How does the brain INITIATE a signal?

As a biochemistry graduate student, I understand a lot about neurons (action potentials, neurotransmitters, all that good stuff), white/grey matter, different lobes of the brain. The question I am asking is, how is a signal the brain sends to the rest of the body (lets use skeletal muscles in this example - somatic system) initiated? Simplified, how is my brain initiating a signal to the muscles in my hand/arms to move? How does the thought of a movement physiologically translate to the act of the movement? Furthermore, how is that thought even initiated? What neurons do we have active voluntary control over in the brain to initiate a signal? If possible, please link me to a site with more information about this. Thank you.