Hold your hand out and try to keep it perfectly still. After a moment, it will probably begin to shake. Hold that same hand out just before you give a major presentation to your new boss, and you'll notice that those normal small tremors increase substantially. Why?
Why do we shake?
The answer has to do with how nervous feelings influence a response cycle between two parts of your brain. These parts are called the basal ganglia and the cortex.
In order for your brain to control the steadiness of your hand, the cortex and the basal ganglia exchange signals with each other. First, the cortex sends a signal to the basal ganglia; the basal ganglia, in turn, responds returning a signal to the cortex. These signals are the basis of motor control.
Cortex and the basal ganglia
By exchanging signals, the cortex and the basal ganglia allow your hand muscles to hold their position. This cycle of signals repeats continuously the entire time your hand is extended.
But, like most mysteries of the brain, this one involving motor control is not fully understood by scientists.
However, scientists believe that the mental effort required to hold your hand still causes the muscles in your hand to contract a bit at regular intervals and therefore experience a tremor, even under normal circumstances.
When you're nervous, the part of your brain that regulates your anxiety interferes with the response cycle between the cortex and the basal ganglia, increasing the severity of these muscle contractions and thereby altering your ability to accurately control the position of your hand.
Though your hand will shake a little bit under normal conditions, it will shake a lot more when these nervous feelings influence your hand position control.