Download L6. Thalamus (László Acsády) All cortical areas receive thalamic

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