Update articleFunctional significance of the cortico–subthalamo–pallidal ‘hyperdirect’ pathway
Section snippets
Position of the subthalamic nucleus in the basal ganglia circuitry
The subthalamic nucleus (STN, corpus Luysi) is a relatively small nucleus of the basal ganglia and is located ventrally to the zona incerta and dorsally to the cerebral peduncle. In early 1980s, the STN has been considered to be part of a minor and closed ancillary loop in the basal ganglia circuitry that includes the external segment of the globus pallidus (GPe) (DeLong and Georgopoulos, 1981). In a more recent view, the striatum receives direct excitatory cortical inputs, and projects to the
STN as an input station of the basal ganglia
The STN receives direct cortical projections from wide areas of the frontal lobe (Hartmann-von Monakow et al., 1978). Such cortico–STN projections were considered to be sparse, and therefore have been regarded less important. However, recent anterograde double-labeling studies have shown that the monkey STN receives substantial somatotopically organized projections from the primary motor cortex (MI), the supplementary motor area (SMA), and the dorsal (PMd) and ventral (PMv) divisions of the
Cortico–STN–pallidal ‘hyperdirect’ pathway
The STN receives inputs from the cerebral cortex and, in turn, sends outputs to the GPi/SNr. Cortical stimulation induces an early, short-latency excitation, followed by an inhibition and a late excitation in pallidal neurons of monkeys. The early excitation is considered to be derived from the cortico–STN–pallidal pathway based on the following findings (Nambu et al., 2000b). Simultaneous recordings of neuronal activity in the pallidal complex and STN have shown that the cortical stimulation
Functional significance of the cortico–STN–pallidal ‘hyperdirect’ pathway
Recent anatomical studies have shown that STN–pallidal fibers arborize more widely and terminate on more proximal neuronal elements than striato–pallidal fibers (Hazrati and Parent, 1992a, Hazrati and Parent, 1992b). This suggests a ‘center-surround model’ of basal ganglia function which proposes focused selection and inhibition of competing motor programs (Mink and Thach, 1993, Mink, 1996, Hikosaka et al., 2000). Actually, analyzing the precise distribution of cortically evoked responses in
Activity of pallidal and STN neurons during movements
The dynamic model of basal ganglia function presented in the previous section has been derived mainly from ‘stimulation’ studies. However, the cortico–STN–pallidal ‘hyperdirect’ pathway may actually contribute to the neuronal activity of the basal ganglia in a physiological state. Pallidal activity during voluntary limb movements always displays an increase rather than a decrease in discharge, and the incidence ratio of an increase to a decrease is 1.6–5.8 (Georgopoulos et al., 1983, Anderson
Unsolved problems
In this article, we have emphasized the functional significance of the cortico–STN–pallidal ‘hyperdirect’ pathway and the dynamic ‘center-surround model’ of basal ganglia function. In the next stage, it should be attested whether this model in relation to the ‘hyperdirect’ pathway actually functions in a physiological state by observing the effect of blocking cortico–STN neurotransmission on pallidal activity during the execution of behavioral tasks.
The contribution of the net inhibitory
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