Lecture 7- Output targets of ganglion cells

Question 1

What are the layers in the retina?

Answer 1

• pigmented epithelium
• outer nuclear layer
• outer plexiform layer
• inner nuclear layer
• inner plexiform layer
• ganglion cell layer

Question 2

What are the six neuron types in the retina?

Answer 2

-rods, cones, horizontal cells, bipolar cells, amacrine cels, ganglion cells

Question 3

What are the two synaptic layers of the retina?

Answer 3

1. Outer plexiform layer= the photoreceptors synapse with second order neurons (bipolar cells) (outer retina) 2. Inner plexiform cells= in inner retina, bipolar cells synapse with ganglion cells

Question 4

What must light go through before hitting the photoreceptors?

Answer 4

-light must pass through all the retinal layers

Question 5

How is the retina wired up?

Answer 5

• have the through pathway= photoreceptors then bipolar cells and then ganglion cells, those send signals to the brain
• this pathway is modulated via lateral interactions
• horizontal cells in the outer retina and amacrine cells in the inner retina modulate

Question 6

What are the ganglion cells?

Answer 6

-output neurons

Question 7

Are there more types of ganglion cells?

Answer 7

-yes, many types -each type of ganglion cell provides information for a different stream of vision

Question 8

What are the two main types of ganglion cells?

Answer 8

1. M ganglion cells (magnocellular or also called parasol), motion, 10% of ganglion cells, can tell if something is moving 2. P (parvocellular or midget ganglion cells) = colour vision, visual acuity

Question 9

What is the problem with John?

Answer 9

his M ganglion cells are dying, due to leucoma, the pressure in his eyes is built up and the big ganglion cells are hit first, can be treated

Question 10

What are the 5 targets of ganglion cell axons?

Answer 10

1. Lateral geniculate nucleus (thalamus): Major target of most GCs (90%), Visual pathway 2. Pretectum (midbrain): Pupil responses 3. Suprachiasmatic nucleus (hypothalamus): Circadian rhythm 4. Superior colliculus: Eye movements 5. Other: various nuclei of thalamus: Photophobia, visual aspects of pain -the points 2-5 are non visual aspects of vision, control of bodily functions depending on light

Question 11

How many layers does the LGN have?

Answer 11

-6 -have one LGN in the left and on in the right hemisphere

Question 12

Which LGN layers do the P ganglion cells input in?

Answer 12

-layers 3-6 (4 layers, 2 for each eye)

Question 13

Which LGN layers do the M ganglion cells input in?

Answer 13

-layers 1 and 2 -one layer for each eye

Question 14

What are the white layers between the M and P input layers in the LGN?

Answer 14

• Koniocellular layers
• in between
• inputs from variety of ganglion cells including bistratified ganglion cells, encode aspects of colour vision

Question 15

Where is each half of the visual field represented?

Answer 15

-on the contralateral visual cortex

Question 16

What are the intrinsically photosensitive ganglion cells? (ipGCs)

Answer 16

• a small population of ganglion cells contain a visual pigment called melanopsin -melanopsin is similar to visual pigments found in invertebrates -light activation of melanopsin leads to depolarisation of ipGCs (so opposite of the photoreceptors) -there are multiple types of the ipGCs–they are sparse -cannot tell brain what you are seeing as span too much of the retina -melanopsin = depolarisatuion (so opposite to photoreceptors) -let people know how much light -mood disorfders when not enough light -that is how blind people can get jet lagged and so on

Question 17

What are the functions of the ipGCs?

Answer 17

1: Circadian rhythm 2. pupil responses 3. photophobia

Question 18

What is the connection between ipGCs and the circadian rhythm?

Answer 18

• In blind people, melatonin levels are suppressed by light (like sighted people). -Activity & Sleep in the blind patients is not disturbed. -melatonin= hormone that controls sleep -affected by when the lights are on and off -so even blind people can have this -SCN is a small nucleus within the hypothalamus that is important for driving circadian rhythm. - ipGCs project to the suprachiasmatic nucleus (SCN) of hypothalamus. -tell the hypothalamus if lights on or off= based on that the SCN produces melatonin

Question 19

What do pupils do when you shine light onto them?

Answer 19

-constrict, even when shine into just one eye the other also constricts

Question 20

What do pupil responses depend on?

Answer 20

Depends on being able to detect light

• Depends on functioning iris:
  a) Muscle for pupil constriction= sphincter pupillae
  b) Muscle for pupil dilation= dilator pupillae
• have two types of muscle across the iris of our eye ring= sphincter pupillae muscle= contracts of the pupil, constrict the pupil
• dilator pupillae= pull up the pupil open= dilation of the pupil
• they are controlled by the light

Question 21

What is the circuit controlling the pupil responses?

Answer 21

midbrain is very important for vision= moving the eyeballs and contrcition and dilation of pupils

• these ganglion cells fdon’t go to LGN where would synapse in thalamus, they exit and go pretectal nucleus in midbrain and froim there to midbrain third nerve nucleus
• and that goes to the sphincter muscle in each eye

Question 22

Do blind people have pupil constriction?

Answer 22

this is all beginning with the intrinsically photosensitive ganglion cells -blind people have a pupil response but smaller than normal people -blind people do but 50% of normal -so pupil responses are not entirely conrolled by melanopsin but a lot

Question 23

What is the connection between the ipGCs and photophobia?

Answer 23

• Migraine aura are often visual
• Light exposure worsens pain
• experiment: even blind people get photophobia during migraines how? = ipGCs
• Migraine pain from dura is communicated via CNV (trigeminal nerve) ultimately to posterior thalamus and then to cortex

␣ ipGCs project to posterior thalamic nucleus

• anothe rplace these ganglion cells target:
• blood vessels= cause the pain of the headache, info goes via trigeminal nerve to the spine and then to thalamus
• input the information about pain in the posteriro nucleus of the thalamus= the part that has the most to do with our ability to detect pain
• it gets input from the eye -that is why light can make pain worse

Question 24

Summary:

Answer 24

Ganglion cells: ␣ Important for image and non-imaging forming functions ␣ Midget, parasol, bistratified: encode different types of information and target different layers of the LGN. ␣ Vision forming targets (Visual pathway) ␣ GCs-optic nerve-chiasm, LGN, optic radiations, visual cortex. ␣ Non-visual functions of GCs ␣ Intrisically photosensitive ganglion cells ␣ Target: ␣Suprachiasmaitc nucleus (circadian rhytham ␣Pretecal nucleus (midbrain)-pupil responses ␣Superior colliculus ␣Thalamic nuclei