1. Pigment epithelium
Photosensitive parts of rods and cones
Outer nuclear layer – Photoreceptor cells (rods and cones)
Rods – spatial acuity at low light levels (one type), more sensitive than cones, work
slower than cones
Cones – high spatial acuity and color vision, 3 types (blue, green, red), less sensitive than
rods, work faster than rods
Inner nuclear layer – Bipolar cells
Outer plexiform layer – photoreceptor cells synapse on bipolar cells; horizontal cells perform
lateral interactions
Inner plexiform layer - bipolar cells synapse ganglion cells ( optic nerve); amacrine cells
perform lateral interactions
Last part of retina reached by light is photosensitive part of rod and cone cells, embedded in
processes of pigment epithelial cells
-Ganglion cells travel along vitreal surface of retina  optic disk  optic nerve with dural
sheath, arachnoid, and SAS; increase intracranial pressure  optic nerve  papilledema
(swelling of optic disk)
- optic disk is the blind spot in the visual field of each eye; optics of eye reverse images on
retina, so blind spot is on horizontal meridian of visual field, lateral to center of field (fovea)
- center of visual field  fovea in pigmented zone called macula; fovea packed with cones, no
rods; approaching fovea: cones increase, rods decrease; approaching periphery: cones decrease
sharply, rods increase then decrease at edges of periphery;
- Fovea’s densely packed cone region specialized for high spatial acuity and color vision, but
only at medium to high levels of illumination
- Region around fovea has many rods/few cones; spatial acuity at low light levels (not good at
color vision)
- Peripheral retina has few rods and few cones; good for telling us is something moving out there
2. Phototransduction
- Rods and cones both have outer segments of membranous disks; cone disks retain connection
to EC space; the connections for rods are pinched-off, free-floating, flattened vesicles; rods have
more disks than cones do, so rods are more sensitive
- Rhodopsin located in disk membranes attached to 11-cis-retinal
- cGMP activated cGMP gated cation channels are open during dark current; depolarized 
release of glutamate
- Photon  cis-retinal to trans-retinal  cGMP PDE  decrease [cGMP] levels  close cGMP
gated cation channels  hyperpolarization  decreased release of glutamate; amplification in
process
- 3. We judge color by comparing relative rate of photon capture by different receptor types;
necessary to compare outputs of three different cone populations; a single cone by itself cannot
tell what kind of photon it has absorbed
4. – Receptive field of single rod or cone is a photon itself  hyperpolarization
- Ganglion cells have on-center cells  fire faster when light falls on center of field, slower
when light falls on periphery; off-center cells  fire slower when light falls on center, faster
when light falls on periphery, i.e. fire faster when illumination at center decreases; either don’t
do much in response to diffuse illumination
This means:
1. Message not related to brightness, but to contrast between different parts of visual world
2. Some ganglion cell will fire faster whether light intensity at any retinal site increases (oncenter cells) or decreases (off-center cells); something fires not matter which way light intensity
changes
- Centers of foveal ganglion cell receptive fields are tiny, corresponding to size of single cone
- Centers of receptive fields in peripheral retina are large, have convergence of outputs of rods
and cones onto a single bipolar cell.
Center-surround receptive fields
- Bipolar cells have on-center and off-center receptive fields
- No APs, but slow potential changes; receptive field center corresponds to photoreceptor input
to bipolar cells
- Glutamate excites off-center bipolar cells so less glutamate release results in bipolar cell
hyperpolarization
-Glutamate inhibits on-center bipolar cells so less glutamate results in bipolar cell
depolarization
- In fovea  each cone synapses on two bipolar cells- an on-center and an off-center
-Photoreceptors release glutamate in the dark onto horizontal cells  release GABA
(inhibitory) onto nearby cone synapses  GABA hyperpolarizes cone synapses (just like light)
-Shining light on surround of bipolar cells receptive fields has effect opposite to shining light in
the center