Physiology of the human eyePhysiology of the human eye

Function of the eye..............

Is to transduce light (visible frequencies of electromagnetic radiation) into patterns of nerve impulses.

These are transmitted to the brain, where the actual process of seeing is performed

The eyes are situated in cavities: orbits

The exposed front surface of each eye is protected by a thin membrane: conjunctiva and by eyelids, eyelashes and lachrymal glands.

Eyebrows: protection from strong sunlight

Gross Anatomy of the Eye -that allows light to enter the eye-black-looking aperture

-colored circular muscle-controls the size of the pupil

[iris color is due to variable amounts of eumelanin (brown/black melanins) and pheomelanin (red/yellow melanins) produced by melanocytes].

Cornea:transparent external surfacecovers both the pupil and the iris

The "white of the eye“forms part of the supporting wall of the eyeball

Human Eye• The eye has an iris

like a camera• Focusing is done by

changing shape of lens

• Retina contains cones (mostly used) and rods (for low light)

• The fovea is small region of high resolution containing mostly cones

• Optic nerve: 1 million flexible fibres

Muscles.....

The human eye is a remarkable organ not only

because of its ability to sense images but especially because

of its ability to process the image before sending a signal

to the brain

http://webvision.med.utah.edu/movies/irisedu.mov

The human eye is wrapped in three layers of tissue:

the sclerotic coat

the choroid coat

the retina

*the sclerotic coat

This tough layer creates the "white" of the eye except in the front where it forms the transparent cornea. The cornea

admits light to the interior of the eye and bends the light rays to that they can be brought to a focus.

The surface of the cornea is kept moist and dust-free by secretions from the tear glands.

*the choroid coat

This middle layer is deeply pigmented with melanin. The choroid coat forms the iris in the front of the eye. This, too, is pigmented and is responsible for eye "color". The size of its opening, the pupil, is variable and under the control of the autonomic nervous system. In dim light (or when danger threatens), the pupil opens wider letting more light into the eye. In bright light the pupil closes down. This not only reduces the amount of light entering the eye but also improves its image-forming ability (as does "stopping down" the iris diaphragm of a camera).

*the retina

• The retina is the inner layer of the eye. It contains the light receptors, the rods and cones (and thus serves as the "film" of the eye). The retina also has many interneurones that process the signals arising in the rods and cones before passing them back to the brain. (Note: the rods and cones are not at the surface of the retina but lie underneath the layer of interneurons.)

Breakdown…

Outer shell

• Sclera is the white of your eyes– 22mm in diameter and 1 mm thick– Gives the eye it’s structural integrity– Totally opaque

• Cornea is the transparent semi-spherical shell that covers the front of your eye where there is a break in the sclera– Radius of curvature of 8mm– Refractive index of 1.37 – causes the convergence of

rays of light within the eye!• Aqueous humor is the fluid behind the cornea

– Basically just water, with refractive index 1.33– Carries nutrients to cornea

• The iris is the the colorful part of the eye, connects with the choroid– The opening centered within the iris is the pupil– The iris consists of delicate muscle which dilates

and contracts to allow various amounts of light through the pupil

– Pupil diameter ranges from about 2mm on a bright day to 8mm under dark conditions

– Both pupils dilate or contract at the same time regardless of whether lighting is consistent for both eyes – called the consensual pupillary reflex

• The lens is also called the crystalline lens

– Refractive index of 1.4 at center to 1.38 at the outside

– Gives fine vision adjustment known as accommodation which helps you focus objects of different distance to the eye

– This is controlled by muscles, and is why you get eye strain from focusing on things too close, and why looking far away helps relax it

Retina• The retina is the light sensitive layer

– Purpose is to form an appropriate real image of the world– The image which “exposes” onto the retina is actually

inverted, but our brain “reverts” it• The fovea is on the central axis of the retina, and is the area

of greatest visual acuity– This is what is “exposed” when we “look at” something– Only 0.25 mm in diameter, so represents very small

portion of our field of view• We all have a blind spot in each eye

– Caused because of the optic nerve about 5mm from central axis

• Rods and cones are actual light receptors which make up retina– Each retina has about 120 million rods and 6 to 7 million

cones

                                     

You can demonstrate the presence of the blind spot. Cover your right eye with your hand and stare at the red circle as you move closer to the screen (the square will disappear). Or cover your left eye and stare at the red square as you move.

More on Rods and Cones• They are about 0.05 mm long and 1 to 3

micrometers in diameter• Necessarily small because we need resolution!• Cones are responsible for

– High light level vision (photopic)– High resolution– Color

• Rods are responsible for– Low light and night vision (scotopic)

• Changing between the two is called adaptation– Adaptation from light to dark can take 20 to 30

minutes– From dark to light takes a few tenths of a second

rods three kinds of cones, each "tuned" to absorb light from a portion of the spectrum of visible light

-cones that absorb long-wavelength light (red) -cones that absorb middle-wavelength light (green) -cones that absorb short-wavelength light (blue)

Rhodopsin is the light-absorbing pigment of the rods. It is incorporated in the membranes of disks that are neatly stacked (some 2000 of them) in the outer portion of the rod. (This arrangement is reminiscent of the organization of thylakoids, another light-absorbing device.)

Even more on Rods and Cones

• Rods and cones contain visual pigment • Rods have one kind of pigment• Cones have 3 kinds of pigment• Somehow the light bleaches out a pigment

when it is exposed, this triggers a nerve impulse

• The pigment in rods is much more sensitive than the kind in cones

So...............

** Transduction of light energy by RODS

-Each rod cell has vesicles containing rhodopsin. Rhodopsin is made of a protein (opsin) and a derivative of Vit A (retinal).

-The light makes retinal amd opsin to separate or break apart: bleaching process

This triggers a series of events which alters the permeabiliry of the rods´membranes.

-if the light stimulation exceeds the threshold level, an action potential is set up ina bipolar neurone, and then passes along a neurone in the optic nerve.

** Colour vision and CONES

-cones are responsible for colour vision-RGB : red, blue and green

-trichromatic theory: different colours are perceived by mixing theinformation from the different types of cones.

** CENTRAL PROCESSING: HOW WE SEE

-visual cortex of the brain is responsible for sorting out all the sensory information from the retina and integratingit with other information, so that we can “see” an object.

1. The lens produces an inverted image on the retina2. The brain turns it the”RIGHT WAY UP”

So the retina is not simply a sheet of photocells, but a

tiny brain center that carries out complex

information processing before sending signals

back along the optic nerve. In fact, the retina really is

part of the brain and grows out from it during

embryonic development.

Rods do not provide a sharp image. However, rods are extremely sensitive to light. A single photon (the minimum unit of light) absorbed by a small cluster of adjacent rods is sufficient to send a signal to the brain. Although cones operate only in relatively bright light, they provide us with our sharpest images and enable us to see colors. Most of the 3 million cones in each retina are confined to a small region just opposite the lens called the fovea. So our sharpest and colorful images are limited to a small area of view.

**

*

The three types of cones provide us the basis of color vision. Cones are "tuned" to different portions of the visible spectrum. -red absorbing cones; those that absorb best at the relatively -long wavelengths peaking at 565 nm -green absorbing cones with a peak absorption at 535 nm -blue absorbing cones with a peak absorption at 440 nm.

**

fibres

Fibres bodies

Neurone cells bodies

Synapse

Next please…

•After the rods and cones, the signal is sent down “nerves”

•The signal hits “bipolar cells”, and then “ganglion cells” which connect to the brain

•Processing is done in the bipolar cells before it is done in the brain

•The nerve bundles from each eye (left and right optic nerves) actually meet at a structure called the optic chiasma and some of the vision from each eye goes into each side of the brain(as a result the left optiv nerve carries information from the right half of the field of vision and vice versa

•Beyond the optic chiasma, the neurons continue to the thalamus,where the information is processed. It is then carried to the Visula cortex at the back of the brain

Vision

• Humans like to see things in 3D – our eyes are already adapted for this– Leads to funny optical illusions

Color Vision

• One clue that we perceive everything in terms of 3 colors (RGB) is that we can construct any other color from combinations of them

• We even get bonus colors! White light is not in the spectrum, nor is purple, but we can get them from combinations of RGB colors

• Still not sure exactly how “color” is transmitted to the brain, though it is thought that there are three color pigments in the cones

• There are other theories with 4 color receptors – red, green, blue, and yellow

• Most scientists believe the tristimulus color theory

Sometimes our brain is tricked

into seeing things that aren’t real.

These are called optical illusions.

This Is a Bunny/duck Illusion. Do You See the Bunny and

the Duck?

How Many Prongs Are on This Fork?

Find the Secret MessageHint: Look at the White.

There Is Not a Triangle Here. There Is Not a Triangle Here.

This Is a Dancing Elephant. How Many Feet and Legs Does It

Have?

Is This Man Playing a Sax, or Is There Something More

Going on Here?

What Does This Say?Now Read It Again!

This Says the Word Liar, but Is There Something More?

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