The Basal Ganglia

Key points

• • The basal ganglia are deep grey-matter nuclei including the caudate nucleus, putamen and globus pallidus, working with the subthalamic nucleus and the substantia nigra.
• • They do not start movement directly; instead they form loops with the cortex that adjust the strength and smoothness of movements and hold back unwanted ones.
• • A direct pathway promotes movement and an indirect pathway suppresses it, and dopamine from the substantia nigra acts on both (through D1 and D2 receptors) to favour movement.
• • Loss of the dopamine-making cells in the substantia nigra causes Parkinson disease, with slowness of movement, rigidity and a resting tremor.
• • Loss of the GABAergic medium spiny neurons of the caudate and putamen, especially those of the indirect pathway, causes Huntington disease, with excess involuntary writhing movements called chorea.
• • Dysfunction of these circuits can produce either excessive involuntary movements, such as chorea, or reduced and rigid movement with bradykinesia, depending on which circuits are affected.