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Movement Disorders Quiz
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A 42-year-old woman presents with progressive involuntary dance-like movements and gradual cognitive decline. A genetic cause is suspected. Which change in basal ganglia signalling best explains her abnormal movements?
- A Reduced inhibitory output from the globus pallidus externus to the subthalamic nucleus You picked ✕
- B Increased dopaminergic input from the substantia nigra pars compacta to the striatum You picked ✕
- C Increased excitatory output from the subthalamic nucleus to the globus pallidus internus You picked ✕
- D Reduced striatal inhibition of the external globus pallidus via the indirect pathway You picked ✓
- E Increased inhibitory output from the striatum to the external segment of the globus pallidus You picked ✕
Correct!
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The answer is D: Reduced striatal inhibition of the external globus pallidus via the indirect pathway
Explanation
Why D is correct
See Motor Disorders. Option D is correct because the picture of dance-like movements (chorea), cognitive decline and a genetic cause suggests Huntington's disease, an autosomal dominant condition (passed on if one parent carries the faulty gene) that mainly kills the inhibitory medium spiny neurones of the striatum. These neurones belong to the direct and indirect pathways, and those of the indirect pathway, which normally suppress movement, are affected early. The indirect pathway neurones normally inhibit the external segment of the globus pallidus (GPe). When they are lost, the GPe becomes overactive and inhibits the subthalamic nucleus more, so the subthalamic nucleus excites the internal pallidum (GPi) less. The GPi then inhibits the thalamus less, and the overactive thalamus drives the motor cortex to produce excess involuntary movement. Symptoms begin typically in mid-life, with chorea, mood change and dementia. A helpful memory aid is that the movement-suppressing pathway fails, so unwanted movements escape.
- Disease: Huntington's disease is autosomal dominant and causes chorea and cognitive decline.
- Cells lost: inhibitory medium spiny neurones of the striatum, especially in the indirect pathway.
- Circuit effect: the globus pallidus internus inhibits the thalamus less, so the thalamus becomes overactive.
- Result: excess involuntary movement.
Why the other options are incorrect
- A. Reduced inhibitory output from the globus pallidus externus to the subthalamic nucleus: This would release the subthalamic nucleus from inhibition, increasing its drive to the internal pallidum and increasing thalamic inhibition. That would reduce movement, the opposite of Huntington's chorea. Your answer
- B. Increased dopaminergic input from the substantia nigra pars compacta to the striatum: Dopamine excess is not the primary lesion in Huntington's disease, in which striatal neurones die. Extra dopamine can worsen chorea, and drugs that deplete dopamine are used to treat it, but the cause is the loss of striatal cells. It would be the answer for a drug-induced dyskinesia. Your answer
- C. Increased excitatory output from the subthalamic nucleus to the globus pallidus internus: This would increase internal pallidal inhibition of the thalamus and reduce movement. In Huntington's disease the subthalamic nucleus is underactive, so this excitatory drive is reduced, not increased. Your answer
- E. Increased inhibitory output from the striatum to the external segment of the globus pallidus: In Huntington's disease the striatal neurones that project to the external pallidum are lost, so this inhibitory output is reduced, not increased. Increased inhibition here would suppress movement rather than cause chorea. Your answer
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1
Which cerebellar peduncle connects the cerebellum to the pons?
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2
A 21-year-old student practises kicking a football. During this voluntary movement, which statement best describes the normal function of dopamine within the basal ganglia motor pathways?
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