1. Anatomy of the Eye

3. Match activity
Part of eye
Description
Function

4. Function of the eye To obtain a focused image
Light must be focused on the retina, this is carried out by the cornea and the lens
To control the amount of light entering the eye
Sufficient light must enter the eye to stimulate the photosensitive cells in the retina to form an image. Too much may damage these cells.

5. Focusing light Most light is focused by the retina
The lens makes further fine adjustment by changing in thickness
This allows light rays to focus no matter what direction they come from.

6. Adjusting the lens thickness
The lens is surrounded by the ciliary body, which contains a ring of muscle callled the ciliary muscle
The lens is attached to the ciliary muscle by the suspensory ligaments
circular ciliary muscles
suspensory ligaments
connect lens and
ciliary body
LENS

7. Focus on distant object
When the circular muscle in the ciliary body relaxes the diameter of the muscle increases
This pulls on the suspensory ligaments making them taught
Which in turn pulls the lens thin
Consequently it bends light less, allowing light from distant objects to be focused.
suspensory ligaments
pulled taught
circular ciliary muscles
relaxed
LENS
pulled thin

8. circular ciliary muscles
Focus on near object
When the circular muscle in the ciliary body contracts the diameter of the muscle decreases
The suspensory ligaments become slack
Which in returns the lens to its thicker normal shape
Consequently it bends light more, allowing light from near objects to be focused.
suspensory ligaments
slack
circular ciliary muscles
contract
LENS
thick thin

9. this process is called accommodation
LENS
circular ciliary muscles
suspensory ligaments
connect lens and
ciliary body
ciliary muscles
contracted
ligaments slack
lens thick
ciliary muscles
relaxed
ligaments taught
lens thin
this process is called accommodation

12. CONTROLLING LIGHT ENTERING THE EYE
If the intensity of the light entering the eye is too small the photosensitive cells of the retina will not be stimulated, if it is too high they will be damaged
The iris contains circular and radial muscles to control the size of the pupil and therefore the light entering the eye.

13. Front view of iris and pupil in low light intensity
in high light intensity
pupil dilated
pupil constricted
circular muscle
relaxed
radial muscle
contracted
circular muscle
contracted
radial muscle
relaxed

14. LOW LIGHT INTENSITY
HIGH LIGHT INTENSITY
CIRCULAR MUSCLE
RELAXES
CONTRACTS
RADIAL MUSCLE
PUPIL DIAMETER
DILATED
CONSTRICTED

15. THE RETINA
The retina contains light sensitive cells called photoreceptors
They act as transducers changing light energy into a nerve impulse
by changing the level of polarisation of the membrane
There are TWO types: rods and cones

16. rod cell cone cell outer segment mitochondria mitochondria inner
Infoldings of
surface membrane
contain the photoreceptive pigment IODOPSIN
membrane-lined vesicles
contain the
photoreceptive pigment
RHODOPSIN
outer
segment
mitochondria
mitochondria
inner
segment
nucleus
nucleus
synaptic
region

17. rods
The pigment found in the membranes of the outer segment is rhodopsin
made of a protein opsin
and a light absorbing compound, retinal (from vitamin A)
Rhodopsin breaks down when stimulated with light changing the membrane potential and creating a generator potential
If threshold is achieved the adjacent bipolar neurone depolarises and conducts an action potential

18. Mitochondria found in the inner segment provide ATP to resynthesise the rhodopsin
Rhodopsin is highly sensitive to light and can be broken down in low light intensities
In bright conditions all of the rhodopsin is broken down (bleached)
Therefore when you go into a dark place it takes time to see cleary, i.e. become dark adapted, because it takes time for rhodopsin to be resynthesised.

19. cones
The pigment found in the membranes of the outer segment is iodopsin
There are THREE types of iodopsin which are sensitive to different wavelengths of light
A cone can contain only one type, resulting in 3 types of cones sensitive to either blue, red or green light.
The combination of the different cones stimulated results in all the different visible colours
This is called the
TRICHROMATIC THEORY OF COLOUR VISION

20. Colour Vision

21. ARRANGEMENT OF THE RODS & CONES
The rods and cones lie with the outer segment, containing pigment, against the choroid layer.
Rods and cones synapse with small bipolar neurones
Which synapse with ganglion cells, neurones whose axons join to form the optic nerve.

22. To optic nerve ganglion cell bipolar neurone rods / cones CHOROID
SCLERA

23. ARRANGEMENT OF THE RODS & CONES
Each cone synapses with one bipolar neurone, which in turn synapses with one ganglion cell. This gives a very precise area on which light falls, giving a high resolution (ability to distinguish between 2 points close together).
This is called visual acuity.

24. VISUAL ACUITY IN CONES
Each cone synapses with one bipolar neurone, which in turn synapses with one ganglion cell. This gives a very precise area on which light falls, giving a high resolution (ability to distinguish between 2 points close together).
This is called visual acuity.

25. VISUAL ACUITY IN RODS
Rods show retinal convergence
A number of rods synapse with a single bipolar neurone
and many bipolar neurones may synapse with a single ganglion cell.
Small generator potentials from different rods combine to reach threshold needed to produce an action potential in the bipolar neurone i.e. summation occurs

26. One rod is not sufficient to produce an action potential but together they can.
This makes rods very sensitive to light,
But results in low visual acuity.
This means that during daylight the light intensity is high enough to breakdown iodopsin and rhodopsin, but at night only rhodopsin will be broken down, limiting colour vision.

27. ACTION POTENTIAL ganglion cell ACTION POTENTIAL bipolar neurone
WHITE LIGHT
GREEN LIGHT
ACTION
POTENTIAL
ganglion
cell
ACTION
POTENTIAL
bipolar
neurone
GENERATOR
POTENTIAL
rods
/ cones
CHOROID
SCLERA

28. LIGHT
RAYS
neurones of
optic nerve
ganglion cell
cell body of
bipolar neurone
synapse
rod cell
cone cell
choroid
sclerotic

29. feature Rod cells Cone cells 120 X 10 6 6 X 10 6
Approximate frequency in a human retina
120 X 10 6
6 X 10 6
Distribution
throughout retina
Evenly
Absent from fovea
Mainly at fovea
Absent from periphery
Shape of outer segment
Rod-shaped
Cone-shaped
Sensitivity to light
Very sensitive therefore operates even in dim intensities
Insensitive to colour (monochromatic vision)
Sensitive only to bright light, therefore operates only in bright light intensities
Sensitive to red or green or blue light
Visual acuity
Produces poorly resolved images
Produces well-resolved images
Light-sensitive pigments
Single pigment called rhodopsin in every rod cell
One of 3 types of iodopsin in any cell
Each type of iodopsin sensitive to red, green or blue light
Stimulation of differemnt combinations of the 3 types of cone cell produces a perception of colour (trichromatic vision)
Synapse with relay cell
Groups of rod cells synapse with one relay cell (retinal convergence)
Each cone cell synapses with an individual relay cell

30. BINOCULAR VISION
Two eyes are used to produce a single image
This allows for 3D vision and accurate judgement of distance
Predators (including humans and primates) have eyes positioned at the front of the head. This provides a narrow field of vision in which the image from which eye overlaps considerably, providing excellent judgement of distance and 3D vision

31. Each eye can see an object from a different position
Each eye can see an object from a different position. The brain measures the angle at which each eye is pointing and calculates the object’s distance.
By merging the two images the brain produces a 3 dimensional image of the object. This is called stereoscopic vision

32. The area which each eye can see is called the field of vision
The area which each eye can see is called the field of vision. The more the areas overlap the better the animal is at judging distance.

33. In prey animals (e.g. rabbits) the eyes are positioned at the side of the head. This gives a very wide field of vision, able to detect movement from all directions.
However as there is little overlap they have poor judgement of distance and 3D vision.

34. Prey animal Eyes at side of head
Little overlap of field of vision from each eye
Wide field of view
Poor distance and depth perception

35. Predator animal Eyes at front of head
Large overlap of field of vision from each eye
Narrow field of view
Good distance and depth perception

36. Distribution of rods & cones in the retina
Most rods & cones are found at the fovea. Both are absent from the blind spot,
where the optic nerve leaves the back of the eye

37. Distribution of rods & cones in the retina
Most rods & cones are found at the fovea. This gives the most detailed, colour images at the centre of our vision.
The total number of rods and cones fall off at the edge of the eye. This area is responsible for our peripheral vision.
The rods that are present allow us to distinguish shapes, but as there are very few cones colour vision is poor.
Both rods and cones are absent from the blind spot, where the optic nerve leaves the back of the eye.

38. Can you see the word STUFF???

39. ishihara test for colour blindness