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
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
Pulvinar: Link areas of occipital and temporal lobes that are involved in the processing of visual
Non-specific relays 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
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
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. 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
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 RS terminals seldom make more than one contact
a number of postsynaptic cells (divergence) 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 Modulators act through metabotropic and
Ensures a fast and short duration EPSP. ionotropic receptors
Faithful transmission of retinal information
Driver cells show very little convergence Modulator cells show very high convergence
Relay cell receives 1-3 retinal inputs at most Relay cell receives >20 inputs
Driver cells have thick axons Modulators have thin axons
Ensuring fast information transmission Cortical afferents
Cortical afferents: 1) Excitatory modulator cortical inputs to the first and higher order thalamic nuclei
originate in layer 6 of the neocortex. 2) Driver inputs of first order thalamic nuclei originate in the
sensory periphery or lower centers of the brain. Driver inputs of higher order thalamic nuclei originate in
layer 5 of the neocortex.
Connectivity between thalamus and cortex: Usually reciprocal: a thalamocortical neuron projects to the
same area of the cortex from which it receives a cortical input (albeit to different cortical layers)
Retinotopic map: Well preserved in visual cortex that receives LGN input. Less preserved in visual
cortical area connected to pulvinar region.
First order nucleus (LGN): Represents the first relay of a particular type of subcortical information to a
first order (primary cortical area)
Higher order nucleus (pulvinar): Represents information from layer 5 of one cortical area to another
cortical area. This relay can be from 1) Primary area to a higher one. 2) Between two higher order corical
Worthy of note: All thalamic nuclei receive a feedback input from layer 6 of cortex, but higher order
nuclei in addition receive a feedforward layer 5 input from cortex. All thalamic relay cells receive inputs
from a cell in the thalamic reticular nucleus. Functionally related corticothalamic neurons from layer 6
also provide collateral innervation to reticular cells that are in turn connected to relay cells. Functional
implication of the reticular-relay cell circuit will be discussed later in the course.
Other afferents: 1) Retinal, cortical and relay cell inputs are all glutamatergic (according to afferents to
LGN) 2) Parabrachial (brainstem) innervation of the reticular, relay cells and interneurons are
modulatory. TRN and interneuron inputs to the relay cell are GABAergic.
Synaptic connections of X and Y cells
Synaptic connections of X and Y cells: Thalamic relay cells receive up to 5000 synapses on their
dendrites. For both X and Y cells of the LGN, 1) Retinal and parabrachial inputs are limited to proximal
dendritic sites. Inputs from interneurons are also concentrated around proximal zone. 2) Cortical and
reticular inputs are located more distally. One of the major differences between the innervation pattern
of X and Y cells is the fact that retinal inputs to X cells are filtered through a complex circuitry of the
glomerulus, while retinal input to Y cells is simpler and more direct in nature.
Dendritic cable properties
Dendritic cable properties: 1) The X and Y relay cells have very compact dendritic structures (even the
most distally located synaptic input have significant impact on the soma and axon). 2) The branching architecture is such that a strong potential generated anywhere in the dendritic arbor will be efficiently
transmitter through the entire dendritic arbor.
Bloomfield and Sherman / Cable modeling experiment: Calculated the degree of voltage attenuation at
various sites in the dendritic tree and cell soma. 1) Relay cells, found that the transmission of distal
dendritic postsynaptic potentials to soma remain very strong. 2) Interneuron, found that transmission of
dendritic PSPs was very poor (not surprising since interneurons have elaborate long and thin dendrites).
Functional significance of elaborate dendritic arbor in interneurons: Unknown. Very inefficient.
Intrinsic firing properties of thalamic neurons
Intrinsic firing properties of thalamic neurons: Governed by 1) Membrane properties of cells. 2) Active