BIOL 428 Vision: the eye and photoreceptors

What are the main structures of the eye?

The 3 layers:

• Uveal tract

• Sclera

• Retina

And the vitreous humor

Uveal Tract:

includes iris, choroid and cilliary body

Sclera:

Fibrous white tissue surrounding the eye. Transitions to clear cornea

Retina

Layer or light sensitive neurons

How does light trave though the eye to the retina?

Light is refracted by the cornea and lens before stimulating photoreceptors in the retina

How does the eye accomodate for long distance vision

cillia is relaxed

How does the eye accomodate for short distance vision

Cillia is contracted

Myopia

Near sighted

Hyperopia

far sighted

Main structural features of the retina

• several layers with different processes

• Retina is considered part of the CNS since it has microglia and recovers similarly to CNS after injury

What are the cells/layers of the retina?

• retinal ganglion cell

• amacrine cell

• bipolar cell

• horizontal cell

• photoreceptors (cones and rods)

• pigment cell

How does the pigment epithilium support the retina?

• removes photoreceptor disks

• helps with retinoid cycle

How does the pigment epithilium remove photorceptor disks?

1. disks curl

2. tip becomes spherical

3. tip separates from photoreceptor

4. tip is englufed by pigment epithilium

5. new disk grows from soma

What is the photoreceptor response of to light?

photoreceptors are hyperpolarized by light

what mediates hyperpolarization of photoreceptors?

Cyclic GMP

How does light affect cGMP?

light reduced cGMP

How does cGMP levels affect hyper/depolarization?

increased cGMP causes an influx of NA+ and Ca+ while not changing K+ efflux

Mechanism of phototransduction in photoreceptors

1. light goes through photopigment (like rhodopsion)

2. light breaks carbon double bond of cis retinal and turns it trans

3. G protein transducin is activated

4. Phosphodiesterase hydrolyzes cGMP

5. Na+ channels close and cause hyperpolarization

Differences between rods and cones

• shape

• photopigment

• distribution

• pattern of synaptic connections

Cone sensitivities

low light sensitivity and high spatial sensitivity

Rod sensitivities

low spatial resolution; high sensitivity to light

What is the reitnoid cycle

replenishing of cis retinol from trans retinol

What is Inter-photoreceptor retinoid binding protein (IRBP)

chaperone protein that helps transport retinol in and out of rod/pigment epithilium

Why do rods have a sensitivity advantage over cones?

many rods converge on one bipolar cell meanwhile there is only one cone per bipolar cell

what structures/cells/circuits enhance visual acuitiy?

• convergence of rods onto bipolar cells

• Distribution of photoreceptors in retina

• The optic disk

What is the distribution of rods in the retina?

there are more rods and they are concentrated on the edges of the retina

What is the distribution of cones in the retina?

cones are concentrated in the fovea but there are less than rods

Fovea

displacement of inner ayers and blood vessels to reduce light scattering

Properties of the fovea

• rod free

• highest acuitiy due to receptor packing

• avascular zone (no capillaries)

Optic disk

blind spot through which ganglion cell axons to the brain

How is color achieved in cones?

different opsins to detect different wavelenghts of light

Different cone photoreceptors

Short, Medium and Large

Short (S) cones

detect blue light; least abundant

Medium (M) cones

detect green light

Large (L) cones

detect red light

Color can provide context to light intensity

same color appearing different depending on its surrounding colors

Dichromatic color blindness

• M or L damage

• S damage

M or L cone damage

red/green colorblindness

S cone damage

blue/yellow colorblindness

Anomolous trichromatics

overlap in green/red wavelengths