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Chapter 2

PSYB51 Chapter 2.docx

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Matthias Niemeier

PSYB51 Chapter 2 -light is energy produced by vibrations of electrically charged material >two ways to discuss light. As a wave (oscillation traveling through medium by transferring energy from one particle/point to next) or photons (tiny particles consisting of a quantum of energy each). >a wave of light moves around the world, a photon is absorbed -electromagnetic spectrum goes from gamma rays to radio/television waves (short to long wavelengths) >visible light waves have wavelengths b/w 400-700 nanometers -photons of light can be absorbed (take up light, not transmit). -light hits objects and is reflected (bounces off surface, accounts for that surfaces light appearance) -sometimes light is neither reflected or absorbed, its transmitted through the surface. -sometimes when light goes from air into glass some of the light rays become bent or refracted as its transmitted -eyes can form images of the world -the first tissue that light will encounter in the eye is the cornea (thin film on top of the eyeball). Its transparent, most light photons are transmitted through it rather than reflected/absorbed. It contains no blood vessels or blood (this absorbs light), has lots of sensory nerve endings that force eye to close and produce tears if cornea is scratched. -aqueous humor: fills space right behind cornea, supplying oxygen and nutrients to/removing waste from the cornea -to get to the lens, the light passes through the pupil (opening at the center of the eye) which is basically a hole in iris (gives eye its color, surrounds pupil. Regulates light that enters by expanding or contracting the pupil < pupillary light reflex) >when you come out of a dark room into bright light, pupil constricts. -the light then enters vitreous chamber (between the lens and retina, here its refracted again by vitreous humor. It’s a gel-like, viscous substance (like egg white). -after travelling though vitrous chamber, light is brought into focus at the retina (light-sensitive membrane at back of eye, contains rods and cones). -cornea is the most powerful refractive surface in the eye. Lens alter refractive power by changing in shape, accommodation. This is accomplished through contractions of the ciliary muscle which is attached to the lens (by zonules of Zinn). >when ciliary muscle is relaxed, zonules are stretched, lens is relatively flat. The eye is now focused on distant objects. >to focus on something closer, the ciliary muscle contracts, reducing tension on zonules and the lens bulge. -presbyopia: “old sight”, loss of near vision bc of insufficient accommodation. >lens become harder, the thing that allows lens to change shape loses elasticity. -4 places where light is refracted: cornea, lens, aqueous and vitrous humor -emmetropia: the refractive power of the eye is perfectly matched to length of eyeball. Errors occur when eyeball is too long or short -myopia: when eyeball is too long, image of something will be focused IN FRONT of retina. This is nearsightedness. Corrected with lenses that diverge the rays of light before they enter eye. -hyperopia: eyeball Is too short, imagine is focused BEHIND the retina. Farsightedness. Corrected with lenses that converge rays before they enter eye -astigmatism: cornea is not spherical, vertical lines mare focused slightly in front of retina and horizontal lines are slightly behind (or vice versa) -at retina, light is transduced into neural energy so it can be interpreted by brain -eye doctors use ophthalmoscope to look at the back surface of your eye called the fundus, the back layer of the retina >optic disc is where the arteries and veins enter the eye and where axons of ganglion cells leave the eye. Theres no photoreceptors here, BLIND SPOT. -retina is layered sheet of clear neurons, half the thickness of a credit card. Also has another layer of darker cells (pigment epithelium) behind the final layer -transduction of light energy intro neural energy starts at the back layer of retina, made up of photoreceptors. When they sense light they stimulate neurons in the intermediate layers like the bipolar, horizontal and amacrine cells. These neurons then connect with the front most layer of the retina, where the ganglion cells are. The axons of the ganglion cells pass through optic nerve, to the brain. -photoreceptors are at the back so light passes through all the neuron layers first before making contact with the photoreceptors. The neurons are transparent so light goes through them -retina has A LOT of photoreceptors (100 million), they are neurons that capture light and produce chemical signals. Has TWO types of photoreceptors, rods and cons. That’s why humans have duplex retinas, some animals have mostly rods (night animals) and some have mostly cons (like lizards) -cons are specialized for daylight vision, fine visual activity and colour. Rods are specialized for night vision. >both have an outer segment (close to the pigment epithelium, behind retina) where visual pigments are stored, and they’re made in the inner segment. Also have synaptic terminals >visual pigments have opsin which determines wavelengths of light they absorb and chromophore that captures the photons >4 kinds of visual pigments. Rhodopsin is one kind, only found in RODS. Concentrated in the membranous disks in the outer segment. Cones have one of the other three pigments (long medium and short wavelengths) -recently found another photoreceptor that have melanopsin that are sensitive to light level, helps brains circadian clock. -when photons get to outer segment of rod and is absorbed by molecule of rhodopsin, it transfers its energy to the chromophore art of the visual pigment molecule. This is called photoactivation. >channels in cell membrane that normally allow ions to flow INTO rod outer segment, close. >this closing makes inside of cell more negative. Hyperpolarization. An inc. in membrane potential. When this happens, calcium channels close at the synaptic terminal, reducing concentration of free Ca in cells, which reduces concentration of glutamate molecules. This change signals bipolar cell that the rod has captured photon. Cones act in a similar way. >amount of glutamate in the photoreceptor-bipolar cell synapse is inversely proportional to # of photons being absorbed by photoreceptor. Photoreceptors pass info to bipolar cells via graded potentials -human
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