What causes is your arm to shake?

A 67-year-old man came into my office and said, “My hands shake when I lift up my coffee cup. This has been going on for many years, but recently has been getting worse. What causes that?”

Shaky hands could be many things.  Many people notice that they have tremor in their hands.  The most common form is called essential tremor[1], because it occurs in isolation without other symptoms of brain disease.  This is noticeable when using the hands to do precise movements, such as drinking hot coffee from a mug filled to the top, when drawing lines on paper, or when using a camera to take a picture.  The tremor is most often mild, but in those sensitive circumstances, it can cause spillage of the coffee, wavy lines drawn on the paper, or photographs that are fuzzy.

Here a few facts.  There is a strong genetic component – there is usually a close family member with tremor.  Characteristically, it is in both hands, not just on one side.  Sometimes, also, it affects the head or the voice.  Because it is benign, meaning that it is not a harbinger of serious or devastating brain disease, essential tremor is usually thought of as a condition, rather than a disease.  In that sense, it is a tendency that some people have, much like a tendency to get tension headaches that last an hour and go away on their own or with a weak pain medication.  It is real and bothersome, but it is not a sign of a serious brain disease.  That said, some people experience such severe tremor that it can be disabling.  But, that is rare.

What causes essential tremor?  There are circuits in the brain that are responsible for controlling the coordination and balance of your movement.  Let’s consider how robotic arms are controlled by electronic circuits.  To achieve smooth movement, the engineers construct circuits that have feedback – they adjust the electronic signals controlling the robot’s motors based on sensors that measure the position and speed of movement of the robotic arm.  Tuning of the circuits needs to be exact.  If the tuning is off, the robotic arm can start to oscillate – in essence, the robot develops tremor.

A similar thing happens as the brain controls your arm.  For example, when you reach your arm to pick up the cup of coffee, a number of steps have to happen with precision.  The voluntary decision to reach for the cup is made by your higher thought centers, mostly involving the part of your brain called the prefrontal cortex.   Then, the motor action is planned out in other parts of your frontal lobe as a sequence of smaller movements involving many different muscles that control the shoulder, elbow, wrist, and finger joints.  Then, comes the execution of the plan that involves deeper parts of your brain called the basal ganglia.  Finally, as the movement is being executed, ensuring the smoothness of the movement is the job of the cerebellum.

The cerebellum is the small part of your brain that humans have in common with practically all other vertebrates, from crocodiles to elephants.  It is the part of the brain that specializes in making sure that the executed movements result in the desired outcomes – precisely and effortlessly.  Think of how you are able to reach over to the coffee cup, lift it to your mouth, and maintain its position in exactly the right way so that you can sip the coffee without spilling.  This is no simple feat.  Your nerve circuits in your cerebellum are at work to adjust the nerve commands going to your arm muscles, so that you don’t spill.  As with robotic control, if all the instructions to the muscles were based on the pre-planned sequence alone, the arm would get close to the cup, but there would inevitably be errors  – and spillage of the coffee.  The cerebellum uses feedback signals from special sensors that tell the brain where the arm is at all times.  This feedback involves your vision, special sensors of your joints’ positions, touch sensors, and more.  It is fast and automatic – you do not need to think about it.

When the cerebellum’s circuits are a little off tune, then the arm may have a small tremor.  That is what happens in essential tremor.  It seems that for some people, their cerebellum is a little off tune, much in the way that other people may have difficulty singing on key.  That is just how we are – all of us slightly different.  Each with our strengths and weaknesses.  We learn to live with it, and it is hardly a disability.

I find it remarkable that the brain uses such a precise movement control system and it only rarely is impaired to the point of disability.  We tend to take our health for granted, but the daily function that is the result of continuous coordinated brain activity is nothing short of miraculous.

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Dr. Ely Simon is an adult neurologist working in Modiin, Israel.  He recently published Embracing the Unknown: A Fresh Look at Nature and Science, a book on the wonders of the brain and other aspects of nature. 

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[1] Essential tremor is the most common movement disorder, affecting around 2% of the population.