Visual System 1: Retina and Transduction

Question 1

path of light in the eye

Answer 1

Light reflected from surfaces in the world enters the at the cornea, passes through the anterior chamber, the lens and the vitreous and an optical image in formed in the plane of the photoreceptors at the back of the eye. Light modulates the membrane potential of the photoreceptors, a process called phototransduction

Question 2

refractive surfaces of the eye

Answer 2

“the cornea and the lens

Most of the refractive power comes from the cornea since the difference in refractive index is large between air and water. The lens provides a much smaller refractive component since the difference in refractive index is small but it is adjustable since the lens can change shape as the ciliary muscle contracts and relaxes”

Question 3

power of a lens

Answer 3

“The power of a lens is expressed in diopters which is one over the focal length in meters.

A nearly flat lens will have a very long focal length and will be weak, i.e., its refractive power may be only a few diopters. A sharply curved lens will have a short focal length and will be strong – its refractive power may be 10s or 100s of diopters.”

Question 4

how much of refraction does cornea account for?

Answer 4

“Most of the optical power of the eye is at the air/water interface, i.e., the
cornea, where the change in optical density is maximal. The cornea
accounts for about 52 of the 58 diopters, the remainder coming from the lens.”

Question 5

refractory power of eye

Answer 5

“1/0.017 = 58 dipoters (for far vision)

In a young person, another 12 diopters can be added for near vision by the
process known as accomodation.”

Question 6

presbyopia

Answer 6

“With age, the ability to accommodate diminishes and the near point (the
nearest distance at which objects are in focus) recedes”

Question 7

accommodation

Answer 7

At rest (distant vision), the lens is stretched and flattened by the zonal fibers. During accomodation, the ciliary muscle contracts and releases some of the tension on the lens capsule allowing the lens to become more nearly spherical. This increases the refractive power of the eye and allows for focused images to be formed on the retina by near objects.

Question 8

three layers of cells in retina & 2 additional layers

Answer 8

“1) ganglion cell layer

2) inner nuclear layer
3) outer nuclear layer

There are also 2 layers where synaptic connections are formed:
IPL (inner plexiform layer) between the GCL and the INL.
OPL (outer plexiform layer) between the INL and the ONL.

ganglion cell layer is closest to vitreous”

Question 9

ganglion cell layer of retina

Answer 9

Innermost in the eye (towards the vitreous). Ganglion cells send their axons over the inner surface of the retina where they leave the eye as the optic nerve. Ganglion cells are the only cells in the retina that produce action potentials.

Question 10

inner nuclear layer of retina

Answer 10

3 cell classes: bipolar cells (B), horizontal cells (H), and amacrine cells (A).

Question 11

outer nuclear layer of retina

Answer 11

cell bodies of the rods and cones (the photoreceptors)

Question 12

choroid

Answer 12

“Any photons not absorbed by the photoreceptors are trapped in the heavily pigmented choroid at the back of the eye.
This prevents the scatter of light by reflection and improves acuity.”

Question 13

fovea

Answer 13

“area in center of macular region of retina that is responsible for sharp central vision. contains only cones

The fovea appears as a pit because all the cells of the inner nuclear and ganglion cell layers are pushed to the side. These cells ‘pile up’ around the fovea helping to create the pit. Long processes of the photoreceptors carry signals from foveal photoreceptors to their postsynaptic elements which may be quite far away. These long processes are not really axons since they do not generate action potentials. The synapses between photoreceptors and subsequent cells are located in the outer plexiform layer. “

Question 14

two kinds of photoreceptors

Answer 14

rods & cones

Question 15

rods

Answer 15

“Rods are 1000x more sensitive to light than cones but there is only one rod pigment. Thus, under starlight conditions (also called scotopic vision), you are using your rods only and you have no color vision. There are no rods in the fovea so you are blind in the fovea under these low light conditions.

Rod outer segments contain 1000-1500 free-floating disks each jam packed with rhodopsin, the photosensitive pigment and other proteins. The amount of transmitter released onto 2nd order neurons is continuously variable and depends on the membrane potential at the synapse.”

Question 16

cones

Answer 16

“Less sensitive to light with three subtypes containing 3 slightly different visual pigments (opsins) with peaks in their absorbance spectra at long (red), medium (green) and short (blue) visible wavelengths. This (in association with a great deal of central processing) allows for color vision. At high light levels (photopic vision, e.g., sunlight), the rods play no role in vision

Cones have stacks of lamella but they are not free-floating in the cytoplasm. The amount of transmitter released onto 2nd order neurons is continuously variable and depends on the membrane potential at the synapse.”

Question 17

how are photoreceptors distributed in retina?

Answer 17

unevenly

Question 18

how can you measure visual acuity?

Answer 18

One measure of visual acuity is the spatial frequency (cycles per degree) of a grating (on the left) that can just be distinguished from a uniform field of the same average light level.

Question 19

what is visual acuity in fovea?

Answer 19

“In primates (including man), this is about 60 cycles/degree in the fovea but falls to 5 cycles/degree at an eccentricity of 8 degrees. This peak acuity corresponds to 60 bright stripes and 60 dark stripes on your thumbnail at arm’s length!

This limiting spacing is determined by the diameter of the cones in the fovea.”

Question 20

how is retinal structure different in fovea and parafovea?

Answer 20

“The central retina (fovea) uses cones only and they are tightly packed, situated shoulder to shoulder. Each bipolar cell is driven by a single cone for maximal acuity.

In the parafovea, each bipolar still receives from a single cone but in addition, each bipolar receives from a substantial number of rods. Also, the cones are farther apart and larger so that acuity during bright light vision falls off with eccentricity.”

Question 21

what do amacrine cells do

Answer 21

“The switching between domination of the ganglion cells by either rods or cones takes place at the level of amacrine cells in the inner plexiform layer.

Amacrine cells are a part of inner nuclear layer”

Question 22

describe the process of phototransduction (in rods because easier to study)

Answer 22

“The outer segment is filled with free-floating membraneous disks (about 1000 per rod) in which are embedded proteins that constitute the phototransduction cascade.

1) visual pigment molecule (rhodopsin) absorbs a single photon and undergoes a conformational change (activation). This begins an enzymatic cascade with each step having a significant amplification.
2) activated rhodopsin activates many G-proteins (transducin here) each of which in turn activate many cGMP phosphodiesterases each of which in turn catalyzes the shift of cytosolic cGMP to its 5’ form.
3) This is meaningful since the plasma membrane of the outer segments contain large concentrations of a special cGMP-gated Na+ channel. This channel is kept open by the high concentrations of cGMP IN THE DARK. In the light, the concentration of cGMP falls and the Na+ channels close.
“

Question 23

what is the membrane potential of a photoreceptor in the dark? Why?

Answer 23

”. -30mV

IN THE DARK, the outer segment of the rod has about equal permeabilities for K+ and Na+ and so the resting membrane potential is about -30mV (in between the equilibrium potentials for K+ and Na+). Thus, photreceptors are depolarized in the dark and ARE CONSTANTLY RELEASING GLUTAMATE at their synaptic terminals! “

Question 24

what is the action of light on a photoreceptor?

Answer 24

The action of light is to hyperpolarize photoreceptors (Vm moves towards the reversal potential for K+ since Na+ channels are closed) and to decrease the release of glutamate onto the next cells in the pathway.

Question 25

what are the two kinds of bipolar cells?

Answer 25

“ON & OFF bipolar cells

For ON-bipolars, glutamate is inhibitory. Therefore, these cells are hyperpolarized in the dark and become depolarized as the light level rises.
For OFF-bipolars, glutamate is excitatory. Therefore, these cells are depolarized in the dark and become less depolarized (or hyperpolarized) as the light level rises.”

Question 26

how many bipolar cells does a photorecptor synapse with?

Answer 26

For every photoreceptor, there is one postsynaptic ON-bipolar and one postsynaptic OFF-bipolar. If one is depolarized, the other is hyperpolarized and vice versa. One set of cells (ON-bipolars) constitute a channel that carries information about light increments and the other set of cells (OFF-bipolars) constitutes a channel that carries information about light decrements

Question 27

what is the straight through pathway?

Answer 27

“cones release glutamate and excite or inhibit bipolar ce lls, bipolar cells also release glutamate and excite ganglion cells

note: ganglion cells also divisible into ON- and OFF-types.”

Question 28

which cells in the retina produce action potentials?

Answer 28

“ONLY GANGLION CELLS GENERATE ACTION POTENTIALS – all other retinal cells communicate using graded synaptic release determined by the constantly changing membrane potential.

Ganglion cells must produce spikes since they have to send their signals many centimeters to various parts of the brain.”

Question 29

where do the two lateral pathways occur in the retina?

Answer 29

“inner plexiform layer

outer plexiform layer”

Question 30

describe the inner plexiform layer lateral pathway

Answer 30

amacrine cells carry signals from bipolar cells to relatively distant ganglion cells. This pathway appears to have multiple, poorly understood functions

Question 31

describe the outer plexiform layer lateral pathway

Answer 31

It is mediated by horizontal (H) cells. H cells collect the output of many photoreceptors and are presynaptic to bipolar cells. Horizontal cells are GABAergic and their action is always opposite to that of the photoreceptor input. Thus, for an ON-bipolar, which is depolarized by light, the H-cell input (from surrounding photoreceptors) hyperpolarizes the bipolar cell to light. Similarly, for an OFF-bipolar, which depolarizes to dark, the H-cell input (from surrounding photoreceptors) hyperpolarizes the bipolar cell to dark. In other words, the relationship between photoreceptor input and H-cell input is always antagonistic.

Question 32

what cell type mediates the outer plexiform layer lateral pathway?

Answer 32

Horizontal cells

Question 33

receptive field of visual neuron

Answer 33

region of retina wherein visual stimulation leads to a modification of the firing rate or the membrane potential.

Question 34

RF of a bipolar cell

Answer 34

The RF of a bipolar cell has 2 components of opposite sign. The center component results from activation of a photoreceptor and, in the case of an ON-center bipolar cell, produces depolarization from a small region of space to light. The surround component results from activation of one or many horizontal cells and, in the case of an ON-center bipolar cell, produces hyperpolarization to light. The RF has a center-surround organization and the center and surround are antagonistic. For OFF bipolars, merely substitute the word ‘dark’ for ‘light’.

Question 35

what is the consequence of the two components of a RF of a bipolar cell?

Answer 35

“COMPUTES LOCAL CONTRAST!

a small spot of light in the center of an ON-bipolar RF produces a strong depolarization. But if the spot is increased in diameter, the depolarization decreases as the spot encroaches on the antagonistic surround. A very large spot of light produces nearly no response since the depolarization from the center and the hyperpolarization from the surround approximately cancel.”

Question 36

what signal is conveyed by bipolar cells?

Answer 36

“the difference between the amount of light in the center and surround (ON-cells) or the difference between the amount of dark in the center and the surround (OFF-cells) of their RFs at any instant.

Their Vm at any moment reflects the difference between the activation of the center (straight-thru pathway) and the surround (lateral pathway), i.e., the local contrast.”

Question 37

what is represented in a neural image?

Answer 37

“local contrast is represented, not the absolute amount of light.

It is represented by the membrane potential of the set of cells representing the optical image”

Question 38

what is a consequence of retinal computation?

Answer 38

our visual systems are notoriously bad at representing absolute light levels

Question 39

what do ganglion cells do?

Answer 39

“recode the bipolar signal into spikes.

each of the bipolars excites a single ganglion cell. Thus, the properties of the bipolars are passed on to the ganglion cells: an ON-center ganglion cell and an OFF-center ganglion cell. These also have antagonistic surrounds.”

Question 40

how do ganglion cells represent the local contrast signal?

Answer 40

“by the firing rate of each pair (On & OFF) ganglion cells

in bipolar cells this Is represented by the membrane potential of the two cells”

Question 41

what are the two main types of ganglion cells?

Answer 41

“M-type - parasol

P type -midget “

Question 42

describe midget ganglion cells

Answer 42

“P type, receive input from single bipolars

They have tiny RF centers because they only get a single bipolar input. They are the majority of primate G. cells (80%).
Project to LGN, lamina 3 to 6”

Question 43

describe parasol ganglion cells

Answer 43

“10% of primate G. cells with much larger dendritic trees

They receive from many ON- and OFF- bipolars, have large RFs, and exhibit an impoverished center-surround RF organization. Circuitry in the inner plexiform layer renders them particularly sensitive to motion.

project to : LGN, lamina 1 and 2”

Question 44

what are the three types of cones?

Answer 44

different peak absorptions - red, green, blue (long –> short)

Question 45

are there more short or long wavelength photoreceptors?

Answer 45

long - red vastly outnumbers blue