Lecture 4: Visual systems

Question 1

How does light reach the receptor cells? (pathway)

Answer 1

Ganglion cells –> Amacrine cells –> Bipolar cells –> Horizontal cells –> Receptor cells (rods & cones)

(-> pigment layer –> choroid)

Question 2

What is the blind spot in the eyes?

Answer 2

The blind spot is a small area in the retina where the optic nerve exits the eye and connects to the brain. It lacks photoreceptor cells (rods and cones) making it incapable to detect light.

Question 3

Why do people have red eyes in flash photographs?

Answer 3

A flash is a sudden, intense burst of light. In a dimly lit environment, the light enters the eye through the pupil and reflects the back of the eyes –> retina + choroid. These areas are rich in blood vessels, hence why the eyes will appear red on the photo.

Question 4

What is the ‘tapetum lucidum’ and how does it contribute to night vision?

Answer 4

The tapetum lucidom is a light-reflecting cell later between the retina (receptor cell layer) and the choroid. The layer reflects light back through the retina, effectively giving photoreceptor cells a second chance to detect the light.
This increases the amount of light available for vision, enhancing an animals ability to see in the dark.

Question 5

How are amacrine cells and horizontal cells involved in the organization of the retinal network?

Answer 5

Both cells act as lateral connection, affecting the retinal information processing:
- Amacrine cells receive input from bipolar cells and project to ganglion cells, bipolar cells and other amacrine cells.
- Horizontal cells receive input from photoreceptors and project to other photoreceptors.

Question 6

Describe the shape of rods and cones:

Answer 6

• Rods are large and cylindrical
• Cones are small and tapered

Question 7

What is the function of rods and cones and in what condition do they react?

Answer 7

Rods and cones are the sensory receptor cells in which transduction takes place (light to receptor potential)
- Rods react to weak light
- Cones react to bright light and color photo pigments

Question 8

What are photopigments and what is their function?

Answer 8

Light sensitive molecules located in rods and cones. They play a crucial role in phototransduction.

Question 9

What is the difference in photopigments between rods and cones?

Answer 9

• Rod has a lot of photopigment within multiple discs in the outer segment. All the photopigments are the same.
• Cones have less photopigment. There are 3 types of cones in which the photopigments differ in sensitivy to photons in different wavelengths.

Question 10

Describe the 3 different ranges of vision.

Answer 10

• Scotopic: vision in low-light conditions (night time), no colour vision –> primarly mediated by rods
• Mesopic: vision in intermediate light conditions (indoor lightening), mediated by rods and cones
• Photopic: vision in well-lit conditions (sunlight), primarly mediated by cones

Question 11

How is the rod and cone ratio in the fovea and the peripheral retina?

Answer 11

• Fovea only has cones
• Peripheral retina has both but more rods (NOTE: more sensitive to light - number of photoreceptor per ganglion cell is higher)

Question 12

When light falls on a photoreceptor this causes a …..

Answer 12

Hyperpolarization of the photoreceptor

Question 13

How does hyperpolarization occur when the photoreceptor aborbs light in rods?

Answer 13

• Photons are absorbed bij rhodopsin –>
• rhodopsin activates transducin (GDP exhanged for GTP) –>
• GTP-tranducin bind and activates phosphodiesterase (PDE) –>
• PDE converts cGMP into GMP –> - - [cGMP] decreases –>
• cGMP gated Na+/Ca2+ channels closes –>
• hyperpolarization –> less neurotranmitter release to bipolar and horizontal cells

Question 14

What happens with rods in the dark?

Answer 14

Depolarization of the receptor:
Tranducin stay binded to GDP, which keeps PDE inactive. [cGMP] can not be reduced. The cGMP gated Na+/Ca2+ channels can open en there is a Na+ influx. This causes depolarization which triggers neurotransmitter release to bipolar and horizontal cells

Question 15

How do the different type of cones contribute to discriminating colors of light?

Answer 15

• You have blue, green and red cones. All have a different sensitivity for wavelengths:
• Blue: short wavelengths
• Red: long wavelengts
• Green: intermediate wavelengths

Since they all respond at a different wavelengths, it is possible to distinguish color/intensity.

Question 16

Why can we not discriminate color with rods?

Answer 16

Humans only have one type of rods. Thereby, the receptor will respond the same to different wavelengths.

Question 17

What is trichromacy?

Answer 17

The theory that the color of light in our visual system is defined by the combination of the output of the 3 different types of cone receptor cells.

Question 18

What is wrong with the photoreceptors in the case of ‘color blindness’?

Answer 18

One of the cone types no longer work

Question 19

Make the right match:
A. Contrast
B. Brightness
C. Luminance

1. physical measure of light intensity
2. perception of luminance
3. difference in brightness between objects

Answer 19

A-3
B-2
C-1

Question 20

What is the network anatomy of the vertical and horizontal connections in the visual system?

Answer 20

• Vertical connections:
  photoreceptor –> bipolar cells –> ganglion cells –> CNS
• Horizontal connections:
  
  • photoreceptor/horizontal
    cell/bipolar cell
  • bipolar cell/amacrine
    cell/ganglion cell

Question 21

The receptive field of bipolar and ganglion cells consists of a circular center area and a ring surrounding the area:

What is the difference between on-center and off-center cells?

Answer 21

• on-center cells are activated by light in the center and inhibited when light hits the surround.
• off-center cells are activated by light in the surround and inhibited when light hits the center

Question 22

Why are there on- and off-center cells

Answer 22

On center ganglion cells detect stimuli that are lighter than the background. Off center detect stimuli darker that the background.
This gives 2 streams of information that provide information about changing luminance

Question 23

What happens in the case of a light spot center in a on-center and off center bipolar cell?

Answer 23

(CONE)
On center:
- Photoreceptor hyperpolarizes –> less Glu release –> less Glu can bind to mGluR6 of bipolar cell, so more Na+ channels can be open –> depolarization of bipolar cell –> more Glu release for AMPA kainate in ganglion cell –> triggers AP in ganglion cell

Off center:
- Photoreceptor hyperpolarizes –> less Glu release –> less Glu can bind to AMPA kainate of bipolar cell –> hyperpolarization of bipolar cell –> less Glu release for AMPA kainate in ganglion cell –> no AP in ganglion cell

Question 24

What happens in the case of a dark spot center in a on-center and off center bipolar cell?

Answer 24

CONE
On-center:
- photoreceptor depolarizes –> more Glu release –> more Glu can bind to mGluR6, so less Na+ channels open –> hyperpolarization of bipolar cell –> less Glu release for AMPA kainate in ganglion cell –> no AP

Off-center:
- photoreceptor depolarizes –> more Glu release –> Glu can bind to AMPA kainate –> depolarization of bipolar cell –> more Glu release for AMPA kainate of ganglion cell –> depolarization of ganglion cell –> AP

Question 25

Explain the function of horizontal cells in ‘center’ cells?

Answer 25

Horizontal cells are the link between the center-surround.
- Input of the receptor cells triggers Glu release, depolarizing the horizontal cell. In exchange horizontal cells will release GABA which hyperpolarizes the receptor cell. Thereby having a regulating role in Glu release.