Parkinson's disease

Meaning: Neurons in the substantia nigra, a midbrain region, release dopamine onto the basal ganglia, which help start and smooth voluntary movement. As these neurons degenerate, dopamine falls and movement becomes harder to initiate and control. Core motor signs are resting tremor, muscle rigidity, bradykinesia (slowed movement), and a shuffling gait with poor balance. Mood and cognitive changes, including depression and, in some patients, dementia, can appear later. Age is the main risk factor, and the cause is often unclear.

On the MCAT: A passage describes tremor at rest and slowed movement and asks which neurotransmitter or brain region is involved. A common comparison is dopamine imbalance across disorders: too little in Parkinson's, while the dopamine hypothesis of schizophrenia proposes excess activity. So raising dopamine can ease Parkinson's motor symptoms but may produce hallucination-like symptoms in some patients, and blocking dopamine can produce parkinsonian-type movement problems.

Common confusion: Students mix up Parkinson's with Alzheimer's disease. Alzheimer's is marked early by memory loss and by plaques and tangles, while Parkinson's begins with movement problems from dopamine loss. Also, the Parkinson's tremor occurs at rest. An intention tremor, which appears during purposeful movement, points to cerebellar damage instead.