The mammalian eye powerpoint

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  • Created on: 30-03-13 21:28
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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 and functionally
adapted for controlling the amount of light
entering the eye and for focusing the incoming
light rays onto the photoreceptors in the retina

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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 and radial muscles modify the aperture of
the pupil and hence regulate the amount of light entering the eye
Circular muscles Radial muscles
contract during contract during
pupil constriction pupil dilation and
and relax during…read more

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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 the brain
in the transmission of a high resulting in the transmission of a
frequency of impulses along high frequency of impulses along
parasympathetic motor neurons to sympathetic motor neurons to the
the muscles of…read more

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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
object the correct way up
Focusing and the Lens
Ciliary Body;
circular ciliary
muscle fibres
Cornea form a ring around
the lens
The ciliary
are responsible
for changing
Iris Lens the shape Lens
of…read more

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Focusing on a Distant Object (Accommodation)
Ciliary muscle
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
Suspensory ligaments
Diverging rays
of light from
a near object
Lens becomes
more convex
Light rays focused
onto the retina
Rods and cones
Photoreceptors transduce the energy
Outer segment has a
of light into the code
thin, rod-shaped of the nerve impulse
appearance Outer…read more

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The Human
Retina Sclera
Pigmented choroid
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
Bipolar neuron;
transmits impulses from
the photoreceptor cells
to ganglion cells
Ganglion cell
neurons of
Direction of optic nerve
Incident Light
Transduction in Rod cells are extremely sensitive to light and thus
Rod Cells respond to…read more

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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 neurotransmitter release
from the synaptic base
The generator potential created in the
rods is the result of closure of some of the
sodium ion channels and an increase in
negativity within the rod cells
If…read more

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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 of the rods and
bipolar neurons within the retina
In contrast to the rods, cones provide us with good
visual acuity (sharp images) but poor sensitivity (only
sensitive to high levels of illumination)
N.B.…read more

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Retinal Convergence, Summation and Amplification
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
Retinal convergence, coupled with the sensitivity of the pigment
rhodopsin, explains the high light sensitivity displayed by rods
Retinal Convergence, Summation and Amplification
At low light intensities, the graded generator potentials created…read more

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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 the fovea produces the sharpest colour vision
Retina of the Left Eye as viewed through an Ophthalmoscope
When viewed directly through the front of the eye, the fovea, optic disc
(blind spot) and retinal blood…read more


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