Schematic Diagram of Thalamocortical Connectivity
This schematic diagram presents the major connections between the thalamus and cerebral cortex. Thalamic relay neurons are densely innervated by prethalamic fibers, for example from the retina in the visual system (large green arrow). Thalamocortical cells (green) then send their axons to the cerebral cortex and synapse onto various elements in layer IV, including the apical dendrites of layer VI pyramidal cells (blue). Layer VI pyramidal cells in turn give rise to a dense projection back to the thalamus. In this manner, there exists a loop between the thalamus and cerebral cortex.
As the axons pass through the thalamic reticular nucleus (nRt - colored red), or that part of the nucleus that is adjacent to the dorsal lateral geniculate nucleus and known as the perigeniculate nucleus (PGN - red), they give off axon collaterals. The neurons of the nRt and PGN are GABAergic and give rise to a dense innvervation of the thalamocortical cells. Thus the nRt/PGN forms a feedforward pathway for inhibition in the communication from cerebral cortex to thalamus and a feedback pathway for inhibition for communication between the thalamus and the cerebral cortex.
The thalamus and cerebral cortex are innervated by a number of different modulatory neurotransmitter systems including a cholinergic innervation from the pedunculopontine and lateral dorsal tegemental nuclei in the brainstem; a noradrenergic innervation from the locus coeruleus; a serotoninergic innervation from the raphe nuclei; and finally an interesting histaminergic innervation from the hypothalamus.
Together, these modulatory transmitter system control the state of activity in the thalamus and cerebral cortex.