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Faculty of Medicine
Dr Zaïd Mansour
“The Eye”
The Eye
Light
Light is the electromagnetic radiation that is visible to our eyes
The electromagnetic spectrum detectable by our visual system: visible light
(wavelengths of 400-700 nm)
Isaac Newton: the mix of wavelengths in this range emitted by the sun appears to
humans as white, whereas light of a single wavelength appears as one of the colors of
the rainbow.
Optics:
The study of light rays and their interactions
Gross Anatomy of the Eye
The Eye in cross-section
Ophthalmoscopic Appearance of the Eye
Refraction and Accommodation
The cornea is the site of most of the refractive power of the eyes
Lens: Accommodation which is greater refraction for near objects and
provided by the lens
refractive power: reciprocal of the
focal distance
RF of the cornea = 42 diopters
Accommodation
Strabismus:
Paralytic (incomitant):
acquired defect of the movement of an eye, diplopia is maximal in the direction of
action of the weak muscle.
Non-paralytic (concomitant):
Imbalance in the EOM leading to misalignment or lack of coordination between the
two eyes.
Esotropia: the directions of gaze of the two eyes cross, and the person is said to
be cross-eyed.
Exotropia: the directions of gaze diverge, and the person is said to be wall-eyed.
The deviating eye usually has a defective vision; this is called amblyopia ex anopsia
Exotropia
Myopia: nearsightedness, shortsightedness
Hyperopia: farsightedness, longsightedness or hypermetropia
Hyperopia (farsightedness)
Vision Correction
Cataract:
is a clouding that develops in the crystalline lens of the eye or in its envelope,
varying in degree from slight to complete opacity and obstructing the passage of light
Glaucoma
is an eye disorder in which the optic nerve suffers damage, permanently
impacting vision in the affected eye(s) and progressing to complete
blindness if untreated. It is associated with increased pressure of the fluid
in the eye (aqueous humour).
The Pupillary Light Reflex:
-Direct
-Indirect (consensual)
Visual Acuity:
-The ability of the eye to distinguish two nearby points
-Snellen eye chart
Visual Field:
Microscopic Anatomy of the Retina
Information about light flows from the photoreceptors
to bipolar cells to ganglion cells, which project axons
out of the eye in the optic nerve.
Horizontal cells and amacrine cells modify the responses
of bipolar cells and ganglion cells via lateral connections.
Photoreceptors:
Rods:
Contain more disks, higher photopigment concentration,
they are 1000 times more sensitive to light, they
contain the same photopigment, they contribute to
scotopic conditions
Cones:
Less disks, three types of photopigment, they
contribute to photopic conditions.
The Fovea
Cones are found primarily in the central retina (the fovea).
Rods are absent from the central fovea and are found mainly in the peripheral retina.
In the central retina, relatively few photoreceptors feed information directly to a
ganglion cell; in the peripheral retina, many photoreceptors provide input.
This arrangement makes the peripheral retina better at detecting dim light but the
central retina better for high-resolution vision.
Phototransduction
Conversion of light energy into a membrane potential
Rods to Cones: 20 to 1
Phototransduction in Rods
In complete darkness, the membrane potential of the rod is – 30 mV.
This depolarization is caused by the steady influx of sodium through special channels in
the outer segment membrane.
This movement of positive charge is called the dark current.
Sodium channels are stimulated to open by cGMP
cGMP is continually produced in the photoreceptor by the enzyme guanylyl cyclase,
keeping the sodium channels open.
Light reduces cGMP, causing the sodium channels to close, and the membrane potential
becomes more negative, thus photoreceptors hyperpolarize in response to light.
The hyperpolarization of photoreceptors in response to light
The activation of Rhodopsin by light
Rhodopsin
The activation of Rhodopsin by light
(a) In the dark, cGMP gates a sodium channel, causing an inward Na + current and
depolarization of the cell.
(b) Light causes a change in the conformation of retinal so that it activates the opsin
(this process is called bleaching as the photopigment changes colour from purple to
yellow), the bleaching of rhodopsin stimulates G-protein called transducin which in
turn activates the effector enzyme phosphodiesterase (PDE), which breaks down
the cGMP. The reduction in cGMP causes the sodium channels to close and the
membrane to hyperpolarize.
Photoreceptor cell
Amacrine C.
Horizontal C.
Bipolar cell
Ganglion cell
Transmission: photoreceptor cells, horizontal cells & Amacrine cells
to Bipolar cells by direct electric current flow
Bipolar Cells:
Two types of bipolar cells provide opposing excitatory and inhibitory signals in the
visual pathway:
(1) The depolarizing bipolar cell
(2) (2) the hyperpolarizing bipolar cell.
That is, some bipolar cells depolarize when the rods and cones are excited, and
others hyperpolarize.
There are two possible explanations for this difference.
1) The first is that the two bipolar cells are of entirely different types—one
responding by depolarizing in response to the glutamate neurotransmitter released
by the rods and cones, and the other responding by hyperpolarizing.
2) The other possibility is that one of the bipolar cells receives direct excitation
from the rods and cones, whereas the other receives its signal indirectly through
a horizontal cell or an amacrine cell
Ganglion Cells and Optic Nerve Fibres
- Axons of ganglion cells form the optic nerve.
- Each retina contains about 100 million rods and 3 million cones; yet the number
of ganglion cells is only about 1.6 million.
- Thus, an average of 60 rods and 2 cones converge on each ganglion cell and
the optic nerve fibre.
There are three distinct types of ganglion cells, designated W, X, and Y cells.
The types of Galnglion Cells:
W cells:
- 40% of all ganglion cells
- slow velocity of 8 m/sec
- they receive most of their excitation from rods,
- especially sensitive for detecting directional movement in the field of vision, and
they are probably important for much of our crude rod vision under dark conditions.
X cells:
55% of the total, the most numerous of the ganglion cells
Velocity of transmission at 14 m/sec.
The fine detail of visual image, Colour vision
Y cells:
The least numerous, 5% of all ganglion cells
Velocity of transmission: 50 m/sec
These ganglion cells presumably apprise the central nervous system almost
instantaneously when a new visual event occurs anywhere in the visual field, but without
specifying the details.
Phototransduction in Cones
In bright sunlight, cGMP levels in rods fall to the point where the response to
light becomes saturated; additional light causes no more hyperpolarization.
Thus, vision during the day depends entirely on the cones.
The process of phototransduction in cones is virtually the same as in rods.
3 types of cones:
"blue" cones that are maximally
activated by light with a wavelength of
about 430 nm,
"green" cones that are maximally activated
by light with a wavelength of about 530 nm,
and "red" cones that are maximally activated
by light with a wavelength of about 560 nm
Colour Detection
The Young-Helmholtz trichromacy theory:
- According to the theory, the brain assigns colors based on a comparison of the
readout of the three cone types.
- When all types of cones are equally active, as in broad-spectrum light, we perceive
“white”
- The color that we perceive is largely determined by the relative contributions of
blue, green, and red cones to the retinal signal.
- It is difficult to detect colors at night because only the rods, with a single type of
photopigment, are activated under dim lighting conditions.
- The peak sensitivity of the rods is to a wavelength of about 500 nm, perceived as
blue-green (under photopic conditions).
automobile dashboard indicator lights
Colour Blindness
-Red, Green, Blue
-Three colour system: Trichromats
-Two-colour system: Dichromats
-One-colour system: Monochromats
-Red & Green: X – chromosome
-Blue: chromosome 7
-About 2% of men actually lack either the red or the green pigment (Dichromats)
-The person with loss of Red Cones: Protanope
-The person with loss of Green Cones: Deuteranope
Ishihara colour test
The central visual system
The Central Visual System
The Retinofugal projection, from the optic nerve:
(1) to the suprachiasmatic nucleus of the hypothalamus, presumably to
control circadian rhythms that synchronize various physiologic changes
of the body with night and day
(2) into the pretectal nuclei in the midbrain, to elicit reflex movements of
the eyes to focus on objects of importance and to activate the
pupillary light reflex
(3) into the superior colliculus, to control rapid directional movements of
the two eyes
(4) into the ventral lateral geniculate nucleus of the thalamus
The Retinofugal projection :
The neural pathway that leaves
the eye in the optic nerve
Suprachiasmatic Nucleus of the Hypothalamus
Optic Nerve to Pretectal nucleus of the midbrain
Visual Pathway to the Superior Colliculus
Lateral Geniculate Nucleus (LGN)
LGN:
- 6 layers
- Layers (I, II): Magnocellular layers, they
receive input from the large type Y retinal
ganglion cells and provide a rapidly
conducting pathway to the visual cortex
(transmit black & white information)
- Layers (III, VI): Parvocellular layers,
they receive their input almost entirely from
the type X retinal ganglion cells that
transmit colour and convey accurate pointto-point spatial information at a moderate
velocity of conduction
The Visual (Striate) Cortex
-Located on the medial aspect of the occipital lobes
-Primary visual cortex (V1), area 17, calcarine fissure
-Secondary visual cortex (V2), visual association areas, area 18, lie anterior,
superior, inferior, and lateral the primary visual area
Layers of the Visual Cortex
Colour Vision
slow
Eye Movements and Their Control
Fixation Movements of the Eyes (Horizontal Gaze)
Fixation movements are controlled by two neuronal mechanisms:
1) the voluntary fixation mechanism:
It allows a person to move the eyes voluntarily to find the object on which he or she
wants to fix the vision.
Prefrontal Cortex
2) involuntary fixation mechanism:
It holds the eyes firmly on the object once it has been found.
Superior Colliculus
L
R
III
III
Frontal Cortex
The eyes look toward a hemispheric lesion
and away from a brainstem lesion ??
MLF
VI
PPRF
VI
PPRF
The Occipito-temporal Network
Visual object agnosia: the inability to recognize familiar object.
Prosopagnosia: the ability to recognize faces