Lecture 5: Physiology of the Visual System

Question 1

What is Refraction?

Answer 1

Fact or phenomenon of light, radio waves, etc., being deflected in passing obliquely through the interface between one medium and another or through a medium of varying density

Question 2

What part of the eye is the first involved in refraction of light and what other structure in the eye also plays a role; which is fixed and which is variable?

Answer 2

• The cornea is the first site of refraction (is fixed) = 2/3’s of light bending
• The lens adds variable amount of bending, depending on the curvature of the lens

Question 3

How does a round vs. flat lens affect the refraction of light?

Answer 3

Round = more refraction

Flatter = less refraction

Question 4

What is involved in increasing the curvature of the lens of eye and what kind of vision is this useful for?

Answer 4

• Ciliary muscle contracts, which allows suspensory ligaments to loosen
• Lens assumes a more rounded shape by natural recoil
• Used for near vision

Question 5

What is involved in decreasing the curvature of the lens of eye and what kind of vision is this useful for?

Answer 5

• Ciliary muscle relaxes, which causes the suspensory ligaments to tighten
• The lens is pulled tight, flattening it.
• Used for far vision

Question 6

When an object is far away, what happens to the light rays entering our eyes and how does this play a role in adjusting our vision?

Answer 6

• Light rays have diverged far enough that only parallel light rays enter the eye
• They don’t need to be bent a lot to focus on retina, so the lens is best off flat

Question 7

When an object is nearby, what happens to the light rays entering our eyes and how does this play a role in adjusting our vision?

Answer 7

• Light rays are still diverging and need a lot of bending to focus on the retina
• Requires our body to increase the curvature of the lens to compensate

Question 8

What is the purpose of the increased curvature of the lens of the eye and what occurs with age?

Answer 8

• Allows light rays from a nearby object to be bent sufficiently to bring them into focus on the retina
• With age, the lens stiffens and loses elasticity = presbyopia = near vision becomes more difficult

Question 9

Near visions requires 2 additional changes on top of contraction of the ciliary muscle, what are they?

Answer 9

1) Convergence of the the eyes to the point of focus
2) Constriction of the pupil

Question 10

How does constriction of the pupil contribute to near vision?

Answer 10

• Reduce the opening for light to enter
• Eliminates some of the diverging light rays, so we don’t need to bend as many to focus on the retina
• Allows us to focus better

Question 11

Which 3 cells are in the vertical pathway of the retina and what is this pathway used for?

Answer 11

1) Photoreceptor
2) Bipolar cell
3) Ganglion cell
- This is the pathway for relaying the visual info to the brain

Question 12

Glutamate release within the retina is highest when?

Answer 12

• HIGHEST in the dark
• No stimulation by photons

Question 13

Glutamate release within the retina is lowest when and why?

Answer 13

• LOWEST when there is light
• Stimulation by photons causes the cells to hyperpolarize

Question 14

What are the 3 steps in the activation of the bipolar cells by cone photoreceptors?

Answer 14

1. Photon stimulates photoreceptor
2. Photoreceptor hyperpolarizes
3. Glutamate release onto the bipolar cell DECREASES

Question 15

What are ON-center bipolar cells, how are they activated, and what increases their discharge rate?

Answer 15

• Activation of a photoreceptor in the center of this bipolar cell’s receptive field causes depolarization of this bipolar cell
• Activation in the periphery of this bipolar cell’s receptive field causes hyperpolarization of this bipolar cell
• These cell increase their discharge rate to luminance increments in the receptive field center

Question 16

What are the OFF-center bipolar cells, how are they activated, and what increases their discharge rate?

Answer 16

• Activation of a photoreceptor in the center of this bipolar cell’s receptive field causes hyperpolarization of this bipolar cell
• Activation in the periphery of this bipolar cell’s receptive field causes depolarization of this bipolar cell
• These cell increase their discharge rate to luminance decrements in the receptive field center

Question 17

What is the glutamate receptor in depolarizing bipolar cells and what is its function?

Answer 17

• A metabotropic receptor (Gi, GPCR) that is activated by glutamate
• Closes cGMP-gated ion channels similar to light transduction in photoreceptors = hyperpolarizes cell
• Channels open when less glutamate is present

Question 18

Explain the process of activation of an ON-center bipolar cell by cone photoreceptors in light conditions.

Answer 18

• Photons decrease the presence of glutamate
• Less glutamate = less activation of Gi-GPCR metabotropic receptor on the ON-center bipolar cell
• Less Gi signaling
• Results in an increase in cation influx into the bipolar cells = DEPOLARIZATION

Question 19

What type of glutamate receptor found on OFF-center bipolar cells and what is the effect of light and dark on them?

Answer 19

• An AMPA receptor is activated and glutamate causes an INCREASE in cation influx (Na+ and some Ca²⁺)
• In the dark, the OFF-center cell is depolarized since glutamate release is high
• With light, glutamate release from the photorecptor decreases and AMPA receptor is NOT activated = OFF-center cell hyperpolarized

Question 20

Explain the activation that occurs to the OFF-center bipolar cells when light hits cone photoreceptors?

Answer 20

• Light decreases the presence of glutamate
• Less glutamate = less activation of AMPA receptor on the OFF-center bipolar cell
• Results in decrease in cation influx into the bipolar cell
• Hyperpolarizes the cell

Question 21

Explain the activation that occurs to the ON-center bipolar cells in the dark by cone photoreceptors?

Answer 21

• Glutamate is high and activates the Gi GPCR-metabotropic receptor on the ON-center bipolar cell
• Results in a decrease in cation influx into the bipolar cell
• Hyperpolarizes the cell

Question 22

Explain the activation that occurs to OFF-center bipolar cells by cone photoreceptors in the dark?

Answer 22

• Increased glutamate would activate AMPA receptor on the OFF-center bipolar cell
• Results in an increase in cation influx into the bipolar cell (Na+ and Ca²⁺)
• Depolarizes the cell

Question 23

How are ganglion cells linked to bipolar cells, how are they activated, rely on what NT?

Answer 23

• Ganglion cells are also ON-center and OFF-center varieties (whichever kind of bipolar cell its connected to)
• When bipolar cell is activated, releases glutamate to depolarize the ganglion cell
• Depolarization —> threshold —> AP

Question 24

Ganglion cell axons become ______

Answer 24

the fibers of the optic nerve

Question 25

How is a bipolar cell activated by **rod** photorecptors?

Answer 25

- Many **rods** converge on one **ON-center Bipolar cell** - Which then synapses on an **A11-amacrine cell** and **rod-bipolar cell** - Which then synapses on a **Cone ON-Center bipolar** cell - Then activates a **Ganglion cell**

Question 26

Why do we require many rods to coverge on a bipolar cell?

Answer 26

- Rods do vision in **low light** situation - Many rods converging on a bipolar cells increases our chanches of seeing what we need to see

Question 27

What is the specific function of ON-center and OFF-center cells in refining the signal to the retina (i.e., tells us what)?

Answer 27

- The **ON-center** cell tells us **where** **something is** - The **OFF-center** cell tell us **where it ends**

Question 28

What is the function of amacrine cells and horizontal cells in refining the signal in the retina?

Answer 28

Work by providing **inhibitory (GABA/glycine) modulation** that modifies the activity of neighboring photoreceptors, bipolar cells, or ganglion cells

Question 29

Where is the AP related to vision and activation of the retina actually occuring?

Answer 29

Ganglion cells - axons of these cells become fibers of optic nerves

Question 30

Light from the right visual field hits what part of each retina?

Answer 30

- The **left temporal** retina of **left eye** - The **right nasal** retina of **right eye**

Question 31

What are the 4 main functions of the LGN?

Answer 31

1) Control the motion of the eyes to **converge** on point of interest 2) Control the **focus** of the eyes based on distance 3) Determining relative position of objects to **map** them in space 4) Detect **movement** relative to an object

Question 32

The primary visual cortex has how many layers and what is each responsible for?

Answer 32

**- Six layers** - Layers **1, 2** and **3** allow networking betweem V1 and other parts of cortex - Layer **4** receives **inputs** from the **LGN** - Layers **5 and 6** send information **BACK to LGN**

Question 33

What is the Ocular dominance of columns in primary visual cortex?

Answer 33

- Columns span all 6 layers of the cortex - A slab of cells that **preferentially** respond to input from one eye or the other (ocular dominance) - Each column does a different job and neighboring columns have related jobs

Question 34

What is the major job of the primary visual cortex (V1)?

Answer 34

Identify the **edges/contours** of the objects in our visual image

Question 35

What is the major job of the primary visual cortex V2?

Answer 35

- Identify **disparities** in the visual images presented by two eyes - These disparities are used for **DEPTH PERCEPTION**

Question 36

What is the function of neurons knows as 'blobs' found within the columns of the visual cortex?

Answer 36

- Organized region of neurons that are **sensitive** to **color** - **ALL 3 color-coding** cones are **required for accurate color detection**

Question 37

What is the major job of the primary visual cortex area V3A?

Answer 37

Identification of motion

Question 38

What is the major job of the primary visual cortex area V4?

Answer 38

To **complete** the **processing** of **color inputs**

Question 39

What is the Dorsal pathway for the processing of visual inputs and where does it arise from?

Answer 39

- From **V3** of the primary visual cortex and goes to the parietal/frontal cortex - Enables us to complete motor acts based on visual input

Question 40

What is the Ventral pathway for the processing of visual inputs and where does it arise from?

Answer 40

- From primary visual cortex to the **inferior temporal cortex**, allowing for the integration of visual input with other sensory inputs to accomplish higher order functions - **Primarily** involved in **interpreting images** (recognizing or copying shapes, forms, faces) and **complex patterns**

Question 41

The ventral pathway relays visual information to areas of brain involved in higher processing of sensory inputs such as what; why does damage to this area not always lead to a complete loss of function?

Answer 41

- **C****opying/naming**objects are**separate functions** in the temporal lobe. - Damage to one area is possible **WITHOUT** damaging the others - **Facial** recognition is a **specialized area**