Deficiencies of mitochondrial enzymes affect primarily the brain and muscle. In the brain, mitochondrial disorders cause neuron loss and present as encephalopathy, myoclonus, strokes, and other clinical disorders. Hypoxia, ischemia and hypoglycemia cause irreversible neuronal injury. Injured neurons shrink, become eosinophilic due to condensation of mitochondria, and their nuclei become pyknotic. Such neurons are referred to as anoxic neurons .
Neurons contain numerous lysosomes used for life-long recycling of biomolecules and organelles.
Deficiency of lysosomal enzymes causes unrecycled substrates of these enzymes to accumulate in lysosomes. Abnormal lysosomes gradually fill the cell body and processes, leading to destruction of neurons. The best known example is Tay-Sachs disease. Neurofilament proteins are chemically distinct from intermediate filaments of other cells. Neurotubules are polymers of alpha and beta tubulin.