The Retina (part 1) – A Retina (parte 1)


about the retina proper it’s a
complicated piece of neural machinery and it is neural machinery this is the
interface between the non neural parts of the eye and the neural parts of the
eye and here is what you would see if you took out a little piece of retina
and looked at it in a microscopic section this is obviously not a
microscopic picture but just a diagram but here are the kinds of features that
you would see in the retina at large which is here and in the diagrammatic
representation of the fovea which is here the fovea is about two millimeters
in extent and what you see here is that the eye is layer and we’ll look at these
layers diagrammatically in a minute and describe them but the layers consist of
a choroid layer in the back of the eye which has the pigment epithelium I’ll
describe the importance of that epithelium in a minute and then it has
the photo receptive layer a layer in which the rods and cones that we’ll
discuss are located and then it has zones that have other kinds of nerve
cells and the output layer of ganglion cells whose axons form the optic nerve
that we looked at a minute ago when seeing the optic nerve head so what’s
interesting here and what’s very important is that as you reach the fovea
again this is just a rough diagram it’s look exactly like this but pretty much the layers the overlying layers of the
photoreceptors which are remember back here in this
are reduced they’re taken away and you might expect that because the overlying
layers of the retina diminish the light that’s getting to the retina and in the
area of greatest sensitivity the fovea you want to take away the components
that are going to scatter the light absorb some of the light and diminish
the sensitivity the accuracy of what you would be able to see where those
elements not there that’s why they are not present over in the foveal region so
let’s look at this in some detail here is again a little piece of the retina
and we’re going to take this piece out diagrammatically and blow it up and look
at the elements that are in the retina and let’s look at that in a little bit
more detail than in this picture so light is coming in here and right away
you see something that is odd and seems an unusual arrangement and that is that
the photoreceptors the cells that actually transduce the light by
capturing photons and turning it into the beginnings of the neural
that end up going to the brain are at the back of the eye not the front and
the light therefore has to go through all of this junk well it’s not junk it’s
obviously critical processing machinery but it has to go through all that stuff
that degrades the light by scattering it while absorbing some of it before the
light reaches the photoreceptors that’s why these elements are taken away in the
fovea as I told you a minute ago but in the vast majority of the retina the
light is passing through these other elements so why is that well the reason
is that the photo pigment layer which is here is an absolutely necessary component that has to be adjacent to the
photoreceptors the reason is that the photoreceptors have a high metabolic
rate and the discs that contain the photo pigment molecules are actually
being turned over at a high rate and the discs are being degraded and that
degraded detritus has to be dealt with and removed so that the rods and there
are two types of photoreceptors that we’ll talk more about in a minute that
needs to be removed so that they can continue to function on an ongoing basis
properly that’s the reason that the photoreceptors are in the back of the
eye and the next question we should ask is
well what are all these other cells doing first of all there is a
straight-through pathway of light that’s mediated by bipolar cells that is
basically the pathway that’s forming the high resolution that one gets from
vision but there are also these other cells that are integrating information
across the retinal surface and they are the amacrine cells and the horizontal
cells that are processing information in the
lateral direction the information that is processed in this way by these
retinal layers and there are five of them in the retina ends up in the
ganglion cell layer and the ganglion cells are the output layers of the
retina that send their axons to the optic nerve and form the optic nerve
head that we talked about a minute ago so again the retina as a legitimate part
of the brain is doing a lot of processing we normally think of that is
happening in the cerebral cortex or higher up in the visual system but it’s
beginning in the retina it’s not completely understood people have spent
an enormous amount of time and energy trying to understand the nature of this
processing but suffice it to say that that work is still in progress and one
knows a lot about the retina but not fully what the retinal processing
accomplishes in any event what it clearly does accomplish is providing an
output from photoreceptor cells to ganglion cells and that carries that
information to the rest of the brain

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