How do the basal ganglia loops run? The direct and indirect pathways
Both loops run from the cortex into the striatum and back to the cortex through the thalamus. The direct pathway quiets the internal globus pallidus, freeing the thalamus to excite the cortex; the indirect one detours through the external globus pallidus and subthalamic nucleus and does the reverse. [1]
A circuit has no shape of its own in this atlas; lit: the parts it runs through, in order.
The 3D brain needs JavaScript. The steps below still say where the basal ganglia loops runs, in order.
The brain starts seen from the front, a little from the left and above; the striatum, globus pallidus and thalamus are deep, lit through the faint cortex. Step through lights one step at a time, about 1.2 s each, from the cortex down through the deep nuclei, and back to step 1; tap a step below to light it alone, or Next step to go round the loop one step at a time. Direct pathway or Indirect pathway switches the step list. Drag to turn, pinch or scroll to zoom. Step through is off because your device asks for reduced motion; tap each step instead.
The globus pallidus is one shape in this atlas: its internal and external segments, split by a thin sheet of white matter (the medial medullary lamina), are not drawn apart, so both segments light the same part. The subthalamic nucleus has no shape; its step lights the thalamus and midbrain it lies between. The frontal lobe stands in for the cortex: the motor loop leaves from and returns to its motor and premotor areas, though the somatosensory cortex of the parietal lobe feeds the striatum too. [2][3][4][5][6]
The two loops, step by step
Direct pathway, 4 steps
The shorter route. It disinhibits the thalamus, and its net effect is more activity in the cortex and more movement. [1]
The loop starts in the cortex: the motor and premotor areas send excitatory fibres, releasing glutamate, into the striatum. All input to the basal ganglia arrives this way, from the cortex into the striatum. [1][7]Where is the frontal lobe?
The caudate nucleus and putamen together form the striatum. Its direct-pathway cells are inhibitory: once the cortex excites them they release GABA onto the internal globus pallidus. Dopamine from the substantia nigra pars compacta excites this pathway. [1][7]Where is the caudate nucleus?Where is the putamen?
At rest the internal globus pallidus, with the pars reticulata of the substantia nigra, sends a steady inhibitory (GABA) signal to the thalamus. The striatum’s signal quiets it, which lifts that brake: the thalamus is disinhibited. [7][1]Where is the globus pallidus?
Freed, the ventral anterior and ventral lateral nuclei of the thalamus send excitatory signals back to the motor and premotor cortex, closing the loop at step 1 and amplifying the movement the cortex has chosen. The two nuclei are not drawn apart; the whole thalamus is lit. [8][6][7]Where is the thalamus?
The loop closes: next is step 1, .
Indirect pathway, 6 steps
The longer route, with two extra links. It strengthens the thalamus’s normal inhibition, and its net effect is less activity in the cortex: it holds back movements that would compete with the chosen one. [1][9]
The same motor areas excite a second set of striatal cells at the same time: the indirect pathway also begins with glutamate from the cortex into the striatum. [9][1]Where is the frontal lobe?
Its indirect-pathway cells, medium spiny neurons with D2 dopamine receptors, are inhibitory too, but they send their GABA to the external globus pallidus. Dopamine damps this pathway, where it excites the direct one. [9][1]Where is the caudate nucleus?Where is the putamen?
The external segment normally holds the subthalamic nucleus down with a steady inhibitory signal; quieted by the striatum, it lets that brake off. It receives mainly from the striatum and projects to the subthalamic nucleus. [9][5]Where is the globus pallidus?
The subthalamic nucleus, a small lens-shaped nucleus below the thalamus and above the substantia nigra, is the only part of the basal ganglia whose cells release an excitatory transmitter, glutamate. Freed from the external segment’s brake, it excites the internal globus pallidus. Not a separate shape in this atlas; lit: the two parts it lies between. [4][5]Where is the subthalamic nucleus?
Driven by the subthalamic nucleus, the internal segment, with the substantia nigra pars reticulata, inhibits the thalamus more strongly. It is the same atlas shape as step 3, so this step lights nothing new: the globus pallidus stays lit. [9][3]
Held back, the ventral lateral nucleus sends less excitation to the motor cortex, so cortical areas that would compete with the chosen movement stay quiet; the loop closes at step 1. [9][6]Where is the thalamus?
The loop closes: next is step 1, .
More about the basal ganglia loops
Other names: cortico-basal ganglia-thalamo-cortical loop, direct pathway of movement, indirect pathway of movement [7][9]
The two pathways work as a pair: the direct pathway lets a chosen movement through, while the indirect pathway keeps unwanted contractions from competing with it. [7][9]
Dopamine from the substantia nigra pars compacta is the switch between them: through excitatory D1-type and inhibitory D2-type receptors it excites the direct pathway and inhibits the indirect one, so both effects favour movement. [1]
A third, faster route has been proposed, the hyperdirect pathway: the cortex sends glutamate straight to the subthalamic nucleus, skipping the striatum. [5]
The motor loop is one of several parallel loops: others run from the frontal eye fields and dorsolateral prefrontal cortex through the caudate (the oculomotor and associative loops), and from the anterior cingulate cortex, hippocampus and insula through the ventral striatum (the limbic loop). [5]
Globus pallidus is Latin for “pale globe”: the many myelinated fibres crossing it give it its pale look. [3]
The basal ganglia shape movement; the commands themselves run down the corticospinal tract. Every part of movement, lit together: What part of the brain controls movement?
Facts describe anatomy and normal function only. The 3D model is the adult male BodyParts3D brain (credits and licence); it has no shape for tracts, so a pathway is shown by the parts it runs through. A circuit is lit the same way, one link at a time, and its last step leads back to its first.