The direct, indirect and hyperdirect pathways of basal ganglia information processing

THE DIRECT, INDIRECT AND HYPERDIRECT PATHWAYS OF BASAL GANGLIA INFORMATION PROCESSING 

The distinct striatal projection pathways contribute differentially to the excitatory and inhibitory circuits regulating the basal ganglia output, resulting in functionally opposite effects the direct pathways lead to a disinhibition of the target regions, whereas the indirect pathways lead to their inhibition. Therefore, the roles of the direct and indirect pathways are implicated in the activation and suppression of motor behavior, respectively activation of the direct pathway is thought to facilitate motor behavior, whereas the indirect pathway is thought to inhibit inappropriate motor behavior. 

The STh receives also direct excitatory input from the cerebral cortex, especially from the frontal cortical areas the primary motor cortex, with a minor contribution of prefrontal and premotor areas (see for review Nambu et al., 2002). Cortical afferents from the primary motor cortex in rodents and cats are composed of collaterals of the pyramidal tract or of corticostriatal fibers (reviewed by Hamani et al., 2004). The hyperdirect pathway is a cortico-STN-pallidal pathway, which conveys excitatory input from the motor-related cortical areas to the GP bypassing the striatum, and therefore with shorter conduction time. According to the model of the hyperdirect pathway, the activity of the cortico-STh-pallidal route could result in a wide inhibitory effect on motor programs, with adjustments of signals through the direct cortico-striato-pallidal pathway (Nambu et al., 2002). 

Despite the emphasis on parallel pathways in this conceptual scheme, it should also be considered that information processing through the direct and indirect pathways is subserved by complex synaptic interactions striatal neurons giving origin to the direct pathway are synaptically interconnected with indirect pathway striatal neurons, and the direct and indirect pathways converge at the synaptic level on single output neurons of the basal ganglia (Smith et al., 1998). 

DESCENDING EFFERENTS OF THE MIDBRAIN DOPAMINERGIC CELL GROUPS 

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