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Basal gangliaA group of deep brain structures that help control movement and are affected in Parkinson's disease and by antipsychotic drugs.Read more → circuitry matters because it explains why Parkinson's disease causes slowed movement, while a stroke in one small nearby structure causes wild, flinging movements instead. The basal ganglia are a group of deep brain structures, including the striatum, globus pallidus, subthalamic nucleus, and substantia nigraA deep brain structure rich in dopamine cells whose loss causes Parkinson's disease.Read more →, that fine-tune movement. For example, imagine two patients: one with slow, stiff movement from dopamine loss, and one with sudden violent flinging of one arm after a small stroke. In practice, the second pattern, hemiballismus, points to a completely different structure within this same circuit: the subthalamic nucleus.

The basal gangliaA group of deep brain structures that help control movement and are affected in Parkinson's disease and by antipsychotic drugs.Read more → control movement through two competing pathways. The direct pathway facilitates, or promotes, movement, while the indirect pathway inhibits, or suppresses, unwanted movement. Normal movement depends on a balance between the two. Dopamine from the substantia nigraA deep brain structure rich in dopamine cells whose loss causes Parkinson's disease.Read more → normally favours the direct, movement-facilitating pathway. In Parkinson's disease, loss of these dopamine neurons shifts the balance toward the indirect, inhibitory pathway, producing bradykinesia (slowness of movement), rigidity, and tremor. The subthalamic nucleus is part of the indirect pathway, and a lesion there removes its inhibitory brake, causing hemiballismus, sudden large, flinging movements of one side of the body. Picture a patient with poorly controlled hypertension who has a small stroke in the subthalamic nucleus: they develop sudden violent flinging movements of one arm. In practice, this specific lesion site explains why the movement disorder looks nothing like Parkinson's disease, despite both involving the same basal ganglia circuit.

• A neuroanatomy lecture identifies the two structures from which the basal gangliaA group of deep brain structures that help control movement and are affected in Parkinson's disease and by antipsychotic drugs.Read more → send their main inhibitory signal onward to the thalamusA deep brain structure that relays and filters sensory information on its way to the cortex.Read more →. Which pair forms this output?
• Dopamine from the substantia nigraA deep brain structure rich in dopamine cells whose loss causes Parkinson's disease.Read more → has opposite effects on the two main basal gangliaA group of deep brain structures that help control movement and are affected in Parkinson's disease and by antipsychotic drugs.Read more → pathways. Which combination of receptor actions produces this opposing pattern?
• A man with an inherited condition develops involuntary flowing movements of his limbs. Loss of which cells in the input region best explains these excess movements?
• After a small stroke, a man develops violent flinging movements of the limbs on one side of his body. A lesion of which structure most characteristically produces this?

• Direct pathway, facilitates movement, unlike the indirect pathway which inhibits it.
• Substantia nigra dopamine loss, causes Parkinson's disease's slowness and rigidity, whereas subthalamic nucleus damage causes hemiballismus.
• Hemiballismus, sudden flinging movements from lost inhibition, rather than the slow, stiff movement typical of Parkinson's disease.
• Levodopa, restores the direct pathway's dopamine balance, unlike treatments for hemiballismus which target a different structure.

• That pair forms the main input region rather than the output.
• Those structures belong to the reward-related rather than motor circuit.
• Neither of those structures is a principal output nucleus here.
• The first is an intermediate relay and the second is unrelated to this circuit.
• This reverses both receptor actions and would suppress movement.

• Localise the clue to the relevant anatomical level, cell type, pathway or molecular process.
• Map that structure or process to its normal function.
• Predict the deficit or clinical sign produced when it is disrupted.
• Reject options that belong to a neighbouring structure, pathway or level of explanation.