Basal Ganglia
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Direct answer
The basal ganglia are paired masses of subcortical grey matter deep in each cerebral hemisphere, comprising the caudate nucleus, lentiform nucleus (putamen plus globus pallidus), amygdaloid body and claustrum, with the subthalamic nucleus and substantia nigra functionally included. Together with the cerebellum and cerebral cortex they form the extrapyramidal system that programmes and smooths voluntary movement. Their disorders produce movement abnormalities — poverty of movement with rigidity and tremor, or excess abnormal movement — without paralysis.
What you must remember
- Nomenclature: striatum = caudate nucleus + putamen (the neostriatum, the main input zone); paleostriatum = globus pallidus (the main output zone); corpus striatum = caudate + lentiform nucleus; the caudate and putamen are separated by the internal capsule fibres but joined anteroinferiorly.
- Lentiform nucleus lies lateral to the internal capsule, buried in the white matter, with the putamen lateral and the globus pallidus medial (divided into lateral and medial segments).
- Input: the whole cerebral cortex projects to the striatum; the striatum projects to the globus pallidus.
- Direct pathway: cortex excites striatum, striatum inhibits the medial globus pallidus and substantia nigra reticulata, releasing the thalamus to drive the cortex — facilitates movement; dopamine acting on D1 receptors reinforces it.
- Indirect pathway: striatum (D2 receptors) inhibits the lateral pallidum, which disinhibits the subthalamic nucleus, which excites the output nuclei and suppresses movement; dopamine inhibits this pathway.
- Output: the medial globus pallidus and substantia nigra pars reticulata project via the ansa lenticularis and lenticular fasciculus to the ventral anterior and ventral lateral thalamic nuclei.
- Classical diseases: Parkinson disease — loss of dopaminergic neurons of the substantia nigra pars compacta with Lewy bodies, causing bradykinesia, cogwheel rigidity and resting pill-rolling tremor; hemiballismus — subthalamic nucleus lesion with wild flinging movements; Huntington disease — caudate nucleus atrophy with chorea and dementia; Wilson disease — copper deposition in the putamen with wing-beating tremor and cirrhosis.
Common confusion
Do not blame basal ganglia lesions for paralysis or sensory loss — they alter movement quality, not power. Second, keep the internal capsule straight: it lies between the caudate plus thalamus medially and the lentiform nucleus laterally, so caudate and putamen are sister nuclei of the striatum despite being physically separated. Third, Parkinson disease is a hypokinetic disorder of dopamine deficiency in the nigrostriatal pathway, whereas chorea and hemiballismus are hyperkinetic disorders of indirect pathway structures.
Exam-focused takeaway
Theory questions ask for the components of the basal ganglia in a horizontal section of the cerebrum, the direct and indirect circuits with their neurotransmitters, and the lesion-site-diagnosis matching of parkinsonism, chorea, athetosis, hemiballismus and Wilson disease. In the viva, define the striatum, pallidum and corpus striatum, and contrast pyramidal weakness with extrapyramidal rigidity.
Frequently asked questions
What structures make up the striatum?
The caudate nucleus and putamen, which share embryology, chemistry and connections; together they form the neostriatum, the principal receiving station of the basal ganglia.
Which two nuclei form the lentiform nucleus?
The putamen laterally and the globus pallidus medially; the pallidum is the main outflow nucleus of the basal ganglia to the thalamus.
Why does loss of dopamine cause parkinsonism?
Dopamine from the substantia nigra pars compacta excites the direct pathway through D1 receptors and inhibits the indirect pathway through D2 receptors; its loss both suppresses desired movement and releases rigidity and resting tremor.
Which lesion causes hemiballismus?
A lesion of the contralateral subthalamic nucleus, classically after a small lacunar stroke, producing violent flinging movements of the opposite limbs.
Where do basal ganglia outputs go?
Through the ansa lenticularis and lenticular fasciculus to the ventral anterior and ventral lateral nuclei of the thalamus, and thence to the motor and premotor cortex.