Visual System Physiology

Question 1

What color is RPE

Answer 1

. Black pigment melanin
. Absorbs light not captured by photoreceptors preventing light form being reflected back to photoreceptors that would dull an image

Question 2

Components of photoreceptors

Answer 2

. Outer segment: light sensitive portion where signal transduction takes place
. Inner segment: part of cell w/ nucleus and metabolic machinery
. Synaptic terminal: part of cell that transmits the electrical signal to next neuron in pathway

Question 3

Photopigments

Answer 3

. Outer segments of rods and cones contain membranous discs that have high conc. Photopigments (rhodopsin in Rhodes, other names in cones)
. Disc structure enhances SA available to inc. photopigments available
. Each pigment has small light-absorbing (retinal) molecule attached to large protein (opsin)
. 4 types of pigment (1 in rod, 3 in cones)
. Retinal molecule is the same in all 4 pigments (aldehyde of vit. A) and only opsin proteins differ

Question 4

Opsin protein function

Answer 4

Filtering action that determines which wavelengths of light at filtered and which are absorbed by retinal in a given photoreceptors

Question 5

Phototransduction

Answer 5

. Mechanism where light energy is converted to an electrical signal
. Absorption of light triggers cascade of events that leads to altered ion. Channel activity across cell. Membrane of photoreceptors

Question 6

Retinal response to light

Answer 6

. Retinal in dark exists as 11-cis configuration
. In dark it fits into binding site w/in opsin molecule and inhibits opsin activity
. When light strikes retinal molecule, retinal undergoes conformational change to all-trans-retinal
. This is only light dependent step in vision
. Retinal comes off opsin and diffuses away
. Removal of inhibitory retinal activates opsin molecule

Question 7

Phosphodiesterase activation

Answer 7

. Activated opsin binds to a G-protein (transducin) which activates phosphodiesterase

Question 8

Degradation of cGMP

Answer 8

. Phosphodiesterase hydrolzyes cGMP dec. its intracellular conc.
.cGMP levels are high in dark and low in bright light

Question 9

Closure of cation channels

Answer 9

.cGMP-activated cation channels in photoreceptors membrane are open in dark due to cGMP binding
. Channels closed by dec. cGMP levels in response to light
. Results in reduction in depolarizing current mainly carried by Na

Question 10

Hyperpolarization

Answer 10

. Reduction of depolarizing current results in hyperpolarization of cell

Question 11

Decreased Ca. Channel activity

Answer 11

. Hyperpolarization of photoreceptor membrane spreads to synaptic terminal
. VG Ca channels in membrane are open when photoreceptor is depolarizer and close when it is hyperpolarized

Question 12

Decreased NT release during photo signal transduction

Answer 12

. Hyperpolarization-induced closure of Ca channels dec. Ca influx
. Results in dec. Ca-dependent NT release from synaptic terminal
. In dark retinal goes back to original conformation, cGMP inc., channels open, and membrane potential is restored to -40 mV and NT release is high

Question 13

Rhodopsin

Answer 13

. Rod photopigment
. Doesn’t discriminate wavelengths to rods response to all visible light as shades of gray
. Have huge convergence resulting in poor signal sharpness but higher sensitivity
. More sensitive to scatter light

Question 14

Cone photopigments

Answer 14

. Respond to limited range of wave lengths to varying degrees for that specific wavelength
. Each cone has little convergence to inc. image sharpness but also makes it harder to reach AP threshold
. More sensitive to direct rays

Question 15

Rod vs cone temporal resolution

Answer 15

. Rods: slow response, long integration time

. Cones: faster esponse, short integration time

Question 16

Color blindness

Answer 16

. Inability to perceive differences btw some colors that most people can distinguish
. Usually genetic, more common in men
. Major causes: absence of cones, absence of 1 photopigment, non—functioning photopigment, alteration in spectral sensitive to photopigment

Question 17

Red-green color blindness

Answer 17

. Most common
. Mutation in pigment of green photoreceptors that alters spectral sensitivity (shift to red end of spectrum)
. Individuals have difficulty distinguishing colors in red-yellow-green part of visual spectrum

Question 18

Do bipolar cells fire APs?

Answer 18

. No, only have graded potentials

. Primary cells connecting the photoreceptors to ganglion cells

Question 19

First cells in visual pathway to fire APs

Answer 19

. Ganglion cells

. Axons form optic n. And transmit visual into to brain where more complex integration of visual info occurs

Question 20

What do bipolar cells respond to?

Answer 20

. Characteristics of visual images (color, intensity, shape, and movement)

Question 21

Horizontal and amacrine cell function

Answer 21

. Interconnect photoreceptors

Question 22

What control rate of firing for APs in ganglion cells?

Answer 22

. Patterns fo graded potential generated by lower order cells
. Allows considerable integration before signal leaves retina

Question 23

Modulation of ganglion cell firing

Answer 23

. Ganglion cells fire APs at all times even in the dark
. Only thing that changes in frequency of firing determined by graded potentials impinging on ganglion cells
. Each ganglion cells respond to light in a defined circular area of retina (receptive field)
. Each ganglion cell monitors its receptive field
. Size of receptive field is important component of visual acuity (large field - high sensitivity to low light but low acuity)

Question 24

On-center signaling

Answer 24

. Receptive field has circular center and area around center circle (the surround)
. On-center cells fire infrequently in the dark
. Respond to light signal to the center of the field w/ inc. in firing

Question 25

Off-center signaling

Answer 25

. Cells fire when light shines on the surround

. Firing inhibited by light shining on center of field

Question 26

Importance of on a dm off center signaling

Answer 26

. Provides parallel pathways for the processing of visual info
. The visual system is much better at discriminating contrasts than absolute light intensities

Question 27

What is absolute light intensity determined by?

Answer 27

Ambient illumination

Question 28

Thalamus in visual pathway

Answer 28

. Optic n. Fibers pass through lateral geniculate nucleus in thalamus to get to other parts of brain

Question 29

Primary visual cortex

Answer 29

. Neurons in one subdivision of visual cortex respond only to stimuli oriented in a particular direction in visual field
.in another subdivision the neurons may respond primarily to movement across a visual field
. Other neuronal groups in cortex will respond mainly to color
. Complex interaction of many neurons in this cortex together w/ neurons in visual assoc. areas lead to visual picture