Physiology of Visual System

Question 1

• The eye uses _ to focus image on retina
• Which site accounts for most?
• When the lens is rounder is there more or less?
• When the lens is flatter is there more or less?

Answer 1

• Refraction
• Cornea (also first site)
• Rounder lens=more refraction
• Flatter lens=less refraction

Question 2

• The curvature of the lens requires what muscle and ligaments?
• Increasing curvature occurs with contraction or relaxation of these muscle/ligaments?
• Decreasing curvature occurs with contraction or relaxation of these muscles/ligaments?

Answer 2

• Ciliary m and suspensory L
• Increasing curvature
  
  • Ciliary m contracts
  • Suspensory L relaxes
• Decreasing curvature
  
  • Ciliary m relaxes
  • Suspensory L contracts

Question 3

• Describe the near vision response and how it works in the eye

Answer 3

1. Contraction of ciliary muscles
2. Convergence of eyes to point of focus
3. Constriction of pupil
   
   1. Reduces opening for light to enter
   2. Eliminates diverging rays
   3. Allows better focus

Question 4

• Describe the far vision response and how it works in the eye

Answer 4

• Ciliary muscles relax
• Suspensory ligament pulls on lens
• Lens becomes flatter to focus on distant objects

Question 5

• What are the 5 neuron types in the retina?
• Where do electrical signals begin?

Answer 5

• Vertically oriented
  
  • Receptor cells (rods and cones)
  • Bipolar cells
  • Ganglion cells
    
    • MG subtype of ganglion cells
• Horizontally oriented cells
  
  • Horizontal cells
  • Amacrine cells

Question 6

• Rod system

Answer 6

• Lots of convergence from many rods onto one ganglion cell
• Allows rods to operate in dim light
• Sacrifices acuity to gain sensitivity

Question 7

• Cone system

Answer 7

• Less convergence
• Maximal acuity

Question 8

• The highest density of cones is located in the _ whereas the highest density of rods are _

Answer 8

• Fovea
• Periphery (20 deg away from fovea)

Question 9

• Rods and cones constantly release what NTX?

Answer 9

• Glutamate

Question 10

• Release of glutamate is highest when it is _
• Release of glutamate is lowest when it is _

Answer 10

• Dark
• Light

Question 11

• Activation of bipolar cell by a cone receptor in the light (just as an example)

Answer 11

• Photons will stimulate cone receptor
• Photoreceptor hyperpolarizes
• Less glutamate is released onto the bipolar cell

Question 12

• On center bipolar cells /Depolarizing Bipolars

Answer 12

• Activation of a photoreceptor in the center of this bipolar cell’s receptive field will cause depolarization
• Activation in the periphery of this cell causes hyperpolarization

Question 13

• Off center bipolar cells /Hyperpolarizing Bipolars

Answer 13

• Activation in center of this bipolar cell will cause hyperpolarization
• Activation in periphery of this bipolar cell will cause depolarization

Question 14

• The glutamate receptor in a depolarizing bipolar/on center cell is a _ receptor
• The glutamate receptor in a hyperpolarizing bipolar/off center cell is a _ receptor

Answer 14

• GPCR (Gi)-sign change occurs
• non-NMDA (AMPA and Kainate)-no sign change

Question 15

• Example:
  
  • Activation of an on-center/depolarizing bipolar by a cone receptor in DARK

Answer 15

• Dark-more glutamate
• Glutamate will activate GPCR (Gi) on ON-Center Bipolar Cell
• Results in decrease in cation influx in bipolar cell
• Hyperpolarization of the cell

Question 16

• Example
  
  • Activation of an on-center/depolarizing bipolar cell by a cone photoreceptor in the light

Answer 16

• Light-less glutamate release

1. Light decreases presence of glutamate
2. Less glutamate around
3. Less activation of metabotropic receptor (GPCR-Gi) of on-center cell
4. Less Gi signaling
5. Increase in cation influs into bipolar cell
6. Depolarization of cell

Question 17

• Example:
  
  • Activation of an off-center bipolar cell by a cone photoreceptor in the dark

Answer 17

• Dark-more glutamate

1. More glutamate present (cuz we are in the dark)
2. Glutamate activates AMPA receptor on off center cell
3. Increase cation influx
4. Depolarizaton of the cell

Question 18

• Example
  
  • Activation of an off center bipolar cell by a cone photoreceptor in the light

Answer 18

• Light-less glutamate

1. Light decreases presence of glutamate
2. Less glutamate around
3. Less activation of AMPA receptor of off-center bipolar cell
4. Decrease in cation influx into bipolar cell
5. Hyperpolarization of the cell

Question 19

_ axons become fibers of the optic nerve

Answer 19

• Ganglion cell

Question 20

Ganglion cells release what NTX in the cortex?

Answer 20

• Glutamate

Question 21

• Activation of a bipolar cell by rod photoreceptors

Answer 21

1. Rods signal vision in low light situations
2. Many rods converge onto one on-center bipolar cell
3. COnnects to a rod-bipolar cell and a rod amacrine cell (use glycine or GABA to inhibit cone pathway)

Question 22

• What does the on-center system tell us?
• What does the off-center system tell us?
• What do the amacrine and horizontal cells do?

Answer 22

• On center system tells us
  
  • Where something is
• Off center system tells us
  
  • Where something ends
• Horizontal/Amacrine cells release GABA/Glycine to inhibit nearby cells in retina

Question 23

• Direct targets of the retina

Answer 23

• LGB (Lateral Geniculate Body)
• Superior colliculus
• Pretectum
• Hypothalamus
• AON (Accessory optic nuclei)
• Pulvinar

Question 24

• Properties and functions of the LGB

Answer 24

• Signals from two eyes kept apart
• Functions
  
  • Control motions of eyes to converge on point of interest
  • COntrol focus of eyes based on distance
  • Determine position of objects to map them in space
  • Detect movement relative to an object
• Relay cells via optic radiations to visual cortex on IPSILATERAL side

Question 25

• What layer of the primary visual cortex (V1) receives input from the LGB?
• What layers of V1 send outputs? Where do these outputs go?

Answer 25

• IV
• V and VI
  
  • LGB
  • Thalamus
  • Subcortical regions

Question 26

• Ocular dominance columns
  
  • Where are they located?
  • What do they do?

Answer 26

• Located in V1 (Primary Visual Cortex)
• Columns spand all 6 layers
  
  • Preferentially respond to input from one eye or the other (hence the name ocular dominance)

Question 27

• Orientation columns
  
  • Where are they located?
  • What is their function?

Answer 27

• Also located in V1
• Columns that span all 6 layers of V1
• Excited by visual line stimuli of varying angles
• Oriented perpendicular to the cortical surface

Question 28

• Blobs
  
  • Where are they located?
  • What is their function?

Answer 28

• Located in V1
• Span all 6 layers of V1
• Sensitive to color

Question 29

• Major job of V1

Answer 29

• Identify edges and contours of objects

Question 30

• Function of V2

Answer 30

• Depth perception

Question 31

• Visual cortex (V3a) function

Answer 31

• Identification of motion

Question 32

• Function of V4 area of visual cortex

Answer 32

• Complete processing of color inputs

Question 33

Dorsal pathway (Where pathway)

Answer 33

• From primary visual cortex to parietal/frontal cortex
• Parimary path associating vision with movement
• Completes motor acts based on visual input
• Passes thru V3

Question 34

• Ventral Pathway (What Pathway)

Answer 34

• From primary visual cortex to inferior temporal cortex
• Involved in interpreting images (recognizing or copying shapes, forms, faces) and complex patterns

Aside:

Copying/naming objects are separate functions. Damage to one is possible without damaging the other

Facial recognition is in a specialized area

Question 35

MG Ganglion Cells

Answer 35

• Light, via melanopsin causes changes in Ca2+ levels in MG cells
• non-image forming but sends this information to the hypothalamus (suprachiasmatic nucleus)
• AIDS w. CIRCADIAN RHYTHMS