NSCI 410 Lecture Notes - Optic Disc, Christoph Scheiner, Radiant Energy

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Light
light consists of radiant energy similar to radio waves – oscillates as it is
transmitted from source
wavelength – distance between adjacent waves of radiant energy; in vision,
most clearly associated with perceptual dimension of hue
wavelegnth for visible light ranges from 380-760 nanometres (380 – violet, 760
– red)
entire range of wavelength is called electromagnetic spectrum and part our
eyes can detect is visible spectrum
The Eye and Its Function
cornea – transparent tissue covering front of eye; admits light
sclera – tough white membrane that is outer layer of eye
iris – pigmented muscle of eye that controls size of pupil – contains two bands
of muscle
brain controls these muscles and regulates size of pupil
aqueous humour - space behin cornea is filled with (watery fluid)
constantly produced by tissue behin cornea that filters fluid from blood
nourishes cornea and other portions of eye in place of blood vessels – must
circulate
if produced too quickly or if passage that returns it to blood becomes
blocked, pressure within eye can increase and cause damage to vision
(glaucoma)
no blood vessels – transparency of cornea
lens – transparent organ situated behin iris of eye; helps focus an image on
retina (inner surface of back of eye)
image is upside down and reversed from left to right
brain compensates for this alteration and interprets information
appropriately
contains no blood vessels – functionally dead tissue
shape is flexible – special set of muscles can alter shape so eye can obtain
images of nearby or distant objects
accommodation- change in shape of lens to adjust for distance
length of eye normally matches bending of light rays produced by cornea and
lens so that image of visual scene is sharply focused on retina
for some people length of eye doesn not match bedning of light rays so
iamge is out of focus – need extra lens in front of eye
near sighted - eyes too long (front to back) so need concave lens to
correct focus
far sighted – eyes too short so need convex lens
as people age, lens becomes less flexible and becomes difficult to focus on
objects close to them – need convex lens
retina – tissue at back of inside surface of eye that contains photoreceptors
and associated neurons (performs sensory functions of eye)
embedded in retina are over 130 million photoreceptors
photoreceptors – receptive cell for vision in retina (rod or cone);
specialized neurons transduce light into neural activity
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information from photoreceptors is transmitted to neurons that send
axons toward one point at back of eye (optic disc)
optic disc – circular structure located at exit point from retina of axons
of ganglion cells that form optic nerve; all axons leave eye at this point
and join optic nerve which travels to brain
no photoreceptors in form of optic disc so that portion of retina is blind
retina has three layers – light passes successively through ganglion cells
(front), bipolar cells (middle) and photoreceptor layer (back)
cells located above photoreceptors are transparent
Johannes Kepler – astronomer suggested that retina, not lens, contained
receptive tissue of eye
Christopher Scheiner – 1625 proved that lens is only focusing device
photoreceptors respond to light and pass info to bipolar cells by means of
transmitter substance
bipolar cells – neuron in retina that receives information form
photoreceptors and passes it on to ganglion cells, from which axons
proceed through optic nerves to brain
bipolar cells transmit information to ganglion
ganglion cells – neuron in retina that receives informaiton from
photoreceptors by means of bipolar cells and form which axons proceed to
optiv nerves to brain
photoreceptor – responds to light that reaches its immediate vicinity
ganglion cells – responds to information from many different photoreceptors
retina also contains neurons that interconnect both adjacent photoreceptors
and adjacent ganglion cells – indicates that some kinds of information
processing are performed in retina
retina – contains 2 types of photoreceptors: 125 million rods and 6 millions
cones
rods – sensitive to light but cannot detect changes in hue; function in dim light
cones – responsible for acute daytime vision and for colour perception;
function when light is bright enough to see things clearly
fovea – small pit near centre of retina containing densley packed cones;
responsible for most acut and detailed vision; 1 mm in diameter
cones connected to one ganglion each
when we look at point in visual field, we move our eyes so that image of
that point falls directly on cone-packed fovea
rods – increase as move further from fovea; connected to many ganglion (up to
100 each)
ganglion receiving info from many rods is sensitive to very low levels of light
rods are responsible for sensitivity to dim light but visual information lacks
sharpness
Transduciton of Light by Photoreceptors
molecule responsible for transduction is derived from vitamin A, in absence of
light it is attached to protein and forms photopigment
photopigment – complex molecule found in photoreceptors; when struck
by light, splits apart and stimulates membrane of photoreceptor in which it
resides
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