The rigidity seen in Parkinson’s disease is due to
**Core Concept**
Parkinson's disease is a neurodegenerative disorder characterized by the progressive loss of dopamine-producing neurons in the substantia nigra, leading to motor symptoms such as tremors, rigidity, bradykinesia, and postural instability.
**Why the Correct Answer is Right**
The rigidity in Parkinson's disease is primarily due to the abnormal functioning of the basal ganglia, particularly the globus pallidus and substantia nigra. The loss of dopamine leads to an imbalance in the inhibitory and excitatory signals between these structures, resulting in increased muscle tone and rigidity. The **subthalamic nucleus** and **globus pallidus internus** are key components of the basal ganglia circuitry, and their dysfunction contributes to the development of Parkinsonian symptoms, including rigidity.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the underlying pathophysiology of Parkinson's disease. While muscle tone may be increased in Parkinson's, the primary cause is not a simple increase in muscle tone.
**Option B:** This option is incorrect because it oversimplifies the complex neuroanatomical and neurophysiological changes that occur in Parkinson's disease. Rigidity is not solely due to a decrease in dopamine, but rather a disruption in the normal functioning of the basal ganglia circuitry.
**Option C:** This option is incorrect because it is too vague and does not provide a specific mechanism for the development of rigidity in Parkinson's disease.
**Clinical Pearl / High-Yield Fact**
The **"on-off" phenomenon**, characterized by fluctuations in motor symptoms between "on" periods of relatively normal function and "off" periods of severe rigidity and immobility, is a hallmark of advanced Parkinson's disease.
**Correct Answer:** C.