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Excitable Cells - Lecture 13 (13/03/2018)
Sensory Systems I: Vision and the Eye
Properties of Light
Light may be described both as an electromagnetic wave, and a group of photons.
The wavelength is the shade of the light of the visible light spectrum.
The amplitude is the intensity of the light.
We can see only a narrow wavelength range of electromagnetic radiation which is in the visible spectrum.
Different people may see somewhat different ranges of colours due to colour blindness.
The wavelength can be described as the speed over frequency.
White light is a mixture of radiations with different wavelengths.
The reason we see different colours is that we have photoreceptors that are tuned to different wavelengths of light.
The Vertebral Eye
Light enters the eye through the Cornea that is optically the strongest part (42 Dioptres), and the Lens (12 Dioptres), which together bend the light onto the Retina that has photoreceptors.
The Optic nerve brings blood and contains axons and ganglion cells, which transmit signals to the brain.
The Retina contains photoreceptors and other cells and a lot of image processing, and this sends processed information to the brain.
The Lens is attached by the Zonule fibers to the ciliary muscle, which can be relaxed to focus onto far objects, or contracted, to be focussed onto near objects. When the lens is the the wrong shape, it causes Myopia (short-sightedness) or Hyperopia (far-sightedness).
The Cornea is the first point of refraction with the greatest refracting power in the eye. The lens is the second point and the younger you are, the better you can see close up.
The accommodation is the changing shape of the lens, allowing the organism to see from horizon to about 20 cm, allowing more refractive power.
The image projected is inverted (left-right and up-down) on the retina and then the brain inverts everything again.
When the Cornea and Lens bend the light too much, the image on the retina is blurred causing myopia (short-sightedness). This can be corrected with a concave lens.
When the Cornea and Lens don't bend the light enough, the image is also blurred, instead causing hyperopia (farsightedness). This can be corrected with a convex lens.
When Cornea and Lens are not spherical, but shaped more like a melon, there may be two focal points, or the focus in the vertical plane and the focus in the horizontal plane may be misaligned in Astigmatism (blurred vision at all distances).
Humans and other animals are able to see the world in 3D as the third dimension (depth) is reconstructed by the brain from the two slightly different images that our 2 eyes provide through binocular disparity.
In order to obtain these different images, the eyes need to be some distance apart. This is a problem with very small organisms, such as insects, whose eyes are very close together.
Its recently been discovered that the praying mantis uses a different computational algorithm to extract depth information. The mantis compares not the brightness of images between its two eyes, but the change of brightness (first derivative).
The brain may be tricked into seeing 3D by presenting slightly different 2D images to both eyes, as can be done with red/green or with polarized glasses.
Autostereograms also present another method of seeing 3D from a 2D image in a very similar way.
Light goes through several layers of retinal cells before it reaches the photoreceptors, made up of rod and cone cells. On the back of the eyes there is the sclera and retinal epithelium.
The processed visual information in transmitted in the direction opposite to the incoming light, from the photoreceptors to the retinal ganglion cells.
It is not ideal to have receptors on the back! But more primitive eyes don't have neuronal layers, they only have photoreceptors and RPE (retinal pigment epithelium) - structural component, provides support to the photoreceptors and supplies retinal.
As the eye evolved, the only space for new cells was in front of photoreceptors. These new cells were transparent and don't obscure the light significantly and acts as the first layer of processing.
Rods and cones (and some ganglion cells that express melanin) pick up the light, and the ganglion cells send the signal to the brain through firing action potentials.
All of the other cells are neurons too, but they release neurotransmitters, through graded depolarization. These cells are are small and act over short distances, so they don't need action potentials.
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