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RECEPTORS IN ANIMALS RICHARD LLOPIS GARCIA Adapted by MH A2 BIOLOGY FOCUSING The eye is able to focus all rays of light from one object to a single point on the retina. The ability of the eye to change focus from near to distant objects is called ACCOMODATION FOCUSING The amount of refraction taking place at the cornea is more or less constant if the curvature remains the same. The curvature of the lens is altered by the action of the ciliary muscles and so the amount of refraction changes. The Retina Three main layers Photoreceptors (cones and rods) Associated neurons (bipolar) Sensory neurons (ganglion) Cones and Rods Cones and Rods differ by: • Visual acuity (degree of detail, “pixels”) • Sensitivity ((the intensity of light required to produce a generator potential large enough to trigger an action potential) This is due to their connections with bipolar neurons Differences between cones and rods PROPERTY CONES RODS Sensitivity Low: light energy transduced by a single cone must produce a generator potential large enough to exceed the threshold needed for and action potential (unlikely in low light intensities) High: in low light intensity, generator potential from several rods can combine and so the threshold is more likely to be exceeded and action potential initiated (SUMMATION). Several rods are liked to a single nerve cell (via bipolar cells). This is called retinal convergence. Differences between cones and rods PROPERTY CONES RODS Acuity High: each cone is conected to a single bipolar cell, so in high light intensities each cone stimulated represent a separate part of the image which can be seen in detail Low: several rods are conected to the same bipolar cell, so the individual parts of the image represented by each rod are merged into one (low detail distintion) Distribution of rods and cones Rods and cones are distributed unevenly across the retina (please see graph and diagram) What conclusions can you deduce from the diagrams about the distribution of rods and cones in the eye? Distribution of rods and cones The greatest concentration of cones is found at the fovea in the centre of the retina. Looking straight at an object focuses light intensity from it into the fovea, enabling to be seen in great detail if the light intensity is high Distribution of rods and cones The greatest concentration of rods is about 20 degrees away from the fovea. In low light intensities, looking slightly to the side of an object causes the light rays to fall on this area of the retina. Summation by the rods allows better perception than if the light fell on the fovea. (in dim light) TRICHROMATIC THEORY We see everything by mixing only 3 colours in different proportions. Each cone is sensitive to one of these wavelengths. Any particular colour is experienced because the wavelength stimulates one, two, or all three types TO A DIFFERENT DEGREE. Different types of cones There are 3 different types of cone, sensitive to different wavelenght of light which are broadly equivalent to the 3 primary colours (red, blue and green. Now look again to the graph and suggest where in the graph will we be able to see the yellow colour? Different types of cones Answer: a wavelenght of 575 nm stimulates both red and green cones and it is interpreted by the brain as the yellow colour How rods and cones are stimulated (at the molecular level) When cones and rods are stimulated by light, A change occurs in a photosensitive pigment. This alters the membrane potential of the cell, creating a generator potential. The pigment in RODS is RHODOPSIN Light energy is absorbed by a part of rhodopsin called RETINAL Retinal changes from cis retinal to a trans retinal isomer (bleaching) Trans retinal is not photosensitive Rhodopsin How things go back to normal? Retinal and opsin then join back together in an enzyme catalysed reaction that regenerates the photo senstive cis retinal ready to be used again. The same happens with IODOPSIN in CONES but breaks less easily and joins back together more slowly (cones better for High Light Intensities) Creating a nerve impulse (Action Potential) Bleaching causes excess Na+ channels to close (resting potential is more –ve) 120mV Less neurotransmitter is released at the bipolar synapse This stops the inhibition of the bipolar cell A generator potential is formed in the bipolar cell If threshold is reached in the bipolar cell (through a summation of the generator potentials) then an action potential is produced in the bipolar and ganglion cells So RODS are more sensitive 20 times more rods than cones Found outside the fovea (periphery of the retina) More sensitive because they converge onto the SAME BIPOLAR NEURONE. i.e. even the small responses from rods will be detected by the brain. They are not good at providing clarity or detail. (try to see an object from the corner of your eye) But Cones let you see in more detail. Mostly packed together in the FOVEA They give good VISUAL ACUITY (clarity) And more accurately and in more detail Because each cone synapses with its own individual bipolar synapse. Remember that also let you see in colour. (draw diagram of connections) Click on the hyperlink to see an eye dissection Eye Dissection Complete.wmv