The amount of resistance to movement that exists in a muscle. Muscle tone is increased in Parkinson’s disease as a result of disruptions in the nerve signals from the basal ganglia, particularly the subthalamic nucleus (STN), and other areas of the brain involved with movement to the muscles. Delays and interruptions in these messages cause slowed or inappropriate response in the muscles.
The depletion of dopamine that is the hallmark of Parkinson’s disease allows a disproportion of acetylcholine to exist in the brain. In addition to their separate functions related to brain activity, these two neurotransmitters seem to work as a “check and balance” system to regulate messages from brain neurons to muscle cells. In Parkinson’s disease, the lack of dopamine prevents transmission of certain nerve signals, accounting for symptoms such as bradykinEsia (slowed movement). At the same time the excess of acetyl-choline results in a chaotic jumble as other nerve signals are allowed through; other neuromuscu-lar symptoms such as tremors, rigidity, and cog-wheeling, classic symptoms of Parkinson’s disease result.
The person with Parkinson’s typically does not perceive that muscles are stiff or rigid. In the early and middle stages of the disease, many neuromus-cular symptoms tend to disappear with voluntary movement. Regular physical activity helps the person with Parkinson’s disease take advantage of this tendency, observing the common advice “Use it so you don’t lose it.” Walking and swimming are activities that exercise nearly all muscle groups in the body. As the disease progresses and damage to the pyramidal pathways becomes more extensive, however, this is less likely to occur. Doctors can assess muscle tone through clinical examination that involves passive movement of a muscle group.
ELECTROPHYSIOLOGIC STUDIES such as ELECTROMYLOgram (EMG) can provide measures of the electrical activity within muscle groups, a more direct way to assess nerve function.
Muscle tone changes with anti-parkinson’s medications. It can be near-normal during on-states when medication is at its most effective, and extremely heightened during an off-state, when medication levels in the body are lower than the levels necessary to control symptoms such as rigidity. Surgical treatments such as deep brain stimulation (DBS), pallidotomy, and thalamotomy can block chaotic neuron messages, helping to provide long-term control of symptoms related to muscle tone.