The mammalian eye By Sze Nga CHAN Cecilia

Content 1. Structure and function of eye 2. Properties in focusing an image 3. Features of retina 4. Structure of rod cells and cone cells 5. Role of rod cells and cone cells 6. Sensitivity of vision 7. Acuity of vision 8. Effect on rhodopsin

Features and Functions  Layers 1. Sclera -tough layer -form the transparent cornea -transparent conjunctiva covers the cornea -transmission -Refraction 2. Choroid -rich in blood vessels -black in color -prevent internal reflection of light 3. Retina -photoreceptive -contains rod cells and cone cells

Features and Functions 1. Aqueous humour -behind the cornea -a clear salt solution 2. Vitreous humour -behind the lens -jelly-like dark solution 3. Fovea - contains most of the rod cells and cone cells - light rays are focused onto it 4. Blind spot -the connection point of optic nerve -no rod cells or cone cells can be found

How to adjust the amount of light goes into the eye? Strong light -circular muscle contracts -radial muscle relaxes -pupil becomes smaller -less light rays enter the eye Iris is a muscular diaphragm surrounding the pupil

How to adjust the amount of light goes into the eye? Weak light -circular muscle relaxes -radial muscle contracts -pupil becomes dilate - more light rays enter the eye

How to adjust the focus point? A near target -ciliary muscles contract -suspensory ligament slack -lens more rounded shape(more convex) -light focused on retina

How to adjust the focus point? A far target -ciliary muscles relax -pull the suspensory ligament taut -lens flat (pull in) - light focused on retina

Features of retina Three main layers Photoreceptor rod cells and cone cells Bipolar neurones Sensory neurones

Rod cells and Cone cells

FeatureRod cellsCone cells Distribution-Evenly throughout the retina -absent from the fovea -mainly found in the fovea Sensitivity to light-very sensitive to light, can operate in dim intensities -insensitive to colour -sensitive to bright light, can operate in high light intensities only -sensitive to red, green, blue light Visual acuity (details)-produce poorly resolved images -produce high-resolved images Light-sensitive pigments-rhodopsin-three types of iodopsin (red, green, blue) Synapse with bipolar neurone Group of rod cells synapse with one bipolar synapse Each cone cell synapse individually with a single bipolar synapse

Effect on rhodopsin 1. Light energy is absorbed by retinal 2. Rhodpsin separate to retinal and opsin 3. Opsin causes the closure of Na channel 4. Less inhibitory neurotransmitter is released 5. Less inhibition of bipolar neurons 6. Depolarisation 7. Action potential 8. Optic nerve

Reformation of rhodopsin Catalyst ATP Trans-retinal + opsin  rhodopsin In iodopsin in cone cells, the same reaction happen but it breaks less easily and reforms more slowly

Exam question The diagram shows the distribution of rods and cones in the retina of a human eye. Using information in the diagram, explain how: (i) rod cells enable us to see in conditions of low light intensity; (ii) cone cells enable us to distinguish between objects close together. Several rod cells to each neuron / bipolar cell/each synapse/convergence principle of additive effect of light striking several rod cells/(spatial) summation; Each cone connects to a single neurone/no convergence; brain receiving information from each cone cell individually

Reference 3_eye_anatomy_label_v2_700.jpg 3_eye_anatomy_label_v2_700.jpg _eyes.jpg _eyes.jpg _1a.jpg _1a.jpg png png AB8/7y0hnATxxFg/s400/6a00d41423ae593c7f00d41422a3576a pi.jpg AB8/7y0hnATxxFg/s400/6a00d41423ae593c7f00d41422a3576a pi.jpg All information obtained in the internet on 18 th April,2010

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