Figure 8. The basal ganglia gate information flow between cortical regions.

(A) Schematic of the circuit organization of motor thalamus with respect to cortex and basal ganglia, showing the direct (orange) and transthalamic (blue) pathways through motor thalamus. When basal ganglia projections (red) to motor thalamic relays are active, they potently inhibit (red filled motor thalamic cells) and thereby gate transthalamic information flow between cortical regions. In contrast, when basal ganglia inputs to motor thalamus are themselves inhibited (open fill motor thalamic cells), they allow transthalamic information flow between cortical regions, where it may encounter signals from the direct cortico-cortical pathways. Via this gating function, the basal ganglia can regulate which cortical areas are connected by only direct corticocortical pathways, only transthalamic pathways, both, or neither. Importantly, this regulation is dynamic, such that the pattern of connected areas can change over short time scales (see left, time T₀, versus right, time T_n). (B) Box diagram of the basal ganglia circuitry according to the traditional textbook view (left) and with the novel direct connection between motor cortex layer 5 (L5) and motor thalamus shown (right; asterisk). That is, our data demonstrate that L5 of primary motor cortex (M1_L5) drivers innervate cells receiving internal segment of the globus pallidus (GPi) and substantia nigra pars reticulata (SNr) inputs. Given this organization, M1_L5 neurons could simultaneously disinhibit (via the basal ganglia loop, black arrows) and drive (via the direct corticothalamic projection, blue arrow) motor thalamic relays.