April 22, 2019 Monday
Bedtime Story
Photoreceptors, Bipolar and Retinal Ganglion Cells
But it is not that each retinal ganglion
cell will get input from 100 photoreceptors as this ratio varies from place to
place in the retina.
For instance in the fovea that is visually
most significant and functional part of the retina a single retinal ganglion
cell may get input from as low as 5 photoreceptors whereas in the extreme periphery
near a sight known as the ora serrata which is barely used by us on most
occasions a single retinal ganglion cell gets input of signals from thousands
of photoreceptors.
The bipolar cells when stimulated by the
photoreceptors can either be stimulated or inhibited depending upon the type of
receptor that is acted upon by the glutamate neurotransmitter that is released
from the photoreceptors.
This minor complexity and variability in
the response of bipolar cells to the same neurotransmitter glutamate builds in
a computation mechanism for decoding color, contrast, edges and direction.
This complexity is further enhanced by
horizontal cells and amacrine cells or the interneurons that form complex
synapses between bipolar cells and retinal ganglion cells and between
photoreceptors and bipolar cells and also between the photoreceptors
themselves.
One can consider these different cells and
their synapses equivalent to logic gates that form the foundational basis of
working of computational machines.
Computer science provides us with a far
more superior metaphorical language – perhaps the best and the most apt till
date - to under the computational functions of the brain since in the end the
brains along with neurons has to a
computational device evolved to decode the input streaming into it from the
various senses.
To explain the computational mechanism and
origin of the functionality of the cells of the retina a concept of receptive
field was devised.
The concept of receptive field is not
something abstract but extremely definite and precise that can be objectively
measured through physiological experiments using brains of other species and
microelectrodes.
A receptive field is applicable to a single
sensory neuron that defines it area of interest or activity.
This means that there is a limited and
specific zone around a sensory neuron (which can be a skin surface or a visual
field) and which when acted upon by a stimulus will lead to change in the
firing of that neuron.
In the visual system receptive field of any
sensory neuron is a volume in the visual space.
This volume varies in different parts of
the visual system being the smallest in the fovea which explains the accuracy
of vision of fovea centralis but also compromising its visual field.
The average angle of view of a single cone
cell in fovea is a miniscule 31.46 arc seconds.
When we consider such a single
photoreceptor namely a cone in the fovea right behind the center of a clear
natural crystalline ape lens then its receptive field is extremely tiny but
will stretch long, in fact infinitely long from the center of the lens to
infinity.
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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