page_title: Basal ganglia
section_title: Circuit connections
hierarchical_section_title: Basal ganglia / Structure / Circuit connections
caption_attribution_description: English: Depicted are parts of the basal ganglia and their interconnections as revealed by diffusion spectrum imaging based on thirty subjects from the human connectome project (HCP MGH). Direct, indirect and hyperdirect pathways are visualized in different colors (see legend). Subcortical structures are rendered based on the Harvard-Oxford subcortical (Thalamus) as well as the Atlasing of the Basal Ganglia atlas (other structures). Rendering was generated using TrackVis software. STN: subthalamic nucleus, GPi: internal part of the pallidum, GPe: external part of the pallidum, SNr: substantia nigra, pars reticulata.
context_page_description: The basal ganglia are a group of subcortical nuclei, of varied origin, in the brains of vertebrates, including humans, which are situated at the base of the forebrain and top of the midbrain. There are some differences in the basal ganglia of primates. Basal ganglia are strongly interconnected with the cerebral cortex, thalamus, and brainstem, as well as several other brain areas. The basal ganglia are associated with a variety of functions, including control of voluntary motor movements, procedural learning, habit learning, eye movements, cognition, and emotion.
  The main components of the basal ganglia – as defined functionally – are the striatum, consisting of both the dorsal striatum and the ventral striatum, the globus pallidus, the ventral pallidum, the substantia nigra, and the subthalamic nucleus. Each of these components has a complex internal anatomical and neurochemical organization. The largest component, the striatum, receives input from many brain areas beyond the basal ganglia, but only sends output to other components of the basal ganglia. The pallidum receives input from the striatum, and sends inhibitory output to a number of motor-related areas.
context_section_description: Multiple models of basal ganglia circuits and function have been proposed, however there have been questions raised about the strict divisions of the direct and indirect pathways, their possible overlap and regulation.  The circuitry model has evolved since the first proposed model in the 1990s by DeLong in the parallel processing model, in which the cortex and substantia nigra pars compacta project into the dorsal striatum giving rise to an inhibitory indirect and excitatory direct pathway.  
  The inhibitory indirect pathway involved the inhibition of the globus pallidus externus, allowing for the disinhibition of the globus pallidus internus (through STN) allowing it to inhibit the thalamus.
  The direct or excitatory pathway involved the disinhibition of the thalamus through the inhibition of the GPi/SNr.  However the speed of the direct pathway would not be concordant with the indirect pathway in this model leading to problems with it.  To get over this, a hyperdirect pathway where the cortex sends glutamatergic projections through the subthalamic nucleus exciting the inhibitory GPe under the center surround model, as well as a shorter indirect pathway have been proposed.
  Generally, the basal ganglia circuitry is divided into five pathways: one limbic, two associative (prefrontal), one oculomotor, and one motor pathway.  (The motor and oculomotor pathways are sometimes grouped into one motor pathway.)  The five general pathways are organized as follows:
  The motor loop involving projections from the supplementary motor area, arcuate premotor area, motor cortex and somatosensory cortex into the putamen, which projects into the ventrolateral GPi and caudolateral SNr which projects into the cortex through the ventralis lateralis pars medialis and ventralis lateralis pars oralis.
  The oculomotor loop involved projections from the frontal eye fields, the dorsolateral prefrontal cortex (DLPFC), and the posterior parietal cortex into the caudate, into the caudal dorsomedial GPi and ventrolateral SNr, finally looping back into the cortex through the lateral ventralis anterior pars magnocellularis(VAmc).
  The first cognitive/associative pathway proposes a pathway from the DLPFC, into the dorsolateral caudate, followed by a projection into the lateral dorsomedial GPi, and rostral SNr before projecting into the lateral VAmc and medial pars magnocellularis.
  The second cognitive/associative pathway proposed is a circuit projecting from the lateral orbitofrontal cortex, the temporal gyrus, and anterior cingulate cortex into the ventromedial caudate, followed by a projection into the lateromedial GPi, and rostrolateral SNr before looping into the cortex via the medial VAmc and medial magnocellularis.
  The limbic circuit involving the projections from the ACC, hippocampus, entorhinal cortex, and insula into the ventral striatum, then into the rostrodorsal GPi, ventral palladium and rostrodorsal SNr, followed by a loop back into the cortex through the posteromedial part of the medial dorsal nucleus.  However, more subdivisions of loops have been proposed, up to 20,000.
  The direct pathway, originating in the dorsal striatum inhibits the GPi and SNr, resulting in a net disinhibition or excitation of the thalamus.  This pathway consist of medium spiny neurons (MSNs) that express dopamine receptor D1, muscarinic acetylcholine receptor M4, and adenosine receptor A1.  The direct pathway has been proposed to facilitate motor actions, timing of motor actions, gating of working memory, and motor responses to specific stimuli.
  The (long) indirect pathway originates in the dorsal striatum and inhibits the GPe, resulting in disinhibition of the GPi which is then free to inhibit the thalamus.  This pathway consists of MSNs that express dopamine receptor D2, muscarinic acetylcholine receptor M1, and adenosine receptor A2a.  This pathway has been proposed to result in global motor inhibition(inhibition of all motor activity), and termination of responses.  Another shorter indirect pathway has been proposed, which involves cortical excitation of the subtha
caption: Connectivity of the basal ganglia  as revealed by diffusion spectrum imaging based on thirty subjects from the Human Connectome Project. Direct, indirect and hyperdirect pathways are visualized in different colors (see legend). Subcortical structures are rendered based on the Harvard-Oxford subcortical thalamus as well as the Basal Ganglia atlas (other structures). Rendering was generated using TrackVis software.
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