Lecture 7- Output targets of ganglion cells Flashcards
What are the layers in the retina?
- pigmented epithelium
- outer nuclear layer
- outer plexiform layer
- inner nuclear layer
- inner plexiform layer
- ganglion cell layer
What are the six neuron types in the retina?
-rods, cones, horizontal cells, bipolar cells, amacrine cels, ganglion cells
What are the two synaptic layers of the retina?
- 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
What must light go through before hitting the photoreceptors?
-light must pass through all the retinal layers
How is the retina wired up?
- 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
What are the ganglion cells?
-output neurons
Are there more types of ganglion cells?
-yes, many types -each type of ganglion cell provides information for a different stream of vision
What are the two main types of ganglion cells?
- 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
What is the problem with John?
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
What are the 5 targets of ganglion cell axons?
- 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
How many layers does the LGN have?
-6 -have one LGN in the left and on in the right hemisphere
Which LGN layers do the P ganglion cells input in?
-layers 3-6 (4 layers, 2 for each eye)
Which LGN layers do the M ganglion cells input in?
-layers 1 and 2 -one layer for each eye
What are the white layers between the M and P input layers in the LGN?
- Koniocellular layers
- in between
- inputs from variety of ganglion cells including bistratified ganglion cells, encode aspects of colour vision
Where is each half of the visual field represented?
-on the contralateral visual cortex
What are the intrinsically photosensitive ganglion cells? (ipGCs)
- 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
What are the functions of the ipGCs?
1: Circadian rhythm 2. pupil responses 3. photophobia
What is the connection between ipGCs and the circadian rhythm?
- 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
What do pupils do when you shine light onto them?
-constrict, even when shine into just one eye the other also constricts
What do pupil responses depend on?
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
What is the circuit controlling the pupil responses?
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
Do blind people have pupil constriction?
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
What is the connection between the ipGCs and photophobia?
- 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
Summary:
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
