2.6.2 Anatomy of the basal ganglia

Components and function

• The basal ganglia are the caudate nucleus, putamen, globus pallidus, subthalamic nucleus and substantia nigra, not the nearby limbic and diencephalic structures.

• They are principally motor structures that select, initiate and scale voluntary movement, so their degeneration produces movement disorders.

The striatum

Only the putamen sits in both the dorsal striatum and the lentiform nucleus. It lies lateral to the globus pallidus.

• The caudate nucleus arches with the lateral ventricle: its head sits in the frontal horn, its body beside the thalamus and its tail in the roof of the temporal horn.

• The nucleus accumbens lies where the caudate head and putamen fuse ventrally, under the anterior limb of the internal capsule, and receives mesolimbic dopamine from the ventral tegmental area.

• GABAergic medium spiny projection neurones dominate the striatum, and cholinergic interneurones are sparse.

• Purkinje and granule cells are cerebellar, and pyramidal cells belong to the cortex and hippocampus.

Tap to enlarge

Pallidum, substantia nigra and subthalamic nucleus

The substantia nigra is the only melanin-pigmented basal ganglia nucleus, because its dopaminergic neurones contain neuromelanin.

Direct and indirect pathways

The internal globus pallidus tonically inhibits the thalamus, which excites the cortex.

Dopamine promotes movement by exciting direct pathway cells through D1 receptors and inhibiting indirect pathway cells through D2 receptors.

Hemiballismus

A lesion of the subthalamic nucleus removes excitation of the internal globus pallidus, so the thalamus is released and hemiballismus follows: violent flinging movements of the opposite limbs.

Huntington's disease

• Huntington's disease is an autosomal dominant CAG repeat expansion in the HTT gene that causes chorea.

• Striatal medium spiny neurones die first, the caudate before the putamen, and the enkephalin-bearing indirect pathway cells go earliest, so the brake on movement is lost.

• Selective caudate atrophy is the near-pathognomonic hallmark: the caudate heads flatten and lose their bulge, and the frontal horns widen and box out.

• Wilson's disease, Fahr's disease and the cortical dementias lack this selective caudate atrophy.

Tap to enlarge

Other basal ganglia pathology

• In Parkinson's disease the striatum holds only the nigral terminals, so a dopamine transporter scan shows a presynaptic deficit there, and movement is reduced.

• Striatal medium spiny neurones survive in Parkinson's disease but lose their dopaminergic input, whereas in Huntington's disease they die.