Professional and Linguistic Assessments Board (PLAB) Practice Exam

What happens to the contralateral body as a result of dysfunction of the substantia nigra?

1. Ipsilateral paralysis

2. Contralateral dementia

3. Contralateral parkinsonism

4. Unilateral headaches

The correct answer is: Contralateral parkinsonism

Dysfunction of the substantia nigra primarily impacts the dopaminergic pathways in the brain, which are crucial for regulating voluntary movement. The substantia nigra is part of the basal ganglia, and its failure leads to reduced dopamine availability in areas of the brain that coordinate movement. When the substantia nigra is impaired, especially in conditions like Parkinson's disease, the motor control is affected significantly. The consequence is typically contralateral parkinsonism, such as tremors, rigidity, and bradykinesia, which occurs on the side of the body opposite to the affected hemisphere of the brain. This is due to the way that motor signals are processed and cross over in the neurological pathways. This phenomenon illustrates the importance of neural circuits and how the central nervous system regulates motor functions. While local motor dysfunction might suggest issues like unilateral headaches, they do not align with the specific motor deficiencies caused by substantia nigra dysfunction, which is centered around the broader motor control mechanisms leading to bilateral effects in coordination and movement.