Vision

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Vision
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Three main parts Layer of receptors Lens and
cornea system that focuses light on these
receptors Nerves that conduct impulses from the
receptor to the brain
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Detection of light
400 to 750 nm in wavelength The shortest blue The
longest red
photoreceptors - rods and cones Rods dim
light Cones in brighter conditions - distinguish
red, green or blue light
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The principal structures of the eye
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Pupillary light reflex Excess of light Reflex
for miosis receptor - retina afferent fibres
from visual pathway Central part mesencephalon,
area praetectalis Edinger-Westphal nucleus
(parasympathetic) Efferent- n. oculomotorius
through ncl. Ciliare Effector sphincter muscle
of iris
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Low intensity light Reflex for mydriasis
receptor - retina afferent fibres from visual
pathway Central part mesencephalon, area
praetectalis, RF, spinal cord C8 Efferent
sympathetic fibres from ventral horn to
paravertebral cervical gangl., along a. carotis,
a. ophtalmica Effector dilator muscle of iris
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Aqueous humor is produced in the ciliary body and
reabsorbed into the canal of Schlemm
Glaucoma Open-angle Angle-clousure
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Retina Organized in 10 layers, visual receptors
rod and cones 4 tapes of neurons bipolar,
ganglion, horizontal and amacrine cells
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Optic disc place where the optic nerve leaves
the eye. No rods or cones blind spot Macula
lutea at the posterior pole of the eye marks
location of fovea centralis rod-free, cones in
high density, the greatest visual acuity
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Blind spot PC experiment Neurolab
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Neural pathway
1.N rods and cones 2.N bipolar cells 3. N -
ganglion cells 4.N lateral geniculate body
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Visual defects following cutting of pathways in
various places. Dark areas represent blindness
in that visual field
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The image-forming mechanism
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Refractive power of the lens dioptres
The greater the curvature of a lens, the greater
its refractive power. The refractive power of a
lens is measured in dioptres The number of
dioptres being the reciprocal of the principal
focal distance in meters. Lens with a principal
focal distance of 25 cm has a refractive power of
??? Dioptres What is the refractive power of a
human eye?
Lens with a principal focal distance of 25 cm has
a refractive power of 4 Dioptres 1/0.25
100/25 4 What is the refractive power of a
human eye? 1/0.015 1000/15 66.7 dioptres in
rest
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Accommodation is the process by which the
curvature of the lens is increased
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Presbyopia Near point of vision recede throughout
the life from 9 to 80 cm Loss of accomodation dne
to increasing hardness of the lens Correction by
wearing glasses with convex lenses
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Myopia (nearsightedness) the anteroposterior
diameter of the eyeball is too long, changes in
refraction during development Can be corrected by
glasses with biconcave lenses which make parallel
light rays diverge slightly
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Hyperopia (farsightedness) the anteroposterior
diameter of the eyeball is too short, parallel
rays of light are brouht to a focus behind the
retina The defect can be corrected by using
glasses with convex lenses, which aid the
refractive power of the eye.
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The photoreceptor mechanism
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Sequence of events involved in phototransduction R
hodopsin the photosensitive pigment Opsin
(protein) and retinen (11-cis-retinen) All trans
retinen Activation of transducin (G
protein) Activation of phosphodiesterase
PDE Decreased intacellular c GMP Closure of Na
channels Hyperpolarization Decreased release of
synaptic transmitter Production of a signal in
the next neural unit in retina
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Young-Helmholtz theory of color vision Three
types of photopigment
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Optical illusions PC program