The document summarizes key aspects of rod and cone photoreceptor structure and function in the retina: - Rods and cones differ in structure, visual pigments, and function. Rods are more sensitive to dim light while cones support vision and color perception. - Photoreceptor outer segments contain visual pigment discs that are renewed through the addition of new discs and removal of old discs via phagocytosis. - The density and distribution of rods and cones varies across the retina, with the highest cone density in the fovea and a ring of higher rod density around the fovea.

1. • T wo or three types of cone photoreceptor
and a single type of rod photoreceptor are
present in the norm al m am m alian retina.
Som e non-m am m alian retinas have even
m ore cone types

2. I . LIght mIcroscopy and
uL structure of rods a
tra nd
cones.
• In vertical sections of retina prepared for
light microscopy with the rods and cones
nicely aligned, the rods and cones can be
distinguished rather easily.

4. 2. o uter segment
genera tIon.
•It is from the base of the cilium that membrane
evaginations and invaginations occur to produce
the outer segment or the important visual
pigment-bearing portion of the photoreceptor.
Outer segments of both the rods and cones arises
from outpouching of the photoreceptor cell
plasma membrane at this point.

6. 3. VIsua pIgments and VIsuaL
L
transductIon.
•Vertebrate photoreceptors can respond to li ght by virtue of
their containing a vi sual pigm ent em bed ded in the bilipid
m em branous di scs that m ake up the outer segm ent. The
vi sual pigm ent consists of a protein called opsi n and a
chrom ophore d erived from vitam in A known as retinal.
The vitam in A i s m anufactured from beta-carotene in the
food we eat, and the protein is m anufactured in the
photoreceptor cell. T he opsin and the chrom ophore are
bound together and lie buried in the m em branes of the
outer segm ent discs.

8. 4. phagocytosIs of outer segments by
pIgment epItheLIum.
• T he s ta c ks o f d is cs co nt a in in g
v is ua l p ig me n t mo le c ul es in t he ou te r
s eg me nt s o f t he p ho t or ec e pt or s a re
c on st an t ly r e ne we d. Ne w d is cs a r e
a dd ed a t t he ba se o f t he ou te r
s eg me nt at t h e ci li u m as di sc us s ed
a bo ve . A t th e s am e t im e o ld d is c s ar e
d is pl ac e d up th e ou t er s e gm en t a nd
a re p in c he d o ff a t t he t i ps a nd
e ng ul fe d b y t he a pi c al p r oc es se s o f
t he p ig m en t ep it he l iu m.

10. 5. Different types of cone photorec eptor.
•As we have seen from the morphological appearances
described above, two basic types of photoreceptor, rods and
cones, exist in the vertebrate retina. The rods are
photoreceptors that contain the visual pigment - rhodopsin
and are sensitive to blue-green light with a peak sensitivity
around 500 nm wavelength of light. Rods are highly
sensitive photoreceptors and are used for vision under dark-
dim conditions at night. Cones contain cone opsins as their
visual pigments and, depending on the exact structure of the
opsin molecule, are maximally sensitive to either long
wavelengths of light (red light), medium wavelengths of
light (green light) or short wavelengths of light (blue light).
Cones of different wavelength sensitivity and the
consequent pathways of connectivity to the brain are, of
course, the basis of color perception in our visual image.

12. 6. Morphology of the S-cones.
This is illustrated in the tangential
section of the foveal cone mosaic where
the hexagonal packing is distorted in
many places by a larger-diameter cone
(arrowed cones) breaking up the
perfect mosaic into irregular subunits.
The larger-diameter cones are S-cones.
These cones have their lowest density
in the foveal pit at 3-5% of the cones,
reach a maximum density of 15% on
the foveal slope (1 degree from the
foveal pit) and then form an even 8%
of the total population elsewhere in the
retina

13. 7. Densities of rods and cones in the human retina.
It is important for our understanding of the organization of the visual
connections for us to know the spatial distribution of the different cell
types in the retina. Photoreceptors, we know, are organized in a fairly
exact mosaic. As we saw in the fovea, the mosaic is a hexagonal
packing of cones. Outside the fovea, the rods break up the close
hexagonal packing of the cones but still allow an organized architecture
with cones rather evenly spaced surrounded by rings of rods.

14. Rod sensitivity appears to
Thus in terms of densities of the
different photoreceptor populations in be bought at a price,
the human retina, it is clear that the cone however, since rods are
density is highest in the foveal pit and much slower to respond to
falls rapidly outside the fovea to a fairly light stimulation than
even density into the peripheral retina.
There is a peak of the rod cones. This is one reason
photoreceptors in a ring around the why sporting events such as
fovea at about 4.5 mm or 18 degrees
from the foveal pit. The optic nerve
baseball become
(blind spot) is of course photoreceptor progressively more difficult
free. as daylight fails. Both
electrical recordings and
8. Rods and Night Vision. human observations
Rods convey the ability to see at suggest that signals from
night, under conditions of very rods may arrive as much as
dim illumination. Animals with 1/10 second later than those
high densities of rods tend to be from cones under lighting
nocturnal, whereas those with conditions where both can
mainly cones tend to be diurnal. be simultaneously activated

15. Ultrastructure of rod and cone synaptic endings
The job of the photoreceptor cell in the retina is to
catch quanta of light in the visual pigment-containing
membranes of the outer segment and pass a message,
concerning numbers of quanta of light and
sensitivities to the different wavelengths, to the next
stage of integration and processing at the outer
plexiform layer

16. 10. Interphotoreceptor contacts at gap junctions.
There also appears to be a pathway for crosstalk between cones
and cones and cones and rods in the human retina. Cone pedicles
have small projections from their sides or bases that pass to
neighboring rod spherules and cone pedicles. Where these
projections, called telodendria, meet they have a specialized
junction known to be typical of electrical synaptic transmission.
These are minute gap junctions.