The Lateral Geniculate Nucleus of the Thalamus (A model for all …people.musc.edu/~woodward/LGN...
Transcript of The Lateral Geniculate Nucleus of the Thalamus (A model for all …people.musc.edu/~woodward/LGN...
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Visual cortex
Retina Thalamus
The Lateral Geniculate Nucleus of the Thalamus
(A model for all thalamic ‘relays’)
Why not send projections from retina to visual cortex directly?
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2. Vision gone wrong:
a. Scotoma (lesions along the primary visual pathway from retina through primary visual cortex)
b. Perceptual deficits (motion, depth, places, faces – lesions beyond primary visual cortex)
c. Eye movements (saccades, smooth pursuit, vergence, - lesions in SC, FEF, pons, midbrain, SN, cerebellum)
d. Pupil (reflexes – lesions in pretectum, CN III, EW nucl. & “steady state size” – lesions in hypothalamus, RF, T1-T3)
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Figure 12.1 Central projections of retinal ganglion cells
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Vertical meridian
Horizontal meridian
Visual field
Projections of visual field onto retinas
Inversion and reflection
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(Fp-B-A) (D-C-Fp)
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Optic tract has Three (+1) main targets: 1. Pretectum 2. SC 3. LGN
(4. hypothalamus – light Sensing ganglion cells, So circadian clock resetting is intact in photoreceptor blindness)
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Figure 12.2 The circuitry responsible for the pupillary light reflex
LGN
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2. Superior Colliculus: eye movements & 2nd visual pathway
(LGN)
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up to pulvinar &
visual cortex
2. Superior Colliculus: eye movements & 2nd visual pathway (blindsight)
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LGN: sometimes also called the “dLGN” because it is in the dorsal thalamus. But because there is no similarly large ventral counterpart (at least in primates, probably), usually omit the “d”.
LGN lesions: extremely rare. (significant damage around the thalamus leads to coma etc. and then visual field defects might be the last of your worries).
Can result in incongruous homonymous hemianopias, and sometimes other strange horizontal or hourglass-shaped field defects.
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LGN MGN
Pulvinar
Massa intermedia
Third ventricle
Medial thalamic nuclei
Anterior thalamic nuclei
Lateral and ventral thalamic nuclei
Anterior Posterior
3. Lateral Geniculate
Nucleus: Part of the thalamus
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Human LGN (Nissl stain)
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Figure 12.15 Magno-, parvo-, and koniocellular pathways
LGN
affe
rent
targ
ets
in V
1
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(parvo, high SF)
(magno, larger RF, fast )
Excitatory Neurons (relay cells, project to striate cortex)
Inhibitory
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LGN: 1,4,6 (Contralateral eye!) 2,3,5, stay on the same side (Ipsilateral eye). Magno cells are ventral, layers 1 + 2. (Fast, no color, no high spatial frequencies, respond strongly to low contrasts, large cell bodes, big, fast myelinated axons). Parvo cells are dorsal, layers 3,4,5,6. (Slow, color, high spatial frequencies, respond poorly to low contrasts, smaller cell bodies hence slower conduction through axons).
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Low SF High SF (parvo)
Parvocellular: Specialized for fine spatial details & color. Magnocellular: Specialized for flickering/moving objects
Even though each of the two LGNs receive input from both eyes, each layer and each LGN relay cell only receives feedforward monocular input from the retina.
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2
1, 2: lines of projection
1
Coronal view of Right LGN
Coronal view of left LGN
Cat LGN
Guess the number of layers in the rodent LGN?
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Each LGN layer receives input from one eye only, parallel pathways to cortex
No retinal input
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(RTN = TRN, both abbreviations are commonly used. Reticular Nucleus of Thalamus)
Biggest mystery of LGN: over 90% of its inputs come from cerebral cortex, brainstem, TRN, not retina!
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This is a circuit that is common in sensory systems.
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Kara et al. 2000 Neuron
Spatial receptive field structure of LGN relay cells is very similar to RGCs, but…
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+ -
LGN neuron
Cortical feedback sharpens the spatial tuning of LGN neurons. It accentuates excitation at center of RF...
... and suppression in surround of RF
We know only a little bit about the functions of corticothalamic feedback. For example:
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RGC LGN1 LGN2 V1
Kara et al. 2000 Neuron
Timing of spikes in the LGN
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LGN “A” layers circuitry
Driver-fast ionotropic AMPA receptor input to relay cells Modulator
Most synapses onto relay cells are from non-retinal sources
AMPA, GABAa, nACH
Slow, GABAb, mGLUR, mACH
Retinal afferents = 5-10% of synapses on LGN relay cells
Modulator
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Same depolarizing pulse but opposite effects (‘condition’ membrane for >= 100 ms)
Before/after TTX
Cell 1
Cell 2
After TTX (all-or-none Ca spike)
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Same depolarizing pulse but opposite effects (‘condition’ membrane for >= 100 ms)
Na
Ca Before/after TTX
Cell 1
Cell 2
After TTX (all-or-none Ca spike)
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T channel present in EVERY thalamic relay cell (in all thalamic nuclei) and in every species studied
Activation gate closed, inactivation gate OPEN
Inactivation gate
Repolarize cell
slow
1
Activ. & inactiv. gates have opposite voltage dependency
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The LT calcium spike in thalamic relay cells is qualitatively similar to the Na-K spike found in most/all neurons but quantitatively different:
• Calcium spike is slower (activation and inactivation) • Calcium spike is not present in the axon initial segment, but only in the cell body • It does not propagate down into the axon and affect the postsynaptic target • It operates in a more hyperpolarized regime (i.e., low threshold) • It needs to be in a low or high state for > 100 ms for activation/inactivation
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Linear input/output
Non-linear input/output
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Relay cell responses to sensory stimuli (sine gratings)
Why? Sleep/wake (not exclusively), attention (yes), detecting new stimuli in the environment
Rectified = non-linear, but higher S:N, “wake me up” Faithfully reconstructs stimulus
Bursts punch through the thalamo-cortical synapse effectively
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Iontropic receptors (inputs) CANNOT cause mode switches: mGLUR from cortex or mACH from parabrachial region cause switch from burst to tonic & GABAb from brainstem reticular formation and local interneurons – opposite.
Thalamus = last bottleneck for behavioral states to affect information processing: Much fewer cells (than cortex) to gate
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Two main kinds of eye movements
• Saccades (microsaccades)
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Simplified outline of saccadic system
(PPC and big part of it: LIP)
(SEF)
(FEF)
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• Smooth pursuit
Two main kinds of eye movements
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Outline of smooth pursuit system
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Frontal Eye Field
• At the intersection of prefrontal cortex and premotor/motor cortex.
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Same in monkey
MONKEY
Brodmann’s areas (from 1905)
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Prefrontal Cortex
MONKEY
Brodmann’s areas (from 1905)
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Motor/Premotor Cortex
MONKEY
Brodmann’s areas (from 1905)
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Frontal Eye Field
• This position at juncture of “cognition” area and “movement” area thought to be critical, because saccades are so important for our visual analyses and decisions.
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Exploring the Visual Scene (Yarbus 1967)
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Exploring the Visual Scene Baseline: No Instructions
“Estimate Ages of People”