Eye

Structure

The most important structures to learn the function of are…

• Retina• Lens

– Ciliary Muscles

• Iris

The Iris and Pupil

• The iris controls the amount of light entering the eye and able to reach the retina

• High light intensities are able to damage the retina (rods).

• The iris consists of two types of muscle, Circular (parasympathetic) and radial (sympathetic).

Circular muscles contracted, Radial Relaxed

Circular muscles relaxed, radial contracted

Forming an image

• Most refraction happens at cornea• Lens makes fine adjustments. This is called

accommodation• Visual association area perceives image right way up

Accommodation• Far object• Parallel rays• Little refraction needed• Ciliary muscles relax• More tension in suspensory

ligaments• Lens pulled thinner

• Near object• Diverging rays• Much refraction needed• Ciliary muscles contract• Tension lost in suspensory

ligaments• Lens more spherical (natural

shape)

Eye Problems: myopia (near/short sight)

• Parallel rays refracted too much

• Image focussed in front of retina

• Treated with diverging (concave) lens

• Increases focal length

Hypermetropia (far/long sight)

• Diverging rays refracted too little

• Image focussed behind retina

• Treated with converging (convex) lens

• Decreases focal length

Retina

• 3 layers– Ganglion cells (sensory neurone)– Bipolar neurones– Rods & Cones

• Fovea has more cones• Periphery has more rods• Optic nerve head (blind spot) has neither

Rods & Cones

• Contain light sensitive chemical (pigment)

• Convert light into electrical impulse

Rods and cones

• Like neurones but…• Membrane is depolarized

at rest• Gated Na+ channels are

open• (Inhibitory)

neurotransmitter is constantly released

• Keeps bipolar neurone hyperpolarized

Converting light into electrical impulse

• Light changes 11-cis-retinal into all trans-retinal• Rhodopsin → opsin + all trans-retinal• Opsin closes gated Na+ channels• Membrane is hyperpolarized• Inhibitory neurotransmitter release is reduced• Bipolar neurone is depolarized• If depolarization reaches threshold an action

potential is generated in the bipolar neurone

Adaptation

• Dark adapted– Large amounts of

rhodopsin, so…– low levels of light can

break some down but..,

– High levels of light break down large amounts of rhodopsin (bleaching)

– Bright light hurts

• Light adapted– Low levels of

rhodopsin– Bright light is able to

break down some but…

– Dim light does not break down enough to generate an action potential

In the dark human eyes become “dark adapted” in the light they become “light adapted”

Summary of differencesRods Cones

Pigment is rhodopsin 3 types of iodopsin (pigment B, G, R)

Sensitive to dim light Only sensitive to bright light

Poor visual acuity High visual acuity

No colour vision Colour vision

Share connections to bipolar neurone

Single connection to bipolar neurone

Spread around periphery of retina

Mostly in fovea

RetinƏl convergence

• Cones have individual connection to bipolar neurone

• Several rods share a bipolar neurone

Sensitivity• Rhodopsin is more easily broken down than

iodopsin• Retinal convergence

– In dim light only a small amount of pigment may be broken down in an individual rod or cone

– This may not be enough alone to trigger an action potential in a bipolar neurone

– However, the reduction in neurotransmitter release from several rods at once may be enough to trigger and AP in the bipolar neurone

Visual Acuity

• When an impulse reaches the visual sensory area from a ganglion cell it can either have come from one cone or several rods. The brain cannot tell which

• The area of retina represented by several rods is larger

• The image from rods is less resolved, more blurry (pixelated)

The Trichromatic theory of colour vision

• 3 types of cone• 3 types of iodopsin

– Pigments B, G, R– Sensitive to different

wavelengths of light• Different wavelengths

break down different proportions of pigment

• Brain perceives colour as relative stimulation from each type of cone

• E.g. a mixture of pigment G and R breakdown looks yellow

Parallel processing

• 3 pathways along optic nerve– Colour– Shape– Movement

• Impulse sent to visual sensory area

• Association area processes inputs from sensory area in light of other inputs from memory and other senses

Ageing• Cataracts

– Lens becomes cloudy/opaque– Lens can be surgically

removed or replaced by artificial lens

– Corrective lenses are needed to allow focussing on near/far objects

• Macular degeneration– Loss of rods and cones– Dry

• Cells die• Untreatable

– Wet• Blood vessels grow through

retina and burst• Treatable