Anatomy chapters
Basal ganglia
The nuclei that make up the basal ganglia, the direct and indirect pathways, the role of dopamine, and the movement disorders that follow damage: Parkinson's disease, Huntington's disease, hemiballismus.
- The basal ganglia are grey matter nuclei deep in the hemispheres that help start wanted movements and stop unwanted ones.
- Key parts: caudate, putamen, globus pallidus, with the subthalamic nucleus and substantia nigra working with them.
- Lesions cause movement disorders: Parkinson's (substantia nigra), Huntington's (caudate), hemiballismus (subthalamic nucleus).
On this page
Overview
The basal ganglia (more correctly, basal nuclei) are masses of grey matter deep in each cerebral hemisphere.
They do not connect directly with the spinal cord. Instead, they form loops with the cortex and thalamus that help select and start wanted movements and suppress unwanted ones.
Damage causes either too little movement (Parkinson's disease) or too much (chorea, ballismus).
Parts and names
| Name | Made up of |
|---|---|
| Caudate nucleus | C-shaped nucleus curving along the lateral ventricle (head, body, tail) |
| Putamen | Outer part of the lentiform nucleus |
| Globus pallidus | Inner part of the lentiform nucleus (external and internal segments) |
| Lentiform nucleus | Putamen + globus pallidus |
| Striatum (neostriatum) | Caudate + putamen |
| Corpus striatum | Caudate + lentiform nucleus |
Functionally linked structures:
- Subthalamic nucleus: below the thalamus.
- Substantia nigra: in the midbrain. The pars compacta makes dopamine.
The amygdala is sometimes listed with the basal nuclei but belongs to the limbic system.
The lentiform nucleus is putamen + globus pallidus; the striatum is caudate + putamen. The putamen is in both.
Relations
The internal capsule separates the lentiform nucleus (laterally) from the caudate nucleus and thalamus (medially).
Lateral to the putamen are the external capsule, claustrum, extreme capsule and the insula.
The caudate and putamen are joined by strands of grey matter across the internal capsule, which give the striped appearance ("striatum").
Connections and pathways
The basic loop is: cortex → striatum → globus pallidus (internal) → thalamus → cortex.
The globus pallidus internus normally inhibits the thalamus, which acts as a brake on movement.
Direct pathway (increases movement)
Cortex excites the striatum → striatum inhibits the globus pallidus internus → less inhibition of the thalamus → thalamus excites the cortex → more movement.
Indirect pathway (decreases movement)
Cortex excites the striatum → striatum inhibits the globus pallidus externus → the subthalamic nucleus is released and excites the globus pallidus internus → more inhibition of the thalamus → less movement.
Role of dopamine
Dopamine from the substantia nigra pars compacta acts on the striatum:
- It excites the direct pathway (D1 receptors).
- It inhibits the indirect pathway (D2 receptors).
Both effects increase movement.
Loss of dopamine (Parkinson's disease) weakens the direct pathway and strengthens the indirect pathway, so there is too little movement.
Clinical relevance
Parkinson's disease
How it happens: degeneration of the dopamine neurons of the substantia nigra pars compacta.
What you see:
- Resting tremor ("pill-rolling"), which lessens with movement.
- Rigidity of the "lead-pipe" or "cogwheel" type.
- Bradykinesia (slowness of movement) and difficulty starting movement.
- Postural instability, stooped posture and a shuffling gait.
- An expressionless face and small handwriting.
"TRAP": Tremor, Rigidity, Akinesia (bradykinesia), Postural instability.
Drugs that block dopamine (some antipsychotics) can cause a similar picture (drug-induced parkinsonism).
Huntington's disease
How it happens: an inherited (autosomal dominant) CAG repeat expansion that causes degeneration of the caudate nucleus (and putamen), mainly affecting the indirect pathway.
What you see: chorea (quick, jerky, dance-like movements), personality change and dementia, usually starting in mid-adult life. The lateral ventricles look widened because the caudate is lost.
Hemiballismus
How it happens: a small stroke in the subthalamic nucleus.
What you see: violent, flinging movements of the arm and leg on the opposite side.
Hemiballismus = lesion of the contralateral subthalamic nucleus.
Athetosis
How it happens: damage to the putamen and globus pallidus, for example from brain injury around birth.
What you see: slow, writhing movements, mainly of the hands and fingers.
Wilson's disease
How it happens: an inherited defect in copper handling, so copper builds up in the liver and the lentiform nucleus.
What you see: tremor, rigidity and abnormal movements, liver disease and a brown ring at the edge of the cornea (Kayser-Fleischer ring).
Haemorrhage near the internal capsule
How it happens: rupture of a lenticulostriate artery (see blood supply of the brain and spinal cord).
What you see: opposite-side weakness from internal capsule damage.
Expect: which structures make up the lentiform nucleus, striatum and corpus striatum; the relation of the internal capsule; the role of dopamine; and the lesion site in Parkinson's (substantia nigra), Huntington's (caudate) and hemiballismus (subthalamic nucleus).
Quick revision
- Striatum: caudate + putamen. Lentiform: putamen + globus pallidus.
- Internal capsule between lentiform nucleus and caudate/thalamus.
- Direct pathway increases movement; indirect decreases it.
- Dopamine from substantia nigra pars compacta increases movement.
- Parkinson's: resting tremor, rigidity, bradykinesia, postural instability.
- Huntington's: caudate, chorea, CAG repeat.
- Hemiballismus: opposite subthalamic nucleus.
Written by The House Job from standard neuroanatomy references (Snell's Clinical Neuroanatomy).
FAQ
What are the basal ganglia?
Deep masses of grey matter (caudate, putamen and globus pallidus), working with the subthalamic nucleus and substantia nigra, that help control movement.
What is the lesion in Parkinson's disease?
Loss of dopamine-producing neurons in the substantia nigra pars compacta.
Which nucleus is affected in hemiballismus?
The subthalamic nucleus on the opposite side.
What forms the lentiform nucleus?
The putamen and the globus pallidus.