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Anatomy of the Eye
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• Part of eye
• Description
• Function
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.
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.
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
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 musclesrelaxed
LENSpulled thin
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 musclescontract
LENSthick thin
LENS circular ciliary muscles
suspensory ligaments connect lens andciliary body
ciliary muscles contracted
ligaments slacklens thick
ciliary muscles relaxed
ligaments taughtlens thin
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.
Front view of iris and pupil in high light intensity
Front view of iris and pupil in low light intensity
radial muscle contracted
radial muscle relaxed
circular muscle relaxed
circular muscle contracted
pupil dilated pupil constricted
LOW LIGHT INTENSITY
HIGH LIGHT INTENSITY
CIRCULAR MUSCLE
RELAXES CONTRACTS
RADIAL MUSCLE
CONTRACTS RELAXES
PUPIL DIAMETER
DILATED CONSTRICTED
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
mitochondria
nucleus
membrane-lined vesicles
contain the photoreceptive
pigmentRHODOPSIN
outer segment
inner segment
synaptic region
mitochondria
nucleus
Infoldings ofsurface
membranecontain the
photoreceptive pigment
IODOPSIN
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
• 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.
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
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.
CHOROIDSCLERA
rods
bipolar neurone
ganglion cell
/ cones
To optic nerve
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.
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.
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
• 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.
CHOROIDSCLERA
rods
bipolarneurone
ganglioncell
/ cones
GR
EE
N L
IGH
T
GENERATORPOTENTIAL
ACTIONPOTENTIAL
ACTIONPOTENTIAL
WH
ITE
LIG
HT
neurones of optic nerve
ganglion cell
cell body of bipolar neurone
synapse
rod cell
cone cell
choroid
sclerotic
LIGHT RAYS
feature Rod cells Cone cells
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
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
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
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.
• 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.
Prey animalEyes at side of head
Little overlap of field of vision from each eye
Wide field of viewPoor distance and depth
perception
Predator animalEyes at front of headLarge overlap of field of vision from each eyeNarrow field of viewGood distance and depth perception
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
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.
Distribution of rods & cones in the retina
Can you see the word STUFF???