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Lecture 4

NROC69H3 Lecture Notes - Lecture 4: Premotor Cortex, Neocortex, Ion


Department
Neuroscience
Course Code
NROC69H3
Professor
Rutsuko Ito
Lecture
4

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Lecture 4 Notes
Synaptic organization of the thalamus
Thalamus: Located deep in the brain. Largest component of the diencephalon that surrounds the third
ventricle (alongside the hypothalamus and epithalamus). Subdivided into many anatomically and
functionally distinct nuclei. Functionally distinct, but with a common structural pattern (input / output /
synaptic organization). This allows generalizations to be made across nuclei by investigating one part in
depth.
Thalamus Function: Acts as a relay station for all sensory (except olfactory), cerebellum, basal ganglia
(motor) and limbic (motivation) afferents to the neocortex. Information only reaches the neocortex via
the thalamus. The thalamus is known to actively modulate and regulate information throughout, as well
as controlling states of wakefulness.
Major Thalamic Nuclei
Major thalamic nuclear groups: Each nucleus of the thalamus has defined input/output connections,
which in turn defines the functional role of that nucleus.
Subdivision of nuclei into functionally distinct groups: 1) First order relays (relay thalamic nuclei). 2)
Higher order relays (association thalamic nuclei). 3) Non-specific relays.
Relay thalamic nuclei
First order relays: Carry messages from the periphery and lower brain centers to the neocortex. Nuclei
that fall into this category: 1) Anterior thalamic nuclei, 2) Ventral anterior thalamic nuclei, 3) Ventral
lateral thalamic nuclei, 4) Ventromedial geniculate nucleus, 5) Lateral geniculate nucleus.
Association thalamic nuclei
Higher order relays: Largest part of the thalamus. Receive incoming messages from the cortex and
relays the same messages back to the cortex (not necessarily the same area). Nuclei that fall into this
category: 1) Medial dorsal thalamus, 2) Laterodorsal nucleus. 3) Pulvinar.
Medial dorsal thalamus: Receives input from the prefrontal cortex (PFC) and sends efferents to another
part of the PFC. Source of cortico-cortical communication.
Laterodorsal nucleus: Receives inputs from the cingulate cortex and projects back to the cingulate
cortex.
Pulvinar: Link areas of occipital and temporal lobes that are involved in the processing of visual
information.
Non-specific relays

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Non-specific relays: Receive mixed inputs from cortical and subcortical regions. Project these inputs to a
wide area of the cerebral cortex and striatum. Nuclei in this category: 1) Intralaminar nuclei, 2) Midline
nuclei.
Topographical and parallel organization in lateral geniculate nucleus
Lateral geniculate nucleus (LGN): The visual relay nucleus. Entire visual field can be mapped onto LGN
(retinotopic map). Laminar organization.
Laminar organization: Provides a way in which two streams of information processing can be
anatomically segregated. 1) Two ventral layers, 2) Four dorsal layers, 3) Layers in between major layers.
Two ventral layers: Receive input from magnocellular ganglion cells. Magnocellular ganglion cells are
most sensitive to motion. Ventromotor stream.
Four dorsal layers: Receive input from parvocellular ganglion cells. Parvocellular ganglion cells are most
sensitive to color and form.
Layers in between: Receive input from koniocellular ganglion cells. Innervate areas in between major
layers.
Laminar organization in cat vs monkey: Less evidence for laminar organization providding anatomical
separation of different streams of visual information processing. Where different pathways share a
lamina (in the cat) there is not signficant interaction between them. Therefore the laminar organization
as seen in monkeys is not a necessary feature of functional segregation through the LGN.
Cat relay cells: Differ in morphology. 1) Y cells have larger cell bodies and thicker dendrites. Dendrites
tend to be smooth and contained in a roughly spherical arbor. 2) X cells usually have clustered
appendages on proximal dendrites, often near primary branch points. These appendages mark the
postsynaptic sites of retinal inputs and triads. The arbors of X cells tend to be bipolar in shape, oriented
perpendicular to the layering.
Afferents (inputs) to thalamus
Afferents (inputs) to thalamus: Two different types of afferents: 1) Driver inputs. 2) Modulatory inputs.
Driver inputs: Comprise of less than 10% of the total input to the thalamus. These neurons contain
primary information direct from the periphery to the cortex.
Examples of driver inputs: Driver cells dominate receptive field properties of target cells (for sensory
relays). LGN cells have center surround fields just like retinal ganglion cells such that removal of retinal
input to LGN relay cells eliminates LGN receptive fields.
Modulatory inputs: Comprise of 90% of the total input. These include local GABAergic (interneuron)
inputs, cortical and brainstem inputs (30% each) as well as modulatory input (<5%) from ACh, NA, 5HT
and HA cells.
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Electron microscopic appearance of nerve terminals in LGN
Electron microscopic appearance of nerve terminals in LGN: Zooming in on a glomerulus (small cluster
of nerve fibres) in the A layer of a cat LGN using electron microscope shows organization of nerve
terminals characteristic of thalamic nuclei of most speicies.
There are four major types of synaptic terminal in a thalamic glomerulus: 1) RL (round vesicle and large
profile) terminals. 2) RS (round vesicle and small profile) terminals. 3)F1 (flattened vesicle) terminals. 4)
F2 (flattened vesicle) terminals
RL terminals: Contributes 5-10% of all synaptic contacts. Asymmetric type 1 synapses (excitatory) with
more thickening of the postsynaptic density than the presynaptic zone. Represent glutamatergic driver
cells.
RS terminals: Contribute 50% of all synaptic contacts. Asymmetric synapses. Roughly half of the RS
terminals are cortico-thalamic (glutamatergic). The rest are from the brain stem and likely to be
cholinergic, serotonergic or noradrenergic (modulator inputs)
F1 terminals: Form symmetric GABAergic synapses. Strictly axonal and presynaptic. Axon terminals of
local reticular cells, interneurons and nucleus of optic tract.
F2 terminals: Form symmetric GABAergic synapses. Dendritic terminals and can be both pre and
postsyanptic. Dendritic processes of interneurons.
Triadic 'driver' junction: Distinctive feature of thalamic glomeruli is that synaptic terminals are very
close together with very little or no astrocytic cytoplasm between synaptic profiles. X relay cells in the
cat form a characteristic triadic junction which consists of a driver input and other modulatory inputs
converging upon an X cell.
Functional roles of driver vs modulatory inputs in LGN
Functional roles of driver vs modulatory inputs in LGN:
Driver input
Modulator input
RL terminals make multiple synaptic contacts with
a number of postsynaptic cells (divergence)
RS terminals seldom make more than one contact
with the postsynaptic cell (convergence)
Driver EPSPs are relatively large
Modulator EPSPs are much smaller
Driver terminals restricted to proximal dendrites
Modulator terminals can be located anywhere on
the dendritic arbor
Drivers activate ionotropic glutamate receptors
Ensures a fast and short duration EPSP.
Faithful transmission of retinal information
Modulators act through metabotropic and
ionotropic receptors
Driver cells show very little convergence
Relay cell receives 1-3 retinal inputs at most
Modulator cells show very high convergence
Relay cell receives >20 inputs
Driver cells have thick axons
Ensuring fast information transmission
Modulators have thin axons
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