the mammalian eye

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The mammalian eye. By Sze Nga CHAN Cecilia. Content. Structure and function of eye Properties in focusing an image Features of retina Structure of rod cells and cone cells Role of rod cells and cone cells Sensitivity of vision Acuity of vision Effect on rhodopsin. - PowerPoint PPT Presentation


The eye

The mammalian eye

By Sze Nga CHAN CeciliaHi everyone, today I m going to talk about the mammalian eye. Before we start, lets have a look with the model to see if you remember the things we learned in GCSE. ( bring the model and ask them)2ContentStructure and function of eyeProperties in focusing an imageFeatures of retinaStructure of rod cells and cone cellsRole of rod cells and cone cellsSensitivity of visionAcuity of visionEffect on rhodopsin

Here are the sections we will go through today, and we will focus mainly on the rod cells and cone cells.3

This is the structure of mammalian eye, as you can see, there are three layers4Features and FunctionsLayersSclera-tough layer-form the transparent cornea -transparent conjunctiva covers the cornea-transmission-RefractionChoroid-rich in blood vessels-black in color-prevent internal reflection of lightRetina-photoreceptive-contains rod cells and cone cells

Sclera is a tough layer to protect the eye, which forms the transparent cornea in the front. Also, it is covered by a thinner transparent conjunctiva. They are both transparent, so they can transmits light. Because of the cure shape, cornea can refract light as well.

The middle layer is Choroid, it contains many blood vessels, which can supply nutrients to inner layer (retina). Also, it can prevent internal reflection of light as it is black in colour.

The inner layer is Retina, it is photoreceptive because it contains numerous rod cells and cone cells.

5Features and FunctionsAqueous humour-behind the cornea-a clear salt solutionVitreous humour-behind the lens-jelly-like dark solutionFovea- contains most of the rod cells and cone cells- light rays are focused onto itBlind spot-the connection point of optic nerve-no rod cells or cone cells can be found Moreover, this two guys are work to maintain the shape of the eye ball by hydrostatic pressure.Aqueous humour is a clear salt solution which is located behind the cornea while Vitreous humour is a jelly-like dark solution which is located behind the lens.

Fovea contains most of the rod cells and cone cells, so light rays are focused onto it. While Blind spot is the connection point of the optic nerve, therefore, there is no rod cells and cone cells.

6How 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 pupilIn a sunny day, to prevent too many light rays goes into the eye, circular muscle will contracts and radial muscle will relaxes, it results the pupil becomes smaller, therefore, less amount of light rays can enter the eye.7How 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

On the other hand, when the light is weak, circular muscle will relaxes and radial muscle will contracts to allow the pupil becomes dilate, as a result, more light rays can enter it.

At the bottom, there is a video about how the eye works and some eye related illness. You can go to have a look if you want.8How to adjust the focus point?A near target-ciliary muscles contract-suspensory ligament slack-lens more rounded shape(more convex)-light focused on retina

Cornea is not the only one refract light rays, the lens also take part on it.The lens find-tunes the direction of light by changing their shape, indeed it has to done by the circular ciliary muscle and the suspensory ligaments.

When watching a near object, ciliary muscles contract, resulting the suspensory ligament become slack and then the lens become more rounded shape, and finally the light focused on retina.9How to adjust the focus point? A far target-ciliary muscles relax-pull the suspensory ligament taut-lens flat (pull in)- light focused on retina

The same but opposite way when watching a distant object, ciliary muscles relax, resulting the suspensory ligament become taut and then the lens become flat, and finally the light focused on retina.

10Features of retinaThree main layers

Photoreceptorrod cells and cone cellsBipolar neuronesSensory neuronesThere are three main layers in retina. First, the photoreceptor, which including rod cells and cone cells. In the middle, it is bipolar neurones and the lowest layer is sensory neurones, which is ganglion cells.Another thing we have to look at in this picture is more than 1 rod synapse linked to one bipolar synapse while each cone synapse linked to its own individual bipolar synapse. 11Rod cells and Cone cells

Here are the structure of the rod and cone cell. All we have to know is which part is the outer segment and inner segment. And they contain cilium mitochondria and nucleus. And the location of the synaptic ending.12FeatureRod cellsCone cellsDistribution-Evenly throughout the retina-absent from the fovea-mainly found in the foveaSensitivity 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 lightVisual acuity (details)-produce poorly resolved images-produce high-resolved imagesLight-sensitive pigments

-rhodopsin-three types of iodopsin (red, green, blue)Synapse with bipolar neuroneGroup of rod cells synapse with one bipolar synapseEach cone cell synapse individually with a single bipolar synapse

Effect on rhodopsin

Light energy is absorbed by retinalRhodpsin separate to retinal and opsinOpsin causes the closure of Na channel Less inhibitory neurotransmitter is releasedLess inhibition of bipolar neuronsDepolarisationAction potential Optic nerve

Reformation of rhodopsinCatalystATPTrans-retinal + opsin rhodopsin

In iodopsin in cone cells, the same reaction happenbut it breaks less easily and reforms more slowlyExam 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/convergenceprinciple 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 individuallyReference information obtained in the internet on 18th April,2010

???Good luck in EXAM!


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