Eye. Structure The most important structures to learn the function of are… Retina Lens –Ciliary...
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Transcript of Eye. Structure The most important structures to learn the function of are… Retina Lens –Ciliary...
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