basic visual pathway Flashcards
thalamus
- Large structure
- Divided into multiple nuclei with distinct functions
-some visual
-some non-visual
lateral geniculate nucleus
- Primary visual part of the thalamus
what projects to teh LGN in a primate?
~90% of retinal ganglion cells
role of teh thalamus
- Plays a major gating/ modulatory role in the relay of sensory information
- Integrates information from cerebellum and basal ganglia, sending this information to the motor regions of cortex
- Determines whether information should reach consciousness awareness and is involved with sleep/ wake and attention
-determines whether we ‘notice’ specific pieces of sensory information
thalamic organisation
- The main visual component of the thalamus is the LGN
- Other region of processing are:
-Pulvinar (communicates to higher visual cortex)
-thalamic reticular nucleus
general thalamic properties
- Send information to cerebral cortex for all sensory systems except olfaction
-individual sub-nuclei that process specific senses
-overall communicates with the entire cortex - Every relay nucleus receives information back from the cortex
- Amount of feedback from cortex may equal or exceed that from sense organs
- Feedback may be specific or diffuse, may separately target relay cells and interneurons
what type of structure is the LGN?
laminated
how many layers of laminae in the LGN?
6 layers
which layers are magnocellular?
1 and 2
which layers are parvocellular
3,5,6
what stain shows up cell bodies in the LGN?
nissl stain
large cells
magnocells
small cells
parvocells
what does each cell in the LGN receive most of its input from?
a single retinal ganglion cell
what does each LGN layer contain?
excitatory (relay) cells and local inhibitory interneurons
what is underneath each laminae?
very small excitatory relay cells: koniocellular layers
how are retinal projections ordered?
retinotopically
where does each LGN get its signals from?
RGCs viewing the opposite visual hemifield
what do adjacent cells within each LGN layer view?
adjacent portions of visual space
what representation does the foveal region have in the LGN
a disproportionately large representation: half the mass of the LGN
how many eyes does each layer of the LGN receive input from?
one
what is the effect of each laminae only receiving input from one eye?
there are multiple representations of the same ‘retinotopic’ space in each LGN
cells along the projection line all view…
overlapping region of visual space
what is the receptive field of a sensory neuron
a region of space in which the presence of a stimulus will alter the firing of that neuron
what organisation of receptive field to LGN cells have?
concentric centre-surround antagonistic
what do LGN centre surround receptive fields act as?
local edge detectors
spatial acuity small receptive field
high spatial acuity: more detail
spatial acuity large receptive field
low spatial acuity: less detail
temporal frequency of RFs that respond best to slow stimuli
low preferred temporal frequency
temporal frequency of RFs that respond best to fast stimuli
high preferred temporal frequency
parvocellular LGN neurons (8 points)
- Dorsal 4 layers have smaller cell bodies
- Small receptive fields
- High spatial acuity (can resolve fine detail)
- Prefer low temporal frequencies (slow changes)
- Receptive fields have concentric centre/surround like the retina
- Both On and Off centre subtypes
- Chromatic (red/green) but can respond to brightness
- Input from retinal P ganglion cells
magnocellular LGN neurons (8 points)
- Ventral 2 layers
- Have larger cell bodies
- Lower spatial acuity (can’t resolve fine detail)
- Prefer high temporal frequencies (fast changes)
- Receptive fields have concentric centre surround like the retina
- Both On and Off-centre subtypes
- Achromatic
- Input from retinal M ganglion cells
koniocellular LGN neurons (5 point)
- Very small cells found between laminae
- Direct input from blue/yellow RGC’s
- Indirect input from superior colliculus
- Heterogenous types?
- Functional properties/ function largely unknown
organisation within laminae
eccentricity
what is magnified with LGN retinotopic map?
fovea
what changes with eccentricity
receptive field size/ proportion of M and P cells
relay cells (4 points)
- ~90% of LGN cells are relay, single axon projections to V1
- Use glutamate- therefore are excitatory
- Each has an axon contralateral just above the LGN
where does the contralateral axon to each relay cell (just above the LGN) terminate?
in the visual sector TRN, known as the perigeniculate nucleus (PGN)
what neurotransmitter do all inhibitory cells use?
gamma-aminobutyric acid (GABA) as a neurotransmitter
what are the two populations of inhibitory neurons
feed forward inhibition and feedback inhibition
feed forward inhibition neurons
modulates centre:surround inhibition
intrinsic to the LGN
receives retinal input
project locally
feedback inhibition neurons
global changes in the LGN cell responses
with the perigeniculate nucleus
project widely within the PGN
feedback to LGN
where does the LGN send excitatory input to?
V1
modulatory systems
long projection systems from the brainstem and basal forebrain that adjust thalamic function
noradrenaline
- Produced in locus coeruleus (LC):
-small periaqueductal gray matter cell group ~10,000 cells/side - Maintains vigilance, fight or flight responses
- Extensive projections incl. thalamus and cortex
- Modulates visual sensitivity
acetylcholine
- Basal forebrain groups: innervates entire cerebral cortex including amygdala and hippocampus
- Pontine groups: innervate brainstem reticular formation and thalamus
important for arousal and REM sleep
serotonin
- Serotonergic cells found in the Raphe nuclei
- Pons and midbrain groups project the whole of the forebrain
- Role in mood, cardiovascular control, thermoregulation + modulate thalamic and cortical function
- Specific function largely unknown
dopamine
- Multiple cell groups, some of which provide input to thalamus
not those with Parkinson’s - Roles in reward, neuroendocrine, motor control
- Thalamic projections mainly avoid primary sensory areas
- Dopaminergic modulation of the thalamus
