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Ref: Stephan Schwartz <[log in to unmask]> Subject: Re: Models for PD -Reply -Reply I've changed the subject line from my original post "Models for PD". I realize now that this is a misnomer since we first need a model which represents those elements that sense and control motor movement before we can identify possible causes of failure in that system. Some have suggested in this list that once we stop voluntary movement, such as, moving our finger to our nose, that we are in a static position not affected by dopamine levels. My theory is that we reach a steady state position, but not a static position. Holding any position whether sitting or standing requires fine levels of motor control. Since muscles can only contract or relax, holding position requires multiple muscles to be in some state of contraction. As I stated in a previous post, changing the phase or gain in the feedback loop of an electromechanical system (such as a robotic arm) can cause it to exhibit Parkinsonian symptoms. Phase is simply time delay. It takes longer for the sensing signal to get through the feedback system. Reduced gain is loss of sensitivity. It takes a higher state of contraction of a given muscle before we detect it is in a given state. We either oscillate (tremor) or activate both antagonistic controls at the same time (rigidity) in an attempt to hold position. Holding position is an involuntary action our brain learned during our childhood development. As we moved from sitting in a baby carrier with arms and legs flailing in uncontrolled movements to eventually standing and walking, our brain learned to maintain synergistic levels of muscle contraction for stability. My theory is that the out of phase nature of our sensor inputs is what causes the slowness of movement. We learn to correlate the phase relationship between our visual input and out tactile input. When this relationship changes, it takes more processing to sort out the correct action. As a practical example, if you drive an automobile from the early '50s with a large steering wheel it feels akward since we have grown accustommed to smaller wheels with greater responsiveness to a slight turn. With PD, this relationship can change with the state of our medication. Just think what will happen when I go over and visit that Brits and have to drive on the left-hand side of the road. Don't worry Brian, I'll give you plenty of warning before I come. Now to get to Stephan's post about why we don't tremor when asleep. My theory is that it is because we are not trying to hold any kind of involuntary position. I am just beginning to exhibit a slight tremor, but it is highly positional dependent. My finger will tremor if I try and hold it about 1/4 inch above a keyboard key. If I rest it slightly on the key, no tremor. I'd like to hear from people on the list with persistent tremor as to when it is the worst and when it is at it's lowest. Disregarding medication levels, when is tremor the lowest. Example might be sitting with arms resting on a table, sitting with arms resting on the chair arms, in a totally prone position in bed. Phil Gesotti 47(1) [log in to unmask]