Photoreceptors

variable pupil diameter, different photoreceptors

(photoreceptors are the MAIN reasons though)

We can see a huge range of light, from 10^-6 (threshold) to 10^10 (sun). Why? (2 main reasons)

photopic vision

refers to our ability to perceive visual stimuli under bright light conditions due to the activity of cones; roughly 10⁴ to 10⁸ luminance

**bright lighting conditions mediated by cones

scotopic vision

refers to our ability to perceive visual stimuli in near darkness due to the activity of rods; roughly 10⁻⁵ to 10⁻³ luminance (includes threshold)

**dim lighting conditions mediated by rods

mesopic vision

refers to our ability to perceive visual stimuli at dawn or dusk levels, which involves both rods and cones

**twilight conditions mediated by rods & cones

cones

Which photoreceptors mediate photopic vision?

rods

Which photoreceptors mediate scotopic vision?

rods and cones

Which photoreceptors mediate mesopic vision?

cones
(lowest threshold light is scotopic vision, mediated by rods, so cones have the highest threshold)

Which type of photoreceptors have the higher threshold of detecting light: rods or cones?

rods
(lowest threshold light is scotopic vision, mediated by rods, and lowest threshold = highest sensitivity)

Which type of photoreceptors have the higher sensitivity of detecting light: rods or cones?

ganglion cells

Which cells in the retina encounter light first?

bipolar, horizontal, amacrine

What 3 kinds of cells in the retina are found in the more middle layers as light passes through?

photoreceptors

Which cells in the retina encounter light last?

photoreceptors are in the back but initiate the phototransduction signal

Why is the retina considered to be "inverted"?

retinal pigmented epithelium (RPE)

outermost layer of the retina that provides metabolic support for the retina and absorbs light photos that are not absorbed by photoreceptors

ganglion cell axons
(leave the retina to carry signals from retina to cortex)

What makes up the optic nerve?

photoreceptors, bipolar cells, ganglion cells

What is the order of the 3 main cells in the phototransduction process? (feedforward connection)

horizontal cells

Which cells mediate interactions between photoreceptors and bipolar cells?

amacrine cells

Which cells mediate interactions between bipolar cells and ganglion cells?

ganglion cells to photoreceptors

What is the order of the 2 main cells in the feedback phototransduction process? (i.e. NOT going toward the optic nerve)

discs containing photopigments

What is included in the outer segment of photoreceptors?

photopigments

What do discs in rods and cones contain?

daytime

The outer segment discs in photoreceptors are continuously shed. When do rods mostly shed their outer segment discs?

nighttime

The outer segment discs in photoreceptors are continuously shed. When do cones mostly shed their outer segment discs?

organelles (including nuclei)

What is included in the inner segment of photoreceptors?

spherule

refers to a rod synaptic ending

pedicle

refers to a cone synaptic ending

inner segment

Which part of a photoreceptor does light encounter first: inner or outer segment?

true
(this is why light encounters the inner segment first)

True or false: The retinal layers, including the photoreceptors themselves, are inverted to the direction of incoming light.

inner segment
(since the intended target is the photopigments in discs of the outer segment)

What part of a photoreceptor acts as a "light guide" to direct photons of light towards their intended target, acting like fiber optics?

RPE
(prevents light from bouncing back onto photoreceptor outer segments, which would degrade the retinal image)

Photons that are not absorbed by the outer segments of photoreceptors are mostly absorbed by which retinal layer?

6 million

How many cones are in the human retina?

120 million

How many rods are in the human retina?

rods

Are there more rods or cones in the retina?

1 million

About how many ganglion cell axons leave the retina to comprise the optic nerve?

true
(since there are 6 million cones and 120 million rods, but only 1 million ganglion cells)

True or false: Each retinal ganglion cell receives signals from several photoreceptors.

spatial summation

refers to each retinal ganglion cell receiving signals from multiple photoreceptors

false

True or false: Rods and cones are distributed equally throughout the retina.

peripheral

Are rods more numerous in the central or peripheral retina?

fovea

Where are the most cones found in the retina?

**at 0 degrees eccentricity in the retina

5

What is the width of the fovea in degrees?

foveola

very center of the fovea that has NO rods; peak density of cones here is about 200,000 cones/mm^2

**Note: rest of the fovea outside of this very specific region does still have some rods

1.2

What is the width of the foveola in degrees?

optic disc

region where ganglion cell axons leave the eye as the optic nerve

**at about 15 degrees eccentricity in the nasal retina

5

What is the width of the optic disc in degrees?

20 degrees

Rods are the most densely packed at about how many degrees from the fovea?

**peak density is about 150,000 rods/mm^2

decrease

As we age, does our amount of rods increase, decrease, or remain the same?

decrease
(actually it decreases significantly, even if you just go 0.2 degrees out)

Cones are the mostly highly concentrated in the foveola, with a density of about 200,000 cones/mm^2. As you move out, does the concentration increase or decrease?

1/10

Just 1 degree away from the fovea, what is the cone density relative to its maximum at the foveola?

neither

Does the optic disc contain rods, cones, both, or neither?

wavelength they respond to

There are 3 types of cones, which vary based on what?

red

What color of light do LONG wavelength cones respond to?

green

What color of light do MEDIUM wavelength cones respond to?

blue

What color of light do SHORT wavelength cones respond to?

short

Which wavelength of cones responds to BLUE light: short, medium, or long?

medium

Which wavelength of cones responds to GREEN light: short, medium, or long?

long

Which wavelength of cones responds to RED light: short, medium, or long?

long, red

Typically, which cones are most numerous: long, medium, or short wavelength? Which color do they respond to?

short, blue

Typically, which cones are least numerous: long, medium, or short wavelength? Which color do they respond to?

0.5 degrees from the fovea

Where does S cone density peak?

false
(NO S-cones at the central 0.3-0.4 degrees of the fovea)

True or false: S cones can be found highly packed from 0-0.5 degrees of the fovea.

increases

What happens to the size of the cones with increasing retinal eccentricity? (moving away from the fovea)

increases
(S cones are very widely spaced, about 6 arc min between each)

What happens to the spacing of the cones with increasing retinal eccentricity? (moving away from the fovea)

radial symmetry

pattern of rod and cone distribution around the fovea, showing that the greatest concentration of both is surrounding the optic disc (except for the optic disc itself) and the distribution decreases moving toward the periphery

15 degrees nasal

Where is the blind spot on the retina? (i.e. how many degrees, nasal or temporal)

20 degrees

According to a topographic map, the highest rod density is a ring encircling the fovea at what eccentricity?

Note: there is a little bit of asymmetry

decrease
(more or less radially symmetric)

As eccentricity increases, does cone density increase or decrease?

nasal

(25% more cones in nasal retina)

Are there more cones in the nasal or temporal retina?

**on a topography map, results in a cone ____ streak

rhodopsin

What photopigment(s) are in rods?

cyanolabe, chlorolabe, erythrolabe

What photopigment(s) are in cones?

cyanolabe

photopigment found in S-cones

chlorolabe

photopigment found in M-cones

erythrolabe

photopigment found in L-cones

507 nm

What is the maximal absorption wavelength of rhodopsin?

426 nm

What is the maximal absorption wavelength of cyanolabe? (found in S-cones)

530 nm

What is the maximal absorption wavelength of chlorolabe? (found in M-cones)

557 nm

What is the maximal absorption wavelength of erythrolabe? (found in L-cones)

each cone contains only one photopigment

Why are cones not as sensitive to light as rods?

each rod disc contains about 10,000 molecules of rhodopsin

Why are rods more sensitive to light than cones?

-50 mV

What is the resting membrane potential of rods?

rhodopsin

When light hits a rod, what substance absorbs the photons?

closing of sodium channels

(specifically cGMP-gated Na channels, since this process results in decreased cGMP)

When light hits a rod, rhodopsin absorbs the photons. Photon absorption triggers the process of phototransduction, which leads to the opening or closing of what kind of channels?

hyperpolarization
(between -50 and -70 mV)

In phototransduction, does the closing of sodium channels lead to depolarization or hyperpolarization of the rod?

glutamate

Hyperpolarization of a rod leads to a decrease in its release of what neurotransmitter?

true

True or false: The amount of sodium channels closed in the initiation of phototransduction is directly proportional with how much the membrane potential changes.

rods, neurotransmitter, membrane potential, neurotransmitter

Phototransduction summary:

1. A photon initiates the transduction process...

2. Change in voltage in ____

3. Change in concentration of ____ in the synapse

4. Change in the ____ ____ of the post-synaptic cells

5. Change in the release of ____ at the next synaptic terminal

saturation

process in which all sodium channels are closed when just 10% of a rod's rhodopsin molecules are bleached with light

**further bleaching of rhodopsin does not result in further hyperpolarization, since the number of Na+ channels located in the outer segment is limited

-70 mV

At what membrane potential have we reached saturation of sodium channels?

shine monochromatic light through box with rhodopsin, measure amount of light transmitted through the box with a radiometer

Photopigments absorb photons of a specific wavelength range, but different wavelengths of light have a different probability of being absorbed. How can we measure the absorption probability of different wavelengths?

rhodopsin

In the transmission experiment, what molecule is put in a box for monochromatic light to shine through?

radiometer

In the transmission experiment, what can we use to measure the amount of monochromatic light that is transmitted through a box of rhodopsin?

**this is how we know that wavelengths in the region of 507 nm are most likely to be absorbed by rhodopsin

20 quanta

Transmission experiment example: 100 quanta of a 400 nm light is incident upon a box containing rhodopsin. 80 quanta are transmitted. How much did rhodopsin absorb?

relative absorption vs. wavelength

How do we plot a graph for the transmission experiment? (y vs. x)

lesser
(since rods are MOST sensitive to this wavelength)

If light of wavelength 507 nm hits the retina, is this a greater or lesser threshold for rods to respond?

1

(which is why rhodopsin bleaches faster than cones)

Absorption of how many photons does it take to bleach rhodopsin?

true

True or false: Absorption of one photon is enough to activate rhodopsin.

5 minutes
(means that when a certain number of rhodopsins is bleached, 50% of them will recover in 5 minutes)

What is the half life for rhodopsin regeneration?

50

100 rhodopsins are bleached with photons of light. After 5 minutes, how many have recovered and are now unbleached?

75

100 rhodopsins are bleached with photons of light. After 10 minutes, how many have recovered and are now unbleached?

507 nm
(since this is mediated by rods)

At what wavelength is the scotopic visual system most sensitive?

cones
(but rods bleach faster since they only require 1 photon)

Which photoreceptors recover faster: rods or cones?