Lecture 46: Vision 2

Question 1

Describe the neural components of the retina:

Answer 1

• Considered part of the brain
• Neurons in the retina:
• Photoreceptors
• Ganglion cells
• Interneurons

Question 2

What are ganglion cells?

Answer 2

• Send axons that form optic nerve
• Sends information that has been processed by the neural circuit involving interneurons

Question 3

What are interneurons?

Answer 3

• Bipolar cells: Relays information from photoreceptors to retinal ganglion cells
• Amacrine cells and horizontal cells: Provide lateral inhibition

Question 4

What are photoreceptors?

Answer 4

• Light sensitive cells
• Furthest from incoming light
• Rods and cones

Question 5

Where are light sensitive proteins expressed?

Answer 5

On outer segments of photoreceptors

Question 6

What are rods?

Answer 6

• 120 million per retina
• Few in fovea
• Function in low light
• Do not contribute to colour processing regions- less colour visible in low light

Question 7

Describe the structure of rods:

Answer 7

Outer segment stacked with membranous discs (free floating discs)
* Increases surface area of membrane
* Increased area for light sensitive membrane proteins

Question 8

What are cones?

Answer 8

• 8 million per retina
• Many in fovea
• Require relatively high
  light levels
• 3 types: Each most sensitive to red, green, and blue light

Question 9

Describe the structure of cones:

Answer 9

Outer segment is smaller and formed by continuous folding extensions of the
membrane
* High surface area but less than rods

Question 10

Where are photopigments found and what is the function of photopigments?

Answer 10

Photoreceptors contain photopigments
- Give capacity to respond to light

Question 11

Name the 2 components of photopigment:

Answer 11

1. Opsin
2. Retinal

Question 12

What is opsin and what type is found in rods and cones?

Answer 12

Transmembrane receptor (GPCR)
* Function: Binds retinal and triggers signalling cascade
* Rods: rhodopsin
* Cones: either S(blue), M(green), or L(red) photopsin

Question 13

What is retinal?

Answer 13

A chromophore
Function: Absorbs light and changes shape
* Derived from Vitamin A: Lack of vitamin A leads to “night blindness”

Question 14

Describe the mechanism of colour vision:

Answer 14

1. Cones express one of three types of photopsins: S, M, or L, each tuned to a specific wavelength range
2. Each photopsin creates a unique chemical environment around retinal, shifting its absorption peak
3. Alters the wavelength of light most efficiently absorbed (blue, green, or red)
4. Perception of colour is created by relative activation of the 3 cone types

Question 15

What is colour blindness?

Answer 15

• Can be inherited (congenital) or acquired
  (due to disease)
• Congenital forms affect 8% of males and 0.5% of females
• Genes encoding production of M & L opsins are on the X chromosome
• Various forms of colour blindness exist

Question 16

Describe the overall process of phototransduction:

Answer 16

• Opsins are membrane GPCRs with attachment site for retinal
• Retinal undergoes photoisomerization upon light absorption:
• 11-cis retinal (in dark): inactive form
• All-trans retinal (in light): active form
• Structural change in retinal activates opsin and initiates G-protein cascade

Question 17

Describe phototransduction in the dark:

Answer 17

• No light: Retinal non activated (inactive 11-cis isoform)
• Guanylyl cyclase converts GTP to cGMP.
• cGMP-gated channels are open
• Influx of Na+/Ca2+ Ions → Depolarisation
• Glutamate released onto bipolar cells

Question 18

Describe phototransduction in the light:

Answer 18

• Retinal activated (all-trans isoform)
• Activation of the G-protein transducin and subsequently the enzyme phosphodiesterase
• Phosphodiesterase breaks down cGMP
• Unbinding of cGMP from cation channel
• Hyperpolarisation

Question 19

What does the peak and duration of photoreceptor hyperpolarisation depend on?

Answer 19

Intensity of light

Question 20

What is relative to the peak and duration of hyperpolarisation?

Answer 20

The magnitude of glutamate release inhibition

Question 21

Describe the speed of the response to light intensity:

Answer 21

Relatively slow process

Question 22

Describe retinal processing:

Answer 22

• Signals from photoreceptors and retinal
  interneurons combine
• Electrical responses of ganglion cells depend on spatial and temporal pattern of light stimulation on the retina
• Visual perception is dependent on sensory experience

Question 23

Describe the receptive field of the fovea:

Answer 23

• Small receptive fields
• Enables high acuity and fine spatial discrimination

Question 23

Describe the receptive field of the peripheral retina:

Answer 23

• Larger receptive fields
• Greater light sensitivity

Question 23

What is a receptive field in the retina?

Answer 23

Input to each ganglion cell arises from neighbouring photoreceptors in a defined area of the retina

Question 24

Describe the ganglion cell receptive field:

Answer 24

* Network of photoreceptors are roughly circular * Divided into two parts: - Centre: Circular central zone - Surround: Annulus region around the centre

Question 25

What do ganglion cells respond best to?

Answer 25

* Differences in illumination between the centre and surround * Most action potentials are initiated in response to CONTRAST, not uniform brightness.

Question 26

What are ON bipolar cells?

Answer 26

* Express mGluR6 glutamate receptors (inhibitory) * Light -> ↓glutamate -> ON bipolar cell activation * Synapse onto centre of on-centre field ganglion neuron

Question 27

What are OFF bipolar cells?

Answer 27

* Express AMPA glutamate receptors * Light -> ↓glutamate -> OFF bipolar cell inhibition * Synapse onto centre of off-centre field ganglion neuron

Question 28

What cells fine tune the light that hits the receptive fields?

Answer 28

* Horizontal cells: mediate surround inhibition by process of lateral inhibition (within receptive field) * Amacrine cells inhibit neighbouring ganglion neuron pathways (adjacent receptive fields)

Question 29

What happens when placed in room with no objects?

Answer 29

* Bright but no contrast * No changes in retinal ganglion cell firing * No structured visual perception

Question 30

What happens when objects appear in a room?

Answer 30

1. Contrast introduced (edges and shapes) 2. Receptive fields detect more light in centre or surround 3. Ganglion cells change their firing rates depending on: - their location in the retina (part of the visual space) - on/off receptive fields 4. Brain integrates input to construct visual experience of objects in space

Question 30

What type of ganglionic cells are found in the peripheral retina?

Answer 30

* High convergence ganglion cells * Large receptive fields → low acuity * Rods and cones (high light sensitivity) * Parasol ganglion cell (large dendritic tree)

Question 30

Describe the visual pathway from the ganglionic cells?

Answer 30

1. Axons of retinal ganglion cells form optic nerves 2. Leave eye at optic disc 3. Optic chiasm, axons partially decussate: - Nasal axons decussate to contralateral hemisphere - Temporal axons remain on ipsilateral hemisphere 4. Ensures that: - Left visual field project to right brain - Right visual field project to left brain 5. Axons synapse onto neurons in the lateral genicular nucleus (LGN) of thalamus 6. LGN neurons project to primary visual cortex in occipital lobe

Question 31

What type of ganglionic cells are found in the fovea?

Answer 31

* Low convergence ganglion cells * Small receptive fields → high acuity * Cones (requires bright light) * Midget ganglion cell (small dendritic tree)

Question 32

Where does ganglion cell axons project?

Answer 32

4 main subcortical visual areas 1. Superior colliculus * Eye movements and orientation to visual stimuli for stable visual image 2. Lateral Geniculate Nucleus * Sensation of vision 3. Pretectum * Control of pupils (autonomic NS) 4. Suprachiasmatic nucleus * Control of diurnal rhythms (light entrainment)