Subcortical Nuclei · Deep Cerebral Nuclei
The basal ganglia are a group of interconnected nuclei buried deep within the cerebrum, working closely with the motor cortex and thalamus to initiate, scale, and smooth voluntary movement. They also contribute to habit formation and reward-based learning.
The basal ganglia are a cluster of nuclei - chiefly the caudate nucleus, putamen, and globus pallidus - buried deep within each cerebral hemisphere, near the thalamus. The caudate and putamen together are often called the striatum, the basal ganglia's main input structure. Functionally, the circuit also includes the substantia nigra, a dopamine-producing structure in the midbrain portion of the brain stem, even though it sits outside the basal ganglia proper.
The basal ganglia sit in a loop with the motor cortex and thalamus, and their main job is selection rather than execution: when several possible movements compete, the basal ganglia help suppress the ones not being used and allow the intended one through, via parallel "direct" and "indirect" pathways that respectively promote and inhibit movement. Dopamine from the substantia nigra tunes the balance between these two pathways, which is why dopamine loss has such a dramatic effect on movement. Beyond motor control, the same selection mechanism is thought to apply to habits and reward-based learning - repeated behaviours that become progressively more automatic with practice.
The link between the basal ganglia's dopamine pathway and movement disorders was established through 20th-century pharmacology and neurochemistry, work that earned Arvid Carlsson a share of the 2000 Nobel Prize in Physiology or Medicine for identifying dopamine's role and helping establish levodopa as a treatment for Parkinson's disease. Later research into deep brain stimulation - implanting electrodes into basal ganglia structures to modulate abnormal circuit activity - built directly on this understanding of the basal ganglia as a tunable movement-selection circuit rather than a simple relay.
Loss of dopamine input from the substantia nigra, as in Parkinson's disease, shifts the basal ganglia circuit toward suppressing movement, producing slowness and rigidity. Degeneration of the striatum itself, as in Huntington's disease, has roughly the opposite effect on the circuit's balance, contributing to the involuntary, excessive movements (chorea) that characterise the condition. Basal ganglia circuits have also been implicated in obsessive-compulsive disorder and tic disorders, reflecting their broader role in filtering and selecting both movements and, it's thought, some repetitive thought patterns.
The basal ganglia is discussed in connection with the following conditions on this site:
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