phototransduction

Question 1

photoreceptors saturation

Answer 1

rods can saturate cones can not
saturation- as the amount of light that reaches the retina is increased rods response increases to a limit- and the saturation level is low (normal room light)

Question 2

Rods vs cones

Answer 2

Rods (scotopic): more sensitive, slow integration time, saturate at high light intensity, poor spatial resolution, more pigment

Cones (photopic): less sensitive, fast integration time, dont saturate, high spatial resolution, less pigment

Question 3

photoreceptors response to light

Answer 3

hyperpolarize! and release less neurotransmitters, and turns them off

photoreceptors hyperpolarize in a graded fashion, they dont produce APs. the more depolarized they are the more NT they release, the more hyper polarized, the less NT they release

light centered on the photoreceptor is an Off- center cell

when a cone is depolarized it releases the typically excitatory NT, glutamate

Question 4

photopigments

Answer 4

photopigment= chromophore (11-cis retinal) + protein (opsin)

photopigment is embedded in lipid membrane of the outer segment

11-cis retinal normally absorbs light at 375 nm, opsin is responsible for the red shift

rods absorb most strongly in blu green, 3 types of cones (red green and blue)

Question 5

univariance

Answer 5

photoreceptors cant register the wavelength of the photons they catch only the number of which

an individual photoreceptor cant differentiate between a change in wavelength type vs intensity

Wavelength information (color vision) is deciphered not by individual photoreceptors but by comparing the responses across different photoreceptors

Question 6

excitatory cascade

Answer 6

visual pigments are GPCRs, activated by light instead of a ligand

absrption of light activates the photopigment molecule; isomerization of chromophore

the active photopigment molecule stimulates G proteins transdusin Amplification 700x in 100 ms

the activated G protein activates cGMP phosphodiesterase

the phosphodiesterase catalyzes the breakdown of cGMP which makes even more amplification

cGMP maintains the cGMP gated channels in an open configuration and the decrease in cGMP closes them

Na levels fall–> hyperpolarization

the decrease in Ca depresses the PDE activity and cause an increase in cGMP

Question 7

viagra

Answer 7

PDE inhibitor (specifically PDE6) and PDE 5

Question 8

photo transduction defects

Answer 8

retinitis pigmentosa (affects rods via rhodopsin), achromatopsia (rod monochromacyvia GMP gated ion channel)
red green (affects cones via cone opsin)

Question 9

receptive fields

Answer 9

every neuron that is higher order than than the photoreceptors has a receptive field that is really the field of photoreceptors that provide input to that neuron

retinal area theat when stimulated influences the activity of neurons

Question 10

photoreceptors hyperpolarization

Answer 10

they do not produce APs, but hyperpolarize and depolarize in a graded fashion and release more or less NT

Phtoreceptors are Off center cells

neurons in the retina and at the next higher levels of the CNS are termed off center

cones release glutamate

Question 11

parallel pathways

Answer 11

bipolar cells 2 types with Cones:
ON center (metabotropic): light in the center of this receptive field truns them on, sign reversing synapse, invaginating contacts with cones (glutamate hyperpolarizes the ON center, which is usually excitatory)

Off center (ionotropic): flat/ basal contacts with cones, classical excitatory glutamate activity

the reason to have parallel channels: improved sensitivity

Question 12

Connection of bipolar cells Amacrine cells

Answer 12

provide lateral connections and produce transient depolarizing response

Question 13

Connection of bipolar cells Ganglion cells

Answer 13

produce AP, physiological types (On center and off center)

Parasol ganglion cells (exhibit M cell behavior)- large cells with large receptive fields, more transient response

Midget ganglion cells (exhibit P cell behavior)- smaller cells with small receptive fields
bipolar cells always make sign conserving response with ganglion cells in the LGN

Question 14

horizontal cells

Answer 14

photoreceptors are completely synaptically interconnected by an elaborate system of inhibitory interneurons called horizontal cells
interconnect the cones with cones and rods with rods

when a cone is excited (depolarized), it excites the horizontal cell (depolarized), it inhibits/hyperpolarizes the neighboring cones

when a horizontal cell is depolarized it sends GABA (in a graded fashion)

photoreceptors are OFF-center and ON-surrounding

horizontal cells improve resolution

Question 15

retinitis pigmentosa

Answer 15

rhodopsin mutations can alter rod function- with night blindness in the begining but then cones
inherited X linked

Usher syndrome= RP w/ hearing loss
motor proteins

Question 16

Achromatopsia

Answer 16

Cones do not hyperpolarize

congenital defects where one of the cone types are altered so you only have three and a region of the color spectrum is not seen well

Question 17

Acquired vs congential defects

Answer 17

Congental: present ar birth, type and severity constant thruought life, both eyes are affected, visual acuity us unaffected (protan or deutan), males are more affected

Acquired: onset after birth, type and severity can fluctuate, monocular defects, visual acquity is often reduced (tritan) males=females