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from Guyton and Hall Textbook of Medical Physiology (4th South Asia Edition) write a proper 5 mark short answer on Parkinsonism

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Parkinsonism (Parkinson Disease) - Short Answer

Definition/Etiology Parkinson disease (paralysis agitans) results from widespread degeneration of the pars compacta of the substantia nigra, which normally sends dopamine-secreting nerve fibers to the caudate nucleus and putamen (corpus striatum). The exact cause is uncertain, but a combination of genetic and environmental factors (e.g., pesticide toxins) is implicated. It affects over 10 million people worldwide, with men affected 1.5 times more than women.
Pathophysiology Dopamine is an inhibitory neurotransmitter. Destruction of dopaminergic neurons in the substantia nigra removes this inhibition, allowing the caudate nucleus and putamen to become overactive, producing continuous excitatory output to the corticospinal motor system. This overexcitation of muscles explains the rigidity. Loss of inhibition also raises feedback gain in motor circuits, causing them to oscillate, producing the resting tremor. Reduced dopamine in the limbic system (nucleus accumbens) is thought to reduce psychic drive for motor activity, contributing to akinesia.
Clinical Features
  1. Rigidity of much of the body musculature.
  2. Tremor - involuntary, at a fixed rate of 3-6 cycles/sec, present even at rest (unlike cerebellar "intention" tremor which occurs only with voluntary movement).
  3. Akinesia - serious difficulty initiating movement; movements when they occur are stiff and staccato rather than smooth.
  4. Postural instability from impaired postural reflexes, causing poor balance and falls.
  5. Other motor features: dysphagia, speech disorders, gait disturbance, fatigue.
  6. Nonmotor features: sleep disturbances, depression, anxiety, autonomic dysfunction, and cognitive impairment (advanced stages).
Treatment L-dopa is administered because it crosses the blood-brain barrier and is converted to dopamine within the brain (dopamine itself cannot cross the barrier). This restores the balance between excitation and inhibition in the caudate nucleus and putamen, improving rigidity and akinesia significantly, though it has little effect on nonmotor symptoms.
  • Guyton and Hall Textbook of Medical Physiology, p. 711-712
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