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The Eye - Coggle Diagram
The Eye
Structure
complex sense
organ
small section of eye contains
photoreceptors sensitive to light
rest of eye = structures to ensure
receptors in retina receive:
focused light rays
at correct intensity
to form image
definitions
QUIZLET
Function
Obtaining A
Focused Image
light rays must be
focused on retina
method
2.light rays pass through
LENS
further refraction
degree of refraction = adjust THICKNESS of lens
ACCOMODATION
(reflex action)
adjustment of lens thickness
ensure light rays focus on retina
irrespective of angle reach eye
light rays pass through
CORNEA
most refraction automatically
occurs here
3.ciliary body &
suspensory ligaments
CB
contains ring of ciliary muscle
run around inside of eye, surrounding lens
SL
attach lens to CB
tough + flexible - do
NOT stretch
focusing on
DISTANT
object
2.CB
relaxes
springs out
form LARGE diameter
3.tension in wall of eye
transferred through SL
to lens
∴ SL pulled taut
4.lens pulled into
THINNER
shape
less refractive power
bends light rays less
1.light rays arrive
PARALLEL
from distant object
focusing on
NEAR
objects
1.light rays
DIVERGE
from near objects
2.CB
contracts
forms tighter circle
w/ SMALL diameter
3.SL not pulled taut
so slacken
4.less pressure on lens
∴ lens springs out
into
THICKER
shape
more refractive power
bends rays more
Controlling The Amount Of Light
That Enters The Eye
important = correct INTENSITY of
light enters eye + reaches retina
too much or too little =
prevent image being formed
damage sensitive light receptor cells in retina
LOW LIGHT
INTENSITIES
radial contract
circular relax
PUPIL DIAMETER
LARGER
large pupil -
allows
as much light as possible
to enter eye
ensure sufficient light
to
stimulate photoreceptors
in retina
HIGH LIGHT
INTENSITIES
radial relax
circular contract
PUPIL DIAMETER
SMALLER
small pupil -
restrict amount of light
entering
prevents damage
of photoreceptors in retina
size of pupil = direct consequence
of size of iris
muscles of iris contract/relax
change size of pupil
types of muscles
in iris
RADIAL MUSCLES
spokes of bicycle wheel
move out from edge of pupil through iris
CIRCULAR MUSCLES
form rings within iris
around pupil
The Retina
contains millions of
light sensitive cells
neurones
which the synapse with
rods + cones
act as TRANSDUCERS
convert light stimulus
into nerve impulse
in their associated neurones
Rod Cells
RHODOPSIN
light sensitive
pigment
packed into array of
membranes
outer part of cell
formed from
opsin - protein
retinal - light absorbing compound
(from vitamin A)
responds to light
1.stimulated by light
breaks down
into
retinal
opsin
2.breakdown changes membrane potential
creates
GENERATOR POTENTIAL
D: degree of depolarisation that a stimulated receptor can produce
3.if
threshold potential
reached
generator potential cause adjacent linking neurone =
DEOPLARISED
now
conduct AP
4.Rh then
re-synthesised
from retinal+opsin
ADAPTIONS
for vision in LOW
LIGHT INTENSITIES
Rods have
HIGH SENSITIVITY
Rh breakdown readily in LOW LIGHT INTENSITIES
requires only SMALL amount of light energy
leads to "
phenomenon of dark adaption"
in bright light - ALL Rh bleached
∴ takes time be re-synthesised
when move
well lit to dark room
vision INITIALLY POOR
gradually improves
why?
initially = no functional rods (no Rh)
over time = Rh re-synthesised
so
sensitivity INCREASES
D: dark adaption in rods involves re-synthesis of Rh been broken down during exposure to light
provide MONOCHROMATIC vision
NOT sensitive to colour
different shades of grey from black to white
INCREASED VISUAL
SENSITIVITY
retinal convergence
no. rods = common bipolar neurone
allows for summation
of light stimuli
generator potential from individual rods
combine together (summation)
to reach threshold potential
produce AP
also
ability of Rh
breakdown more easily than IO
DECREASED
VISUAL ACUITY
lack of high
resolution
all of R together in each convergence unit only provide as much detail as 1 C
Cone Cells
IODOPSIN
situated in membrane
of outer segment
less readily broken
down
than Rh
don't work in dark
only produces generator potential
in bright light
of high enough intensity
provide COLOUR vision
IO exists
3 different forms
each
sensitive to different wavelengths of light
absorption peaks of
3 types of cone cells
correspond to BLUE, GREEN + RED
trichromatic theory of colour vision
pure blue light only breakdown
BLUE iodopsin
(not all light pure b/g/r)
∴ degree of stimulation of each type of cone cell
determines colour vision
ADAPTIONS
discs of membrane
large SA
allow max light absorption
mitochondria
ATP
iodopsin/NT synthesis
vesicles
contain NT
allow for transmission
branches in cell body
enable connection
w bipolar neurone
allows vision in high light intensities
exists in 3 diff forms = sensitive to diff wavelengths of light
colour blind
=
red or green cones not function
unable distinguish red/green colours
INCREASED VISUAL
ACUITY
can provide highly
precise colour vision of
high resolution
each C individually synapses
with own bipolar neurone
provide OWN DISCRETE IMAGE
stimulation of each C separately
brain distinguish 2 points as separate if close together
Arrangement of Rods +
Cones in Retina
layers
choroid
R+C layer
bipolar neurones
ganglion cells (sensory cells)
optic nerve
axons of ganglion cells
group together
makes optic nerve
carry nerve impulses retina to brain
Cone Cells
monosynaptic with
single bipolar neurone
individually synapse with
1 bipolar neurone
INCREASED VISIAL ACUITY
Rod Cells
retinal convergence
several rods synapse with
singular bipolar neurone
AND
more than 1 rod bipolar neurone
synapses with single ganglion cell
INCREASED VISUAL SENSITIVITY
(ability to operate in low light intensities)
DECREASED VISUAL ACUITY
Arrangement of Photosensitive
Cells + Neurones in Retina
neurones
on
INSIDE
of retina
photosensitive cells
(located behind neurones) closer to
OUTSIDE
edge of eye
means cells NOT at front of
retina where light enters
light rays have to pass
through layers of neurones
BEFORE reach light sensitive cells
inverted arrangement = LESS EFFICIENT
detection of light = compromised
why?
due to evolutionary
development
Distribution of Cones +
Rods Across Retina
CONES
LOW density
across retina
SHARP PEAK @ centre - Fovea
gives clearest daylight colour vision
RODS
HIGH density
across retina
SHARP DECLINE
@ centre - Fovea
most @ edge of retina
∴ used when viewing objects in periphery of vision
why we cannot distinguish colour
Blind Spot
=
no C/R
not sensitive to light
part of retina where sensory neurones unite to form optic nerve leave eye
Left Eye
=
more R+C RIGHT SIDE of eye
(closer to centre of head)
facilitates peripheral vision on left side of head
