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PL3020/FM2101/PL2033 Physiology Vision 1
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The visual system Lecture 1 Structure of the eye
Optics, visual acuity and refractive errors Photoreceptors: transduction and adaptation 2
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The visual system Lecture 1 Structure of the eye
Optics, visual acuity and refractive errors Photoreceptors: transduction and adaptation Lecture 2 Retinal processing and LGN Primary visual cortex: simple and complex cells, edge and feature detection Secondary visual areas, colour processing, stereopsis etc 3
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Gordon Reid Physiology 4
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The simplest camera 5
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Diaphragm: adjust amount of light
Lens: more light Diaphragm: adjust amount of light 6
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The eye as a camera 7
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The eye as a camera cornea + lens (eye) lens (camera)
retina (eye) film (camera) iris (eye) diaphragm (camera) 8
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Structure of the eye Aqueous humour Lens Cornea Sclera Vitreous humour
(proteoglycan gel) Cornea Sclera Lens 9
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Structure of the eye Iris Aqueous humour Lens Cornea Sclera Ciliary
muscle Suspensory ligaments Vitreous humour 10
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Structure of the eye Iris Aqueous humour Lens Cornea Sclera Ciliary
muscle Suspensory ligaments Retina Fovea Optic disc Vitreous humour Optic nerve 11
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Aqueous humour flow Ciliary body 12
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Aqueous humour flow Blocking canal of Schlemm glaucoma
Normal pressure 15 mm Hg Can increase to mm Hg 13
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Refraction 14
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Refractive index Low High Speed of light reduced light rays bent 15
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Bending of light and refractive index
Snell’s law: N1 sin a1 = N2 sin a2 if refractive index N becomes larger then angle a becomes smaller This is why water bends light 16
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Lenses, focusing and refractive power
Power (diopters) = 1/focal length (metres) e.g. focal length = 0.5 m: power = 2 D 17
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Lenses, focusing and refractive power
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Refracting structures in the eye
Four refracting surfaces: Front of cornea +48 D; back of cornea -5 D Front of lens +5 D; back of lens +8 D Four refracting surfaces: Front of cornea +48 D; back of cornea -5 D Front of lens +5 D; back of lens +8 D 19
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Widely used model for human optics
Reduced eye Widely used model for human optics 20
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Visual acuity 21
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Limit of visual acuity Limit ~0.5 minutes (0.008°) = 1 mm at 10 m
= 2 μm on retina (foveal cone diameter ~1.5 μm) 22
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Normal visual acuity “Normal” is considered to be 1 minute (0.017°)
1.75 mm 5 mm 6 m (20 ft) “Normal” is considered to be 1 minute (0.017°) = 1.75 mm at 6 m = 5 μm on retina: If you can resolve 1.75 mm at 6 m you have 6/6 vision (USA: 20/20 vision) 23
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6 m (6/6 vision if resolved at 6 m)
Landolt C test 1.75 mm 6 m (6/6 vision if resolved at 6 m) 3.5 mm 12 m (6/12 vision if resolved at 6 m) 24
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Accommodation 25
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You need increased refractive power to focus on something close up
Accommodation: You need increased refractive power to focus on something close up 26
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The lens becomes more curved
Accommodation: The lens becomes more curved 27
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1. Suspensory ligaments keep the lens stretched
Accommodation: 1. Suspensory ligaments keep the lens stretched 2. Contraction of the ciliary muscle (parasympathetic) allows the lens to relax 28
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Range of accommodation (D) =
1/near point (m) If d = 12.5 cm then accommodation = 8 D If d = 25 cm then accommodation = 4 D 29
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Decline in accommodation with age
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Refractive errors 31
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“Far-sighted” Eyeball too short
Refractive errors Normal Hyperopia “Far-sighted” Eyeball too short Myopia “Short-sighted” Eyeball too long 32
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Correcting refractive errors
Myopia Use a concave lens (negative) Hyperopia Use a convex lens (positive) 33
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Astigmatism Means that refracting power is not homogeneous: e.g. more in vertical (BD) than horizontal (AC) plane 34
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Astigmatism Correction: cylindrical lens to increase refractive power of AC 35
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Contact lenses can repair more complex defects
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Depth of field (near and far objects simultaneously in focus)
Maximised when pupil is constricted 37
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Pupil constriction Depends on antagonistic iris muscles
Sympathetic: pupil dilation Parasympathetic: pupil constriction 38
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The retina 39
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The retina 40
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The retina Peripheral LIGHT Fovea Photoreceptors 41
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The retina Peripheral 42
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Rods and cones Rod Cone 43
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Rods and cones Rod Cone Rod Cone 44
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Rods and cones Rods Operate in dim light: saturated in daylight
Not involved in colour vision Present only in peripheral retina Cones Operate only in bright light: inoperative at night Involved in colour vision (red, green, blue cones) High density in fovea 45
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Spectral sensitivity of rods and cones
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Spectral sensitivity of cones (2)
Three broad groups Inter-individual variation 47
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Rod and cone distribution in the retina
Peripheral Fovea Many rods Relatively few cones Maximally close packing of cones 48
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Rod and cone distribution in the retina (2)
Fovea 49
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Visual transduction (1)
Rhodopsin = opsin + retinal: Opsin Retinal attachment 50
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Visual transduction (2)
A single “flip” in retinal conformational change of opsin 51
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Visual transduction (3)
Opsin is a 7-transmembrane-helix receptor It couples to a G protein (transducin) Transducin activates phosphodiesterase (PDE) PDE breaks down cGMP 52
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Visual transduction (4)
PDE breaks down cGMP Closure of cGMP-gated ion channels Hyperpolarisation 53
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Visual transduction (5):
Amplification Single photon flashes Two photons 54
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Spectral sensitivity of rods and cones
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...depends on sequence differences in opsins
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Dark adaptation After exposure to bright light, subjects were given dim red or green spots as stimuli Threshold measured (i.e. dimmest spot that subject could see) Red spot Green spot Red light stimulates only cones: green both cones and rods Cones adapt faster but rods are more sensitive 57
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Retinal processing of light-induced signals
What next? Retinal processing of light-induced signals 58
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