SN2
SHORT NOTE
5M
★ High-yield
Enumerate the components and functions of the basal ganglia. Describe the features of Parkinson disease.
Components
Basal ganglia = subcortical grey masses concerned with the control of movement, muscle tone and posture.
| Structure | Parts |
|---|---|
| Corpus striatum | Caudate nucleus + lentiform nucleus |
| Lentiform nucleus | Putamen + globus pallidus |
| Neostriatum (striatum) | Caudate + putamen — the chief input nuclei |
| Palaeostriatum | Globus pallidus — the chief output nucleus |
| Functionally associated | Substantia nigra and subthalamic nucleus |
Pathways
| Pathway | Effect on movement |
|---|---|
| Direct | Facilitates movement — disinhibits the thalamus |
| Indirect | Inhibits unwanted movement |
| Nigrostriatal (dopamine) | Excites the direct and inhibits the indirect pathway → net effect is to facilitate movement |
Functions
- Planning and initiation of voluntary movement
- Regulation of muscle tone — inhibitory
- Control of associated (automatic) movements — arm swing while walking, facial expression
- Suppression of unwanted involuntary movements
- Cognitive and emotional functions through frontal lobe connections
Parkinson Disease
Degeneration of substantia nigra pars compacta → ↓ Dopamine in the striatum → Direct pathway underactive, indirect pathway overactive → ↑ Inhibition of thalamus and cortex → poverty OF movement
Cardinal features
- Tremor — at rest, 4–6 Hz, "pill-rolling"; decreases on voluntary movement (the opposite of cerebellar tremor)
- Rigidity — lead-pipe, or cogwheel when tremor is superimposed; affects both flexors and extensors equally
- Bradykinesia and akinesia — slow initiation, mask-like face, micrographia, loss of arm swing
- Postural instability — stooped posture, festinant (shuffling) gait
- Symptoms appear only after 60–80% of dopaminergic neurones are lost
- Lewy bodies are the pathological hallmark
Other Basal Ganglia Disorders
| Disorder | Lesion | Movement |
|---|---|---|
| Hemiballismus | Subthalamic nucleus (contralateral) | Violent flinging of the limbs |
| Chorea (Huntington) | Caudate and putamen | Rapid, jerky, purposeless movements |
| Athetosis | Putamen | Slow, writhing movements |
| Wilson disease | Lentiform nucleus (copper) | Tremor, rigidity, dystonia; Kayser–Fleischer ring |
Applied Aspects
- Treatment — levodopa with carbidopa; dopamine itself cannot cross the blood–brain barrier
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