PS231: Vision II (The Retina)

The Retina

CNS tissue in the periphery. a photosensitive sheet in the back of the eye

Macula lutea

An oval spot containing yellow pigment, supports the highest visual acuity (ability to resolve fine details/sharpness)

Fovea

A small pit at the center of the macula where visual acuity (sharpness) is highest, we make sure light rays hit this by moving our eyes

Optic disk

Site where retinal nerve fibers (axons) leave the eye and travel to the brain via the optic nerve (blindspot)

Light

comes up from the bottom and is absorbed by rods/cones at the very end

Pigment epithelium

absorb excess photons, dark color in humans because we are dinural, plenty of photons and photoreceptors

Photoreceptors

GRADED (do not fire action potentials, de/hyperpolarize graded potentials), NOT neurons, specialized for visual transduction

Bipolar cells

input from photoreceptors, output to ganglion cells, on-center or off-center, release glutamate, excitatory, GRADED (do not fire action potentials)

Retinal ganglion cells

only cell in the retina that produces APs, axons form the optic nerve

Tapetum lucidum

reflective pigment epithelium that helps animals forage in low light

Amacrine cells

input from bipolar cells, influence ganglion cells (release GABA), inhibitory, NO APs

Photoreceptors

only light sensitive cells in the retina (rods and cones), outputs to the bipolar and horizontal cells, release glutamate, excitatory, NO APs

Eye orientation

directing light onto fovea which has highest visual acuity

Why does the fovea have high visual acuity?

the center of the fovea has displaced blood vessels, ganglion and bipolar cells, which allows light to directly strike cones (less light scattering by blood vessels and other cells)

Acuity

a measure of the ability to resolve fine details from a given distance

Convergence

how many photoreceptors and BPCs signal a single retinal ganglion cell

Center of the fovea

low ratio of cones to RGCs, (1-1) in very center, good for acuity, preserves spatial information

Rods on outer periphery (nasal and temporal)

high ratio (pooling) of rods to RGCs, low acuity, 1:6, good for detecting light in dim conditions, but spatial information is lost

low convergence

high acuity

high convergence

good for low light conditions (sensitivity), low acuity