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L6. Thalamus (László Acsády) All cortical areas receive thalamic inputs and no cortical area is functional without intact thalamocortical connections. The thalamus has multiple functions. It may be thought of as a kind of hub of information. The thalamus is generally believed to act as a relay between different subcortical areas and the cerebral cortex, but thalamic nuclei have strong reciprocal connections with the cerebral cortex, forming thalamo-cortico-thalamic circuits that are believed to be involved with consciousness. The thalamus plays a major role in regulating arousal, the level of awareness, and activity. The thalamic module: - Subcortical input: large subcortical terminals on the thalamic cell (bottom-up), which are powerful, complex and multisynaptic terminals for faithful relay. These terminals act like dynamic low-pass filters. Sensory transmission needs to be very precise, and transfer information only if it’s needed, if not it’s better to cut off the signal → short term depression - TRN (Thalamic reticular nucleus): The thalamic reticular nucleus receives input from the cerebral cortex (layer 6) and thalamocortical cells (feed-forward and feedback inhibition). TRN efferent fibers project to TC, but never to the cerebral cortex (the only thalamic nucleus that does not project to the cerebral cortex). The TRN neurons can inhibit the TC, thus it’s viewed as a functional networking filter to regulate conscious perception TRN operates as a low pass filter in action – maintaining selective attention Attend: ↓ ↓ ↑ - TRN - short term depression - faithful transfer Don’t attend: ↑ ↑ ↓ - Sensory cortico-thalamico-cortico circuits: Large parts of the thalamus are dedicated to process cortical signals back to the cortex. The L5 driven thalamus loop is required for primary cortical responses. It has the same large terminals (top-down) as the subcortical input. Similar to sensory inputs L5 inputs lack TRN collaterals. The principle of connections between frontal cortex and thalamus are little explored but we know some differences compared to the sensory circuit - combine highly diverse inputs - use highly distributed motor maps - maintain persistent activity in the absence of sensory stimulus - extract silent features of transient signals - faithfully represent two dimensional sensory surfaces - activity linked to the presence of stimulus