What is the pathophysiology of motor neuron disease?

• A. Progressive degeneration of upper and/or lower motor neurons leading to muscle atrophy.
• B. Defect in peripheral nerve myelination only.
• C. Autoimmune attack on neuromuscular junction.
• D. Degeneration of cerebellar Purkinje cells.

Answer: (A)

Motor neuron disease is a neurodegenerative process in which motor neurons in the central nervous system gradually deteriorate. When upper motor neurons (in the cortex and corticospinal tract) and/or lower motor neurons (in the brainstem and spinal cord anterior horn) die, the brain’s signals to muscles are lost, leading to progressive weakness and muscle wasting. The pattern of signs reflects which neurons are affected: upper motor neuron loss causes spasticity and brisk reflexes, while lower motor neuron loss causes muscle weakness with fasciculations and atrophy. In many conditions within this group, both neuron types are involved, producing a combination of UMN and LMN findings.

The underlying pathophysiology involves several converging processes, including glutamate-mediated excitotoxicity, oxidative stress, mitochondrial dysfunction, and abnormal protein folding or aggregation (such as TDP-43 or SOD1), along with neuroinflammation from glial cells. This all contributes to selective degeneration of motor neurons while other neurons remain relatively preserved.

Other choices describe different disorders: peripheral nerve demyelination, autoimmune attack on the neuromuscular junction, or degeneration of cerebellar Purkinje cells, which do not capture the loss of motor neurons driving motor neuron disease.