Question

Degeneration of neurons in which structure is primarily responsible for idiopathic Parkinson's disease? What is the main neurotransmitter of these neurons (Review Figure 16.7, p. 750 in Neuroanatomy through Clinical Cases, 2nd Ed.)? How does the loss of these neurons result in a hypokinetic movement disorder?

Answer 1

The Parkinson's disease is caused by the fegenedegene of dopaminergic receptors of the substantia nigra which projects to the striatum ie the nigristatial pathway. The neurotransmitter that runs through this neuron is the dopamine.

In the Substantia nigra (SNc), the two hallmark histopathologic features of IPD are depigmentation of dopamine-producing neurons (i.e., loss of SNc neurons) and presence of Lewy bodies in the remaining SNe neurons. Lewy bodies are neuronal cytoplasmic filamentous aggregates composed of the presynaptic protein - synuclein. Clinical features donot emerge until >60-80% dopamine lost.

Compensatory changes include hyperactivity in remaining neurones (increased transmitter turnover), increase in dopamine receptors and receptor supersensitivity.

Other pigmented nuclei are also affected (locus ceruleus and tape). Also cortex and other structures affected are affected.

Basal ganglia of the brain control movement of the body. Dopamine is an inhibitory neurotransmitter in the basal ganglia. Acetylcholine is the excitatory neurotransmitter in the ganglia. Without dopamine, inhibitory intluences are lost and excitatory mechanisms are unopposed This lead to over stimulation of basal ganglia causing excessive muscle tone, tremors rigidity. Thus due to the rigity the motion will be less leading to hypokinetic movement.