Rods or Cones?

Theophilus Samuels theophilus.samuels at btinternet.com
Tue Nov 14 17:59:38 EST 2000


> - they do not work in bright light at all - although I can't recall the
> reason why exactly. if I had to guess, like cones, they are probably
> "shut off" by bright light and turned on by darkness but cones have a
> system which inverts this signal and I doubt that rods do (maybe someone
> can comment on this)... so there is probably no signal at all from rods
> in bright light - i.e., they are very hyperpolarized, but I cant emember
> if this is the exact reason why they do not function in bright light.

In general, a typical neuron has a resting potential of ~ -65 mV, which is
close to the equilibrium potential for potassium ions. In complete darkness
the membrane potential of the rod outer segment is around -30 mV. It was
discovered that this was due to a steady influx of sodium ions into the rod
through specialised channels and was called the 'dark current'. It was found
that these channels are stimulated to open by cGMP, i.e. they are
cGMP-gated. Therefore, these channels are kept open through the continual
production of cGMP by the enzyme guanylate cyclase. To remove the necessity
of describing the rhodopsin cascade in detail, what daylight ('light')
actually does is reduce the levels of cGMP by stimulating phosphodiesterase
enzymes which in turn breakdown cGMP. Thus, the sodium channels close and
the membrane potential becomes more negative, amazingly photoreceptors
hyperpolarise in response to light.

> > Rods are more light sensitive (obviously, thats why they help night
> > vision), but i am still confused as to why they outnumber cones, would
> > there increased senstivity to light be of any advantage in the daytime?

This is intuitively a good suggestion, however it is not the case. What
happens in prolonged illumination prevents this from happening. By this I
mean, that the cGMP levels in the rods fall to the point that their response
to light becomes saturated. Hyperpolarisation no longer occurs even is the
light levels are increased. Thus vision during the day depends almost
entirely on the cones. Phototransduction in cones is almost identical to
that in rods, the types of opsins are what differs.

T.L.S.

'A single idea can turn the world around. Good thing they only come about
once in a blue moon.'


SA <nospam at nospam.net> wrote in message
news:nospam-8EC8CC.01200213112000 at nntp.we.mediaone.net...
> In article
> <mike-anderson-1311000059280001 at dhcp083108.res-hall.nwu.edu>,
> mike-anderson at nwu.edu (mike) wrote:
>
> > In article <mike-anderson-1211002340380001 at dhcp083108.res-hall.nwu.edu>,
> > mike-anderson at nwu.edu (mike) wrote:
> >
> > > Cones in the retina are responsible for our color vision i believe.
> > > Rods
> > > allow us to perceive black and white.  Intuitively i would think that
> > > we
> > > have much more cones than rods in our eyes, however, in many diagrams
i
> > > have seen there are more rods present, is this just a coincidence or
> > > are
> > > there more rods than cones?
> >
> > more rods, solved through a little research
> > >
> > > Rods allow our night vision, however we see much better in daylight,
so
> > > if
> > > rods are more plentiful than cones do they also contribute to our
> > > processing visual information during the day?
> > >
> >
> > Rods are more light sensitive (obviously, thats why they help night
> > vision), but i am still confused as to why they outnumber cones, would
> > there increased senstivity to light be of any advantage in the daytime?
>
>
> This is a little complicated...
>
> As I recall, there are about 120 million rods, and about 6 million
> cones. So you are right there are way more rods.  However, the rods
> provide less information.... they are pooled together onto neurons which
> signal to the brain. This way you get a really hightened sensitivity to
> light, but the information is "blurry".  The cones are much more
> preserved as the signal moves back to the brain, one-to-one and in
> fashions which contrast the different colors, etc.
>
> Moreover, the rods are more concentrated in the peripherary whereas the
> cones are in the tiny area of the fovea (about one half one one
> MILLIMETER).. so the cones are responsible for your high acuity vision,
> because:
>
> (1) they are in the fovea where most important images are viewed
> (2) the signal is conserved on the way back to the brain
>
> the rods are responsible for blurry night vision and peripheral vision
> (1) they are in the peripherary (part of the reason they are more
> numerous is that the peripherary of the retina is simply a much larger
> area... in fact they are physically larger than cones and are not nearly
> as densely packed).
> (2) the signal is pooled - many many rods converge onto single neurons
> which signal back to the brain.
> - they do not work in bright light at all - although I can't recall the
> reason why exactly. if I had to guess, like cones, they are probably
> "shut off" by bright light and turned on by darkness but cones have a
> system which inverts this signal and I doubt that rods do (maybe someone
> can comment on this)... so there is probably no signal at all from rods
> in bright light - i.e., they are very hyperpolarized, but I cant emember
> if this is the exact reason why they do not function in bright light.
>
> Hope this helps,
> S







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