SENSORY RECEPTORS.

SENSORY RECEPTORS.

Objectives:E - label the parts of the eyeD – Describe their functions

C – explain how parts function to adjust focus and the amount of light entering

the eye

The eye as a receptor

cornea

iris

pupil

aqueous humour

ciliary body

lens

choroid choroid

retina

vitreous humour

optic nerve

Soper 2 p593

• Draw eye diagram and label functions

• Iris• Cornea• Sclera• Choroid• Ciliary muscles• Conjunctiva• Adjustment• Longitudinal

muscles• Radial muscles• Suspensory

ligaments

• Lens• Pupil• Eyelid• Aqueous

humour• Vitreous

humour• Retina• Fovea• Optic nerve• Dilation• Constriction• Far object• Close object

Accomodation

• This is the focusing of light by the lens

Dilation & constriction

• Iris• Cornea• Sclera• Choroid• Ciliary muscles• Conjunctiva• Rhodopsin • Longitudinal

muscles• Radial muscles• Suspensory

ligaments• cones

• Lens• Pupil• Eyelid• Aqueous

humour• Vitreous

humour• Retina• Fovea• Optic nerve• Visual accuity• Constriction• Far object• Close object• Rods

The retina

Blood vessel in choroid

choroid

retinal rod and cone cells

optic nerve fibres

synapses

nuclei of retinal cells

nuclei of bipolar cells

nuclei of ganglion cells

LIGHT

Inner segments of rod and cone cells

cell bodies of rod and cone cells

rod cell

cone cell

rod cellcone cell

cell body

bipolar cell

ganglion cell

optic nerve fibres

synapse

Structure of the retina

Axons of ganglion cells optic nerve visual area of the brain

Photoreceptor cells(Rods and cones)

Bipolar neurones

connect photoreceptors to optic nerve

•rods connected in groups

•cones connected singly

Melanin – absorbs light to prevent internal reflection

fovea

Approx 1mm diameter on visual axis of eyeCones only

Point of maximum intensity of visionMain point of interest in visual field focused

here

blind spot

fovea

All axons of ganlion cells converge here to form the optic nerveNo photoreceptors so not capable of forming an image

Rhodopsin located in membrane of discs

Photosensitive pigment

Rod cell is a modified bipolar neurone

How cone cells work; colour vision

• Cone cells work in basically the same way as rod cells.

• However bleaching requires a much higher light intensity (so cone cells cannot function in dim light).

There are three different types of rhodopsin (the opsin protein part differs), which have maximum absorption at different wavelengths so cones cells allow for the discrimination of colour; there are•blue-absorbing cones (max absorption at 445nm)

•green-absorbing cones (max absorption at 535 nm)•red-absorbing cones (max absorption at 570 nm)

• this is the trichromatic theory of colour vision

• different colours are perceived as a result of the degree of stimulation of the blue + green + red cones (i.e. colours are the result of the mixture of inputs from all three cone types)

• By population, about 64% of the cones are red-sensitive, about 32% green sensitive, and about 2% are blue sensitive

for example: % refers to the light absorbed as a % of the maximum*

0% blue + 83% green + 83% red yellow

31% blue + 67% green + 26% red green

•*hence the size of the generator potentials produced, which in turn determines the number of nerve impulses from each type of cone cell, which is then interpreted by the brain as a particular colour