nervous system

Last time we talked about the layers of the retina, it has about 10 layers
including the pigmented layer, photoreceptor layer (where rods and cones
reside), bipolar cell layer and the ganglion cell layer.
The outermost layer of the retina is the pigmented layer.
Receptors of vision are located at the very back of the retina, unlike other
receptors which reside on the surface.
When light hits the retina, it hits and passes through all the layers of retina
until it reaches the last one (the pigmented layer). The pigmented layer
contains melanin so that when light reaches it, extra light gets absorbed.

weak vision compared to normal people. They must get the books closer to
their eyes to control the amount of light in order for them to read so
when light reaches this layer (pigmented layer) it will be reflected instead
of absorbed leading to hazy vision.
Photoreceptor layer this is where the receptors (rods and cones) reside.
The plasma membrane of the photoreceptor cells, which are rods and
cones, is divided into two compartments: the outer segment and the inner
segment. The outer segment contains photopigments. Transduction of light
-

energy into receptor potential occurs here. The inner segment: contains the
2 -
nucleus, Golgi complex and
mitochondria.
When light passes to reach the
receptors it must pass through all
the layers of the retina mentioned
earlier, so refraction of light must
occur during this trip, but that

these layers are located on the
-

sides, so that when light passes it
- -


hits the fovea centralis (macula
=
lutea) where only cones exist and
sharp, coloured vision occurs.
"




The outer segment of rods and cones synapses with a neuron called bipolar
ITTF Tag
neuron, which then synapses with other cells called ganglion cells, then the
I t

axons of the ganglion cells will leave the eyeball by going backwards
f-

to reach the optic nerve. Then nerve impulses propagate along optic nerve
toward the optic disk to exit the eye.
*Axons leave from the optic nerve in the optic disc area.
Optic disc = blind spot no receptors at all

, There areoinhibitory interneurons between the bipolar and ganglion cells to
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cause lateral inhibition. This inhibition of light occurs on the edges, but is
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present in the middle, which serves in the sharpness of vision.
Tip




Rods and Cones:-

-Are named after the shapes of their outer segments

-Rods for dark vision, around 100 million in each
retina. Whereas cones are for day light vision, has three types: red, green and
blue, concerned with colour vision, they are around 3 million.

-To conclude, in each retina we have 100 million rod, 3 million cone, and 1.6
million ganglion cells so every 10-20 million rod will do convergence on one
ganglion cell. The importance of this convergence is having a lot of stimulus
-
from each rod which then summate and make up a strong stimulus. In
contrast, cones have less convergence and this is the reason why rod vision is
-
more sensitive than cone vision
see by rods, while during the day we see by cones.

Transduction mechanism:
-The membrane of rods and cones is rounded forming discs in the outer
segment and a nucleus in the inner segment.
-40% of the membrane is proteins because proteins are the ones that will
decompose and cause the transmission of signals.
- -



=
photopsin or actually photopsins because we have 3 kinds of cones (again:
red, green, and blue) blue cones sensitive to blue light
red cones sensitive to red light.
green cones sensitive to green light.
*Rhodopsins and photopsins consist of a protein called opsin and 11-cis
Vitamin A
-_ (an aldehyde derivativeT of vitamin A). -
me

retinal
is
okay so what exactly is the phototransduction cascade? What are the
important
proteins found in the membrane of rods and cones? And how are signals
for vision. :)
transmitted? To understand this concept clearly lets first take a closer
look at the structure of the rod in the next page:

*Keep in mind that rods and cones take the light energy and convert it into a
neural impulse