which makes up bulk of eyeball. Give eye structure and

maintains ocular pressure.

Aqueous humor

Vitreous humor

On the back of the eye is the retina.The retina contains thousands of photoreceptors.

Photoreceptors are nerve cells that receive light.

Light passing through eyeball

Retina(& photoreceptors)

The role of photoreceptors…

•Photoreceptors are distributed all over the entire retina, at the back of the eye (except where the optic nerve leavesthe eyeball = optic disc/“blind spot”).

•When light from an object is focused on the optic disc, it disappearsfrom our view.

“blind spot”

There are two types of photoreceptors:

a) Rods:slender, elongated neurons. Rods allow us to see in black, white, and shades of gray.

Rods also allows for peripheral vision.

b) Cones: fatter, more pointed neurons (blue cones, green cones, and red cones).

Cones allow us to see in color.

Also allows us to focus clearly on objects in the center of our view.

Rods (skinny)

Cones(fatter)

rods

cones

Vision results from:

Light enters eye as wavelengths. These stimulate neurons at back of eye (retina). These neurons start an action potential (electrical impulse) which pass from the photoreceptors to bipolar cells, then to ganglion cells, and then leave the optic nerve.

Photoreceptors on retina get stimulateand pass electrical impulses to brain viaoptic nerve.

The nerve impulses travel very quickly along optic nerve to the occipital lobe of the the cerebral cortex which allows us to process and interpret these electrical signals, resulting in “vision”.

eyes occipital lobe

Yes, I see!

The End of Part ne !