The basal ganglia receive inputs from the cerebral cortex, send efferents to it, and inter-communicate extensively. Much of the information received from the cortex contains somatotopically-arranged data. In addition, there are thalamic connections. Most afferent projections to the basal ganglia terminate within the neo-striatum. The three major contributors to the basal ganglia are the cerebral cortex, the intralaminar nuclei of the thalamus, and dopaminergic pathways from the midbrain. The dopaminergic striatum inputs from the mesencephalon (midbrain) are known to be involved in at least one motor disease. The destruction of this pathway is the cause of Parkinson's disease (see p. 370).
The basal ganglia send major efferent inputs to the cortex via the thalamus. The thalamus is sometimes referred to as a 'relay' station whose role is the passive transmission of information received from areas such as the basal ganglia, but evidence is growing that the thalamus integrates information received from different sources. The thalamic ventral lateral nucleus receives inputs from the reticular formation, and the ventral anterior nucleus receives cortical inputs, which may be integrated with those from the basal ganglia; the possible functional significance of these interactions is unclear, but may include a role in the control of, for example, the level of consciousness.
Inputs to the basal ganglia from the thalamus include the important afferents from the intralaminar nuclei of the thalamus to the neostriatum. This information is topographically organized; the fibers arise mainly in the thalamic cortico-median nucleus, which receives fiber bundles from the cerebral cortex, and the fibers to the basal ganglia from the thalamic corticomedian nucleus terminate in the putamen.
There appear to be separate basal ganglia parallel circuits; for example, there are two basic circuits, consisting of direct and indirect pathways. Generally speaking, the direct pathway facilitates the streaming of impulses through the thalamus, while the indirect pathway inhibits impulse passage through the thalamus; the pathways therefore provide a regulator of thalamus activation.
The direct pathway: excitatory corticofugal fibers project to the striatum, from where inhibitory fibers pass to the internal segment of the globus pallidus and the pars reticulata. From the internal segment and the pars reticulata, inhibitory fibers project to the thalamus, which sends excitatory fibers back to the cortex. Inhibitory pars reticulata and internal segment cells discharge spontaneously, unless inhibited, and therefore tonically inhibit thalamic discharge to the cortex. Therefore, discharge of the direct pathway causes thalamic disinhibition and cortical excitation.
The indirect pathway: inhibits thalamic discharge through an inhibitory striatopallidal pathway that projects to the external segment of the globus pallidus. The external segment cells fire spontaneously and send tonically inhibitory fibers to the subthalamic nucleus. The striatum, via the pallidum, thus disinhibits the subthalamic nucleus, which also receives an excitatory projection from the cortex. The net result is the inhibition of thalamocortical pathways.
At least five parallel circuits of information flow have been identified in the basal ganglia. These are the motor loop, limbic loop, dorsolateral prefrontal loop, lateral orbitofrontal loop, and oculomotor loop.
putamen thalamus motor circuit of the basal ganglia putamen motor circuit of the basal ganglia
direct pathway direct and indirect pathways
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