April 21, 2019 Sunday
Bedtime Story
Photoreceptors and Retinal Pigment Epithelium
Never forget that our eyes are merely an
extension of the brain in the central nervous system and contains nerve tissues
in its innermost layer which is called the retina.
Analogously our vision or rather the
simulative perception of the outer world occurs not in the eye but rather in
the visual cortex in collaboration with its connections to a wide array of
cortical and subcortical brain areas.
Most of the computations for creating the
simulation that we call visual perception occurs in the cortex located in the
occipital lobe though some argue, and very rightly so, that some computation might
be taking place in the lower visual pathway.
For instance now there is enough evidence
to explain spatial encoding occurring in the retina itself.
Let us see briefly how mathematical
computation can be carried out by biological cells without actually wanting to
do so.
In the retina the chief cells that perceive
the light and convert photons to electrical impulses are the photoreceptors
that are embedded in the retinal pigment epithelium and of all the other
retinal cells are the last ones to receive and “perceive” the photons.
It means that the retinal layer is build
such that the light has to pass through all the other cells of the retina
before it finally reaches the photoreceptors.
The photoreceptors need vast amount of
energy for carrying out their operation – namely converting the photons into
electrical impulses - that is known as visual phototransduction the prime
source of which are these supporting and nourishing retinal pigment epithelial
cells.
Just like every Euler needs his Yekaterina
and every Gauss needs his Ferdinand similar every photoreceptor needs it
retinal pigment epithelial cells to supply its much needed nutrition in the
form of oxygen, glucose, fatty acids and retinaldehyde through the vessels in
the choroid that lie on the other side of the retinal pigment epithelial cells.
It is strange that even though retina has
its own personal blood vessels almost 75% of its nutrition is taken care by the
blood vessels of the choroid via the medium of retinal pigment epithelial cells.
These photoreceptors are highly metabolic
active cells with information capacity of 500,000 bits per second without color
and about 600,000 bits per second when color is being processed (estimated by
Janglin Chen, Wayne Cranton and Mark Fihn).
When a photoreceptor cell undergoes
phototransduction it relays the signal to the bipolar cells in a proportional
manner.
The bipolar cells in turn relay the electrical
impulse to the ganglion cells whose long axons all bunch together and leave the
eye in the form of optic nerve and later as it enters the brain form the optic
chiasm and optic tract.
There is a gross mismatch between the
numbers of photoreceptors and the number of retinal ganglion cells.
In human retina there are about 0.7 to 1.5
million retinal ganglion cells but 99.6 million photoreceptor cells thus
accounting for roughly about 100 photoreceptors for each retinal ganglion cell.
Stay tuned to the voice of an
average story storytelling chimpanzee or login at http://panarrans.blogspot.com
Good night Mon Ami and my fellow cousin ape.
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Another great educator and a teacher that I am aware of is
Professor Subhashish Chattopadhyay in Bangalore, India.
While I narrate stories, Professor Subhashish an electronic
engineer and a former professor at BARC, does and teaches real mathematics and
physics.
He started the participation of Indian students at the
International Physics Olympiad.
Do visit him here:
All his books can be downloaded for free through this link:
For edutainment and English education of your children, I
recommend this large collection of Halloween Songs for Kids:
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