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Vision
 

Vision

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    Vision Vision Presentation Transcript

    • PHYSIOLOGY OF VISION Prof. Vajira Weerasinghe Dept of Physiology, Faculty of Medicine, University of Peradeniya
    • Vision Eye receives light stimulus & transforms it into a nerve impulse which runs along the optic nerve reaching the visual cortex & gives rise to visual sensation Eyeball is a spherical structure with a diameter of 24 mm
    • Coverings of the eye ball There are 3 layers sclera choroid retina
    • Outer coat this is protective anteriorly (1/6) it is transparent - cornea posteriorly it is white, opaque, avascular - sclera sclerocorneal junction
    • Middle coat vascular anteriorly (iris) - circular diaphragm with pupil middle - ciliary body (intraocular muscle) inner aspect contain ciliary processes which secrete aqueous humour posteriorly - choroid
    • Inner coat this is sensory retina contains nerves transparent
    • retina optic disc: where optic nerve comes out of the eyeball, blind spot, no vision at this point macula: the most sensitive spot, cones concentrated. fovea is the centre of the macula blind spot macula fovea
    • retina ophthalmoscopy (examination of the eye using an illuminated source) inside of the retina can be seen optic disc containing blind spot macula & fovea retinal blood vessels
    • Lens crystalline structure biconvex lens, posterior surface more convex suspended from the ciliary body by fine delicate fibres called zonule or suspensory ligament of the lens covered by a capsule
    • posterior compartment lens & zonule divide eyeball into posterior compartment containing a transparent jelly-like structure called vitreous humour
    • anterior compartment lens & zonule divide eyeball into anterior compartment contains aqueous humour subdivided by iris into anterior chamber posterior chamber communicated by pupil
    • seeing from front pupil iris cornea sclera
    • Aqueous humour clear fluid, volume is about 250 ul water 98.9% other: protein, non-protein N, glucose, Na, K, Cl, ascorbic acid, pyruvate, lactate, dissolve O2 lower conc of protein, urea & glucose than plasma osmotic pressure higher than plasma secreted by ciliary processes, ultrafiltrate of plasma pass through posterior chamber -> anterior chamber absorbed back into canal of Schlemm in sclera
    • intra ocular pressure this is about 10-20 mmHg maintained by aqueous humour measured using a tonometer elevated intraocular pressure occurs in glaucoma glaucoma may cause blindness
    • PHYSICS OF VISION OPTICS
    • eye as a camera eye acts as a camera in a camera light rays coming from an object passes through the aperture & forms an image on a film pinhole camera box camera
    • eye as a camera in the eye pupil act as the aperture & its size can vary lens can change its curvature
    • f = focal length power of a lens = 1 ---------- f (m) f = 1 m: power = 1 D f = 2 m: power = 0.5 D f = 0.5 m: power = 2 D + 1 D: converging lens - 1 D: diverging lens
    • as light passes through the lens system several interfaces are traversed their refractive indices are different Air Cornea Aqueous humour lens vitreous humour 1.0 1.38 1.33 1.40 1.34
    • Reduced eye if all the refractive surfaces are added together & represented by a single lens it is known as the ‘reduced eye’ focal length = 24 mm power = + 59 D Nodal point = 17 mm in front of retina
    • Air/cornea interface (1.0/1.38): produces a significant refractive power Aqu hum/lens/vit hum interfaces (1.33/1.4/1.38): produces only a minimum refractive power Power of the lens is only 20 D But it has the ability to vary this power by
    • Accommodation power of the lens can be increased from 20D to 34D in a young child suspensory ligaments in the zonule pulls the lens & make it less convex lens ciliary muscles zonule
    • parasympathetic activity -> -> contracts ciliary muscles -> relaxes suspensory ligaments in the zonule -> lens become more convex -> power of the lens increases -> subject can focus near objects lens ciliary muscles zonule
    • with age this ability decreases power of accommodation decreases 14 D up to 40 yrs 2D at 40-50 yrs 0 D at 70 yrs thereafter constant focal length presbyopia: is the lack of accommodation, occurs with age, requires + glass to increase power lens ciliary muscles zonule
    • errors of refraction emmetropia is the normal eye refractive errors myopia hypermetropia presbyopia astigmatism
    • MYOPIA shortsightedness near objects can be focussed far objects focuses in front of retina this could be due to lens having more refractive power eyeball being longer than normal correction is done by -D lenses (concave lenses) these lenses will move the image back to retina
    • emmetropia: unaccommodated eye emmetropia: accommodated eye myopia: distant objects, forms in front of retina correction: - lens, decreases power MYOPIA
    • HYPERMETROPIA farsightedness objects are focused behind the retina this could be due to lens having less refractive power eyeball being shorter than normal correction is done by +D lenses (convex lenses) these lenses will bring the image on to retina
    • emmetropia: unaccommodated eye emmetropia: accommodated eye hypermetropia: image forms behind the retina correction: + lens, increases power HYPERMETROPIA
    • ASTIGMATISM spherical aberration of the cornea (& lens) resulting in an image with mutiple focal points which is not clear correction is done by spherical or cylindrical lenses these lenses will correct the disparity in corneal curvature
    • emmtropia: unaccommodated eye emmtropia: accommodated eye presbyopia: lack of accommodation presbyopia: + lens, increases accommodation PRESBYOPIA
    • Contact lenses at present contact lenses are widely used
    • Photochemistry of vision photochemicals: rods contain rhodopsin, cones contain similar chemicals rhodospin outer segment contain rhodopsin or visual purple consists of protein scotopsin & carotenoid pigment retinal (or retinene). this is 11-cis retinal
    • decomposition of rhodopsin by light 11-cis retinal combines with scotopsin to form rhodospin when light is absorbed by rhodopsin decomposition of rhodopsin starts extremely unstable barthorhodopsin->lumirhodopsin-> metarhodopsin I -> metarhodopsin II (metarhodopsin II also called activated rhodopsin starts neural activity) in few seconds it is converted to sotopsin & all trans retinal
    • Neural activity
    • reformation of rhodopsin conversion of all-trans retinal into 11-cis retinal in dark this reaction is catalysed by retinal isomerase once 11-cis retinal is formed, it combines with socotpsin to form rhodospin wait until light is absorbed again
    • role of vitamin A alternative route of reformation of rhodospin all-trans retinal is first converted to all-trans retin o l (vitamin A) all-trans retin o l is converted to 11-cis retin o l by enzyme isomerase then 11-cis retin o l is converted to 11-cis retinal when there is excessive retinal in the retina it is converted to retinol (vitamin A)
    • Night blindness vitamin A deficiency not enough quantities of retinal to reform rhodopsin but in daytime cones can still be excited
    • Action potentials excitation of rods causes hyperpolarisation rather than depolarisation increased negativity of the membrane this is due to decreased permeability to Na inner segment pumps Na out outer segment is very leaky to Na normally membrane is -40mV (inside) when excited outer segment prevents Na influx inner segment continually pumps Na out increased negativity inside -> hyperpolarisation inside becomes -80mV
    • a rod outer segment inner segment in light when light strikes the outer segment, Na+ channels close Na+ influx ceases inner segment pumps Na+ out leads to hyperpolarised membrane Na + Na + Na + - 80 mV Na + in dark Na + Na + Na + membrane potential - 40 mV
    • Neurotransmitter Neurotransmitter in the visual receptor cells glutamate
    • Pigments in the cones photochemicals in cones are similar to rhodopsin (scotopsin + retinal) cones contain photopsin + retinal 3 different types of photochemicals are present in cones, their light absorption spectra are different cone pigment wavelength of peak absorption (nm) blue-sensitive pigment 445 green-sensitive pigment 535 red-sensitive pigment 570 rods have peak sensitivity at 505 nm
    • wavelength 400 700 500 600 light absorption spectrum Ultra violet violet indigo blue green yellow orange red Infra red visible spectrum rods
    • Light Adaptation retinal sensitivity depends on the amount of chemical pigment if a person is in bright light for some time, large amount of photochemical is reduced to retinal and opsin retinal converted to vitamin A this reduces the sensitivity of the retina this is known as light adaptation now if the person goes into a dark room he cannot see any object reason: severe reduction in retinal sensitivity
    • Dark Adaptation if the person remains in dark for some time then the retinal sensitivity increases this increases exponentially this consists of two parts initial quick phase: due to adaptation of cones later slow phase: due to adaptation of rods
    • 0 10 20 30 40 50 1 10 100 1000 10000 100000 cone adaptation rod adaptation minutes in dark retinal sensitivity
    • 0 10 20 30 40 50 minutes in dark retinal threshold
    • Colour Vision human eye can see any colour due to a combination of red, green and blue monochromatic light in different proportions
    • Colour Vision since the 3 different types of cones are sensitive to different colours differential stimulation of 3 types of cones determine the colour combination seen eg: orange stimulate R:G:B cones in 99:42:0 % blue 0:0:97 yellow 83:83:0 white light stimulate 3 types of cones equally
    • Colour Vision
    • Colour Vision Tested using Ishihara’s isochromatic charts
    • Colour Blindness Total colour bilndness is extremely rare Impaired appreciation of colour can happen Red green blindness is the commonest type of colour blindness cannot distinguish red from green Transmission is genetical X linked recessive There are different types of colour blindness Monochromacy Have only one type of cones Dichormacy Have only two types of cones in the retina protanopia   a person with loss of red cones deuteranopia a person with loss of green cones tritanopia a person with loss of blue cones
    • Visual pathway visual field is divided into temporal (lateral) and nasal (medial) halves,overlap of nasal halves Retina temporal field corresponds to medial half of retina & vice versa optic nerve lateral & medial retinal fibres maintain spatial arrangement optic chiasma at the level of pituitary, only medial retinal fibres cross to the other side optic tract up to the geniculate lateral geniculate body synapse occipital cortex optic tract continues as geniculocalcarine tract up to the occipital cortex
    • visual field retina optic nerve optic chiasma optic tract lateral geniculate body occipital cortex
    • Lesions along the visual pathway a lesion may arise at different points along the visual pathway gives rise to different types of visual field defects known as hemianopia (half blindness) perimetry is a test which can detect visual field defects
    • Left Right normal visual fields left eye blindness bitemporal hemianopia right homonymous hemianopia right homonymous hemianopia
    • Pupillary light reflex Pupil undergoes the change in size reflexely in response to a change in illumination This reflex is useful in increasing the amount of light entering the eye when the illumination is dim which helps dark adaptation It also makes the pupil narrow in bright light which improves the depth of focus
    • Two type of light reflexes Direct light reflex Constriction of pupil of the eye in which the light is directed is called direct light reflex Consensual light reflex Constriction of pupil of the other eye is called consensual light reflex
    • Pathway Light -> retina -> optic nerve -> optic tract -> collateral from the optic tract -> superior colliculi and pretectal area (midbrain) -> efferent originates in the parasympathetic part of the oculomotor nucleus (Edinger-Westphal nucleus) -> ciliary ganglion -> sphincter pupillae
    • Visual Acuity Acuteness or clearness of vision It is the degree to which the details and contours of objects are perceived It is defined in terms of the minimum separable (shortest) distance by which two lines can be separated and still be perceived as two lines Thus the minimum separable in a normal individual corresponds to a visual angle of about 1 minute Clinically Snellen’s charts are used to determine visual acuity