I describe here my understanding of Parkinson's Disease that I hope brings some insight into your PD or that of a loved one. What I describe below is what has come to me recently after contemplating PD symptoms for quite some time. I would like your feedback on whether all this makes sense to you (this will seem ironic after you read it.) You can skip over the Words and Introduction paragraphs.
Words. If ever there was a disease, PD suffers and their loved ones would say it is Parkinson's. The word "disease" originally meant "inconvenience, discomfort and trouble", and it comes from combining the prefix "dis-" and the word "ease". The prefix "dis-" means "away, apart, lacking, opposite and fail"; and the word "ease" as a noun means "comfort, poise and facility" and as a verb "to facilitate, to move gently and to relax" and even in the military "to assume a relaxed position, maintain silence and stay in place". Wow., the word "disease" certainly hits the nail on the head. Parkinson's is not a disease ... it is THE disease!
Introduction. I'm old now, and I have PD. Many years ago, when I was young, I earned an engineering degree, and I worked with robotics and was an expert troubleshooter, isolating problems to the failed mechanical, electrical and software components. I gained an excellent theoretical understanding and working knowledge of the scientific field of "modern control systems".
Even before college, biology had fascinated me, specifically the brain, and so I also studied neural networks and the biological senses. When the field of Artificial Intelligence began, I very happily studied it, especially Artificial Neural Networks (ANNs) that mimic brain cells. It all thoroughly fascinates me.
I have also successfully tutored a wide range of students over the years, so I think I have a sense of how to describe something complicated in easy-to-understand words. I will describe below how our brains are control systems and that PD seems to involve a particular problem with a particular part of it.
Control Systems. Certain sections of our brains are control systems. All control systems have the same basic components:
- #1 – the command and the decision process,
- #2 – the feed-forward process and the actuator, and
- #3 – the sensors and the feedback process.
As an example, let's say you are driving (in the United States) in the right-hand lane on the highway, and you see the exit ahead. You are part of the car's control system. The command (in your mind) is to be in the exit lane, and the decision process realizes that the exit lane is to your right, requiring you to steer to the right. The "feed-forward process" is everything that actually steers the car — your muscle nerves, muscles, tendons, bones and skin, and the steering wheel, steering column, power steering, suspension, rims and wheels (and road!). So far everything is working as desired, and the car begins to move toward the exit lane. The sensors and feedback process are working as you feel the movement and resistance of the steering wheel, and you sense the car changing lanes, seeing it with your eyes and feeling it with your inner ears. As you see the car cross over the dotted line into the exit lane, you bring the steering wheel back to its neutral position. And as you see the car approach the desired center of the exit lane, you actually steer the car to the left momentarily to stop it from going farther to the right and into the guardrail. You see the exit ramp curving ahead of you, so you apply the brakes and steer the car along the ramp. Congratulations!
(Spoiler Alert: It seems to me that PD involves overly-strong signals in the feedback processes of our brains.)
Feedback. In your driving experiences, I hope you have realized that the feedback process is the most essential part of a control system. Without it, you are not "in control" — you have no control whatsoever (even though the feed-forward process works fine: the steering, power train and brakes, and you're able to work all the controls), and other problems with the feedback will cause problems with the success of the control system. This is why clean windows and windshields that are also clear of rain, snow and ice and condensation are essential to safe driving. When we question someone's abilities, we may actually say that they can't "see" what they're doing (both literally and figuratively), especially in the immediate (feedback) sense. Someone outside a car helps a driver to back up — that's feedback. Likewise, a man may love his wife, but if he "never pays any attention to her", does he really love her? (Hint: it's really a feedback problem.)
Insufficient feedback. Too little feedback, and the control system doesn't "know" what it's accomplishing. You could drive a Rolls Royce or a Ferrari, but if you're blindfolded, you won't get far at all. However, you could drive a rickety cantankerous piece of junk, meaning a car with troublesome "feed-forward processes" (steering, brakes, engine, etc), but if you can see and feel what you're doing, you'll probably reach your destination. Years ago, an American car maker (Ford, I think) had become notorious for having developed and marketed a kind of power steering that didn't provide any (or enough) "road feel" (a form of feedback) through the steering wheel. This kind of power steering may have responded quickly and precisely as commanded, but did not provide enough tactile* feedback. (I can just imagine the design engineers responding to the complaints, "The top brass told us they wanted power "steering", not power "feeling!" It turns out, both are needed.)
Excessive feedback. Too much feedback is also troublesome. Imagine driving while looking through a windshield that magnifies like binoculars or a telescope. Too much detail — the opposite of a windshield cluttered with snow and ice or fogged up with condensation. You'll get too much feedback of every little change that occurs when you steer the car. And everyone knows that having a "backseat driver" is too much feedback. It's the same as a boss "micromanaging" employees. It's the same as someone being a "helicopter parent" to their children. Too much feedback! I think this is the problem with Parkinson's.
Human body. Let's consider the classic situation of extending your hand to shake someone's hand. There's feedback from the muscle, called "muscle sense" or "kinesthesia" (the sensation of position, movement, tension etc of parts of the body, perceived through nerve-end organs in muscles, tendons, and joints) and similar feedback from the skin associated with the body part or muscle and even more feedback through the eyes. There's the command/desire to extend the hand, and the decision process that determines that the commanded position differs from the current position and sends a signal to move the hand via the feed-forward process — that is, the muscle's nerves and the muscle, tendons, bones and skin. As the hand moves, its current position approaches the commanded position, and the decision process sends ever-smaller signals to the muscle until the hand reaches the commanded position, the decision process stops sending a signal to the muscle, and the muscle and hand stop moving.
This sounds great on paper, that is, theoretically — however, control systems are usually more sophisticated than this because, of course, a zero signal will not stop a muscle, and the momentum of the hand will overshoot the commanded position. To prevent this overshooting, the control system also uses the movement (ie, speed) of the hand from the muscles, skin and eyes as another kind of negative feedback. Actually, hand speed will act negatively to limit the signal to the muscle for two reasons: #1 – to limit the speed of the hand toward the commanded position (you don't want to extend your hand as though you're throwing a karate chop), and #2 – to not overshoot the commanded position (you don't want to punch the other person in the stomach) ... and you especially don't want to karate chop the other person in the stomach. So, for reason #2, as the hand approaches the commanded position, a high rate of speed will actually cause an opposite effect — the signal to the muscles will basically reverse polarity, slowing and stopping the hand to prevent it from overshooting the commanded position.
Parkinson's Disease. In my model of motion in the body, dopamine seems to lessen the importance of the feedback processes in the brain. Dopamine is in a Goldilocks-and-the-Three-Bears kind of situation, too much, too little and just right. PD patients may be especially aware of this when treating themselves with Sinemet (Carbidopa-Levodpa).
Excessive dopamine. Too much dopamine causes not enough feedback, so there's not enough to prevent the overshoot, and so, the control system habitually overshoots the commanded position — first in one direction, and then the other, again and again. Think of it in terms of a drunk trying to do something such as put their glass on the table, but with insufficient awareness-feedback; they eventually realize they overshot the table, so they move it in the other direction but overshoot again, and so on, dominated by the momentum of the body parts.
It probably affects the torso so extensively because the immense interconnections among the vertebra cause one muscle's control system to affect neighboring vertebra and their muscles and control systems, like one part of a waving flag moving nearby parts. The neighboring vertebra's control systems then react to these changes in muscle sense, and react to correct. Due to their linear arrangement, all of this in concert will cause the torso to writhe in a somewhat sinuous, snake-like fashion ... or perhaps more accurately, like a flag waving in the wind. We call this "dyskinesia" — relatively slow continuous oscillations.
Insufficient dopamine. Too little dopamine allows too much feedback, and the control system reacts too strongly and quickly to it with starts and stops in both directions, never settling on the commanded position, again and again, but relatively quickly due to the strong feedback signals screaming like helicopter parents, Back! Forward! Back! Forward! Back! Forward! We call these oscillations "Parkinsonian tremors" — relatively fast continuous oscillations.
Sufficient dopamine. Just enough dopamine allows our bodies' control systems to function as designed, including a "dead zone" — an appropriate region of position, posture etc that satisfies the control system, resulting in calmness, comfort and poise (ie "ease" – as in lack of "disease"). We call this "normal", and it's also produced by dopamine's "medication window", which is not too much or too little dopamine.
Conclusions. To me and my engineering-troubleshooting mind, my explanation easily explains most of the known symptoms of Parkinson's Disease:
- movement errors in general — dyskinesia and tremors,
- movement on soft/highly-tactile* surfaces — turning over and moving about on beds, couches, chairs and rugs and entering/exiting cars,
- movement against other soft/highly-tactile* objects — dressing (putting body parts into clothes, stuffing shirt tails into pants tops etc), putting hands in pockets, sexual intercourse etc,
- handling small objects — handling food, money, stamps, small hand tools, buttoning buttons, using a computer mouse etc,
- freezing — I haven't studied thoroughly-excessive feedback by modeling it on a computer, but I'm thinking it would cause a control system to "lock-up" or freeze, like a child excessively harassed by a helicopter parent (or for that matter, an excessively hen-pecked husband) unable to do anything but sit and stare as if in a catatonic stupor, as well as
- other symptoms — less frequent eye blinks, less frequent swallowing, and uncoordinated swallowing or seemingly coughing for no apparent reason etc, where excessive tactile sensations can inhibit, interfere, confuse and overwhelm.
It even explains urgent bladder because, if I remember correctly, the feedback of the urge to void actually increases the urge. That is, this feedback has a positive (speed up) effect, and not a negative (slow down) effect, on voiding. The opposite (negative feedback) would be disastrous — the occasional urges would quiet themselves, and they would be easy to ignore or go unnoticed, resulting in a disastrous situation (rupturing-exploding bladder? or kidney failure?). Urination is such a vital function, as are pulse and breathing, that evolution has probably prevented dopamine from affecting it.
Please ask any questions, and I will try to answer them.
* "tactile" means "associated with touch"