Visual Systems
part 1
may be affected by demyelinating conditions, space occupying
lesions like tumours and vascular disorders
Optic nerve
not a peripheral nerve
it behaves like a central tract
it is covered by myelin made by oligodendrocytes
affected by multiple sclerosis
it does not regenerate upon injury, unlike peripheral
nerves
it is an outpouching from the diencephalon
peripheral nerves are covered by collaginous covering
called epineurium. optic nerve doesnt have epineurium but
has dura, arachnoid and pia mater
visual system is mainly responsible for taking information
from the retina
the retina has the receptors rods and cones
(photoreceptors) and these convert the light energy into
electrochemical changes which eventually generate action
potentials in the optic nerve and visual pathway
optic nerve is called special somatic afferent
How does the retina develop?
developed from neuroectoderm
Visual Systems 1
, from the diencephalon
it comes out as an optic
vesicle then the lens
invaginates and it becomes
double layered and forms
the optic cup
the outer part of the cup
becomes the choroid and
the pigment epithelium
optic vesicle coming off from
inner part makes the diencephalon
retina (with the rods and lens (in blue) goes in to
invaginate and form the optic cup
cones)
Structure of the Retina
first important feature is the optic disc (this is not
centrally placed, it is slightly on the nasal side) -
fundoscopy
it is about 3.5 mm from the center of the fundus medially
a healthy optic disc must have clear duct margins and you
should be able to see normal looking retinal vessels
in some diseases, cuping is altered, margins become
blurred and the retinal vessels look abnormal
the optic disc is exactly the point that is acting as the
head of the optic nerve. this means that all the neuronal
fibers are convering towards the optic disc
the optic disc does not have any rods and cone. this means
that it does not have any light receptors
it is also called the physiological blind spot
slightly lateral to the optic disc is a yellowish area.
this is called the macula lutea
the macula lutea is very rich in cones and it has a low
concentration of rods
Visual Systems 2
, the very center of the macula lutea is called the fovea
centralis
the fovea centralis has the maximum concentration of cones
the macula lutea and the fovea is the point of maximum
visual acuity
Photoreceptors
two types of photoreceptors:
rods
cones
Visual Systems 3
, rods are very specialized and are sensitive to electromagnetic
waves (light)
the pigment in the rod is called rhodopsin
this pigment is very sensitive to dim light (scotopic
vision)
rods are present more in the periphery of the retina
as you move centrally the concentration of rods become
lesser and lesser until you reach the fovea where there are
NO rods
rods are for night vision
rods cannot appreciate colour and shapes clearly (eg. at
nights, cones are not working, only the rods are working)
the pigment present in the cones is iodopsin.
sensitive to bright light (photopic vision)
cones are for day vision
this is why when there is bright light the pupils constrict
so the light can be focused on the central part of the
retina where the cones are
👁 rods are more abundant than cones
there is no major artery going to the fovea centralis. it gets
its supplements via simple diffusion because it is a very thin
area. the choroidal area provides capillaries and from these
capillaries, simple diffusion occurs
this area is avascular
layers of the retina
Visual Systems 4
part 1
may be affected by demyelinating conditions, space occupying
lesions like tumours and vascular disorders
Optic nerve
not a peripheral nerve
it behaves like a central tract
it is covered by myelin made by oligodendrocytes
affected by multiple sclerosis
it does not regenerate upon injury, unlike peripheral
nerves
it is an outpouching from the diencephalon
peripheral nerves are covered by collaginous covering
called epineurium. optic nerve doesnt have epineurium but
has dura, arachnoid and pia mater
visual system is mainly responsible for taking information
from the retina
the retina has the receptors rods and cones
(photoreceptors) and these convert the light energy into
electrochemical changes which eventually generate action
potentials in the optic nerve and visual pathway
optic nerve is called special somatic afferent
How does the retina develop?
developed from neuroectoderm
Visual Systems 1
, from the diencephalon
it comes out as an optic
vesicle then the lens
invaginates and it becomes
double layered and forms
the optic cup
the outer part of the cup
becomes the choroid and
the pigment epithelium
optic vesicle coming off from
inner part makes the diencephalon
retina (with the rods and lens (in blue) goes in to
invaginate and form the optic cup
cones)
Structure of the Retina
first important feature is the optic disc (this is not
centrally placed, it is slightly on the nasal side) -
fundoscopy
it is about 3.5 mm from the center of the fundus medially
a healthy optic disc must have clear duct margins and you
should be able to see normal looking retinal vessels
in some diseases, cuping is altered, margins become
blurred and the retinal vessels look abnormal
the optic disc is exactly the point that is acting as the
head of the optic nerve. this means that all the neuronal
fibers are convering towards the optic disc
the optic disc does not have any rods and cone. this means
that it does not have any light receptors
it is also called the physiological blind spot
slightly lateral to the optic disc is a yellowish area.
this is called the macula lutea
the macula lutea is very rich in cones and it has a low
concentration of rods
Visual Systems 2
, the very center of the macula lutea is called the fovea
centralis
the fovea centralis has the maximum concentration of cones
the macula lutea and the fovea is the point of maximum
visual acuity
Photoreceptors
two types of photoreceptors:
rods
cones
Visual Systems 3
, rods are very specialized and are sensitive to electromagnetic
waves (light)
the pigment in the rod is called rhodopsin
this pigment is very sensitive to dim light (scotopic
vision)
rods are present more in the periphery of the retina
as you move centrally the concentration of rods become
lesser and lesser until you reach the fovea where there are
NO rods
rods are for night vision
rods cannot appreciate colour and shapes clearly (eg. at
nights, cones are not working, only the rods are working)
the pigment present in the cones is iodopsin.
sensitive to bright light (photopic vision)
cones are for day vision
this is why when there is bright light the pupils constrict
so the light can be focused on the central part of the
retina where the cones are
👁 rods are more abundant than cones
there is no major artery going to the fovea centralis. it gets
its supplements via simple diffusion because it is a very thin
area. the choroidal area provides capillaries and from these
capillaries, simple diffusion occurs
this area is avascular
layers of the retina
Visual Systems 4
