Lecture 7 - The Visual System (Part 1) Photoreceptors and the Retina

Question 1

What is the lens of the eye?

Answer 1

a biconvex structure that sits just beneath the cornea and focuses light onto the retina

Question 2

What is the ciliary body and muscles?

Answer 2

-the muscles are important for adjusting the tension of the lens in oder to focus light onto the retina

Question 3

What is the choroid?

Answer 3

-part of the vascular layer of the eye and is rich in blood vessels and it provide oxygenated blood to the retina

Question 4

What is the optic disc?

Answer 4

-the blind spot where the optic nerve or CN II exits the eye and no photoreceptors are present in this region

Question 5

What is the optic nerve CN II?

Answer 5

the sensory only cranial nerve that play a role in primary visual perception by relaying an electrical signal from the retina to the central visual cortical and subcortical regions

Question 6

What is the retina?

Answer 6

-the most important structure in the eye and along with the optic nerve is considered to be the CNS
-the sight of phototransduction and is essential for primary visual perception

Question 7

What is myopia or nearsightedness?

Answer 7

-it is condition that is typically caused by a slightly longer shape of the eye ball which causes light to focus anterior or rostral to the retina

Question 8

What is hyperopia or farsightedness?

Answer 8

typically characterized by a shorter shape of the eyeball which causes light to to be focused posterior or caudal to the retina

Question 9

What is astigmatism?

Answer 9

-near and far vision is blurred due to abnormal curvature of the lens or the cornea of the eye

Question 10

What are photoreceptors?

Answer 10

-rods and cones; they are specialized cells in the retina and they are responsible for phototransduction and they are the cellular starting point for primary visual processing

Question 11

What are the rods and what types of vision are they involved in and where are they?

Answer 11

-scotopic is night vision and they are achromatic and are in the peripheral retina

Question 12

What are the cones and what types of vision are they involved in and where are they?

Answer 12

-photopic vision which is day vision and the are chromatic and they are in the fovea of the retina or the center of the retina

Question 13

What pathway are the rods in?

Answer 13

they are involved in the magnocellular pathway or the dorsal or where pathway

Question 14

What pathway are the cones involved in?

Answer 14

they are involved in the parvocellular or what pathway or the ventral

Question 15

What type of sensitivity and acuity do the rods and cones have?

Answer 15

rods - have high sensitivity and low acuity (meaning differentiation for resolution)
cones - have low sensitivity but high acuity meaning they have great resolution

Question 16

What are the three different types of cones and what are they responsible for?

Answer 16

-long-wavelength or L cones: sensitive to red to yellow wavelengths of light due to the type of opsin expressed in these cells
-medium wavelength or M cones they are sensitive to the yellow to green wavelengths of light due to the type of opsin expressed in these cells
-short wavelength or S cones they are sensitive to the blue to violet wavelengths of light due to the type of opsin expressed in these cells

Question 17

How are opsin genes typically linked?

Answer 17

they tend to be x-linked which means they are located on the X chromosome which causes color blindness to be more common in men than in women

Question 18

What are the two divisions of the retinal surface?

Answer 18

-the nasal retina and the temporal retina

Question 19

Where is the nasal retina?

Answer 19

the portion of the retina that is closest to the nose and tends to receive contralateral projections from the visual field

Question 20

Where is the temporal retina?

Answer 20

the portion of the retina that is closest to ones temporal lobes; receives input from the ipsilateral side of the visual field

Question 21

What is the macula?

Answer 21

the center of the retina and the center of the macula is the fovea

Question 22

What are the retinal layers?

Answer 22

1. photoreceptor layer
2. inner nuclear layer
3. ganglion cell layer
4. nerve fiber layer

Question 23

What is found between the layers of the retina?

Answer 23

high density of vasculature which provide oxygenated blood to each layer of the retina as well as the to filter the scattered light to refine phototransduction

Question 24

What is the photoreceptor layer of the retina?

Answer 24

-include the photoreceptor outer segment and the outer nuclear layers and the most posterior layers of the retina and they are where the rods and cones are located

Question 25

What is the inner nuclear layer?

Answer 25

-have bipolar cells which have lateral support horizontal cells and amacrine cells

Question 26

What is the ganglion cell layer?

Answer 26

-has retinal ganglion cells which play an important roll in edge detection

Question 27

What is the nerve fiber layer?

Answer 27

-where the optic nerve exits the eye to project to subcortical and cortical visual areas

Question 28

In what three ways is human vision backwards in nature?

Answer 28

1. when light is focused on the retina, images are flipped upside down and are mirrored which create a complete flipped inverse of what is actually present in the visual field; orientation is corrected along the central visual processing pathways
2. the photoreceptors or the photoreceptors layer is located most posterior to the layer of the retina which means light has to pass through all the other cells layer before it hits the photoreceptors
   3.photoreceptors hyperpolarize in response to light

Question 29

What are bipolar cells?

Answer 29

-there are on-or on-center bipolar cells which depolarize in response to light
-there are off or off-center bipolar cells which hyperpolarize in response to light

Question 30

What is a sign-inverting synapse?

Answer 30

photoreceptor becomes hyperpolarized in response to light which causes the bipolar cell to be depolarized or an oncenter bipolar cell

Question 31

What is a sign conserving synapse?

Answer 31

when a photoreceptor becomes hyperpolarized in response to light which causes the off bipolar cell to hyperpolarize as well

Question 32

What is lateral inhibition and what cells partake in it?

Answer 32

horizontal and amacrine cells in the inner nuclear layer modulate downstream signaling of bipolar and retinal ganglion cells so that erroneous signaling does not occur
-horizontal cells inhibit the bipolar cells and the amacrine cells laterally inhibit RGCs

Question 33

What are retinal ganglion cells able to exhibit?

Answer 33

edge detectors

Question 34

What are the two types of RGCs?

Answer 34

on center - off surround
off center - on surround

Question 35

What happens during phototransduction?

Answer 35

-when a photoreceptor hyperpolarizes the sodium channel closes which causes the opsin or rhodopsin GPCR receptor to become activated in response to light
-activation of the G-protein causes the secondary messenger cyclic GMP phosphodiesterase to activate
-cGMP phosphodiesterase hydrolyzes cGMP which reduces it concentration in the photoreceptor
-the reduction in cGMP causes the voltage gated sodium channel to close leading to hyperpolarization

Question 36

What happens during light and dark adaptation?

Answer 36

your photoreceptors become less or more sensitive based on the environment you are in