Do the motor manifestations of Parkinson disease alter motor axon excitability?

Axonal excitability is altered in common medical conditions such as stroke, multiple sclerosis, and spinal cord injury. Given the motor neuron changes in the presence of rigidity and tremor in Parkinson disease, we examine whether there are also changes in motor axon excitability.

Axonal excitability studies were performed in 15 Parkinson subjects and 12 age-matched control subjects.

There was no significant difference in excitability indices between Parkinson subjects and control subjects.

It is unlikely that the lack of change in the excitability indices reflects a balance between the effects of bradykinesia ("underactivity") and the effects of rigidity and tremor ("overactivity") on the motoneuron and its axon. It is more likely that plastic changes in motoneuron properties do not occur symmetrically with decreases and increases in activity, being more profound when activity levels are interrupted and less obvious when they are enhanced.