PHYSIOLOGY OF

VISION

Prof. Vajira WeerasingheDept 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

maculafovea

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 Df = 2 m: power = 0.5 Df = 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 humour1.0 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 frontof 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 retinol (vitamin A)

• all-trans retinol is converted to 11-cis retinol by enzyme isomerase

• then 11-cis retinol 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 ceasesinner segment

pumps Na+ outleads to

hyperpolarisedmembrane

Na+

Na+ Na+

- 80 mV

Na+

in dark

Na+

Na+ Na+

membranepotential - 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 differentcone 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

wavelength400 700500 600

light absorption spectrum

Ultraviolet 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 40 501

10

100

1000

10000

100000

cone

adap

tatio

n

rod

adap

tatio

n

minutes in dark

retinalsensitivity

0 10 20 30 40 50minutes in dark

retinalthreshold

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 0:0:97yellow 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

lateralgeniculate

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

left eye blindness

bitemporal hemianopia

right homonymous hemianopia

right homonymous hemianopia

normal visual fields

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