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Lecture 4

Lecture 4 Notes.docx

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Rutsuko Ito

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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 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. 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 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 areas. 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 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 mem
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