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The Mammalian Eye

The human eye is a sense organ in which
are located the photoreceptors that function
as transducers in converting the external
energy of light stimuli into the code of the
nervous impulse

The photoreceptors are the rods and cones
of the retina

The eye is also structurally…

Page 2

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Controlling the amount of light entering the eye
Bright Light Dim Light




The muscles of the iris are responsible for controlling the amount of light
entering the eye by adjusting the size of the pupil accordingly




Controlling the amount of light entering the eye
The actions of the iris circular…

Page 3

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Controlling the amount of light entering the eye
Bright light Dim light




At high light intensities, a large At low light intensities, fewer
number of retinal photoreceptors retinal photoreceptors are
are stimulated; a high frequency of stimulated; a decreased frequency
impulses pass to the brain resulting of impulses pass to…

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Focusing
Light rays from individual point
sources on an object are refracted
and focused onto the retina



When light rays from all points on an object
within the field of view are focused onto the
retina, an inverted image is formed; when
the brain interprets the image we see the…

Page 5

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Focusing on a Distant Object (Accommodation)
Ciliary muscle
relaxed


Suspensory ligaments
pulled taut

Parallel rays of light
from a distant object

Lens becomes
less convex

Light rays focused
onto the retina




Focusing on a Near Object (Accommodation)
Ciliary muscle
contracted


Suspensory ligaments
relaxed

Diverging rays
of light from
a near…

Page 6

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The Human
Retina Sclera
Pigmented choroid
epithelium
Rod cell; Cone cell;
contains the pigment contains the pigment
rhodopsin that is iodopsin that is
only sensitive to sensitive only to high
to low light intensities light intensities and
and is unable to allows for the
distinguish colours discrimination of
colour
Bipolar…

Page 7

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Transduction in When light is absorbed by rhodopsin, cis-retinal is
Rod Cells converted to its isomer trans-retinal and the molecule
becomes unstable and breaks down

Rhodopsin scotopsin + trans-retinal
This reaction results in the creation of a
Low intensity generator potential in the rod cell and a
light REDUCTION of…

Page 8

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Visual Acuity and Sensitivity
Rods display high visual sensitivity
but low visual acuity
High visual sensitivity describes
the ability of the rods to
function at low light intensities
Visual acuity is about the
sharpness of the image produced;
rods display poor visual acuity as a
result of the anatomical
arrangement…

Page 9

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Retinal Convergence, Summation and Amplification

RODS CONES




Cones do NOT show convergence and they display a 1:1 ratio with
ganglion cells; when cones are stimulated, the brain is able to resolve the
impulses generated when light rays from two close objects
strike the retina




Retinal Convergence, Summation and Amplification

RODS…

Page 10

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Density of Rods and Cones in the Retina
Cones are concentrated at the fovea centralis (yellow spot) of the retina
Rods are absent from the fovea region but, a few degrees away, their
density rises to a high value and spreads over a large area of the retina




Light striking…

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