Anatomy of the Eye

Match activity

• 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???