Pigment epithelium Photosensitive parts of rods and cones Outer nuclear layer

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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 (on-center 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

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