Final answer:
To put down and pick up a pen, the nervous system, including the central and peripheral nervous systems, and the somatic division of the PNS, work together to send signals from the brain to the muscles in the hand and arm to carry out the movements.
Step-by-step explanation:
When you put your pen down and then pick it up, various parts of your nervous system coordinate to perform these actions. Initially, your brain, specifically the area known as the cerebral cortex, makes the decision to move the pen. The neurons in the cerebral cortex send a nerve impulse through the central nervous system (CNS), consisting of your brain and spinal cord, down to neurons in your peripheral nervous system (PNS), which includes all the nerves extending from your spinal cord to your muscles. These nerves carry the signal from the CNS to the muscles in your hand and arm. As a result, motor neurons in your PNS activate the muscles to contract or relax, allowing you to perform the movements with precision and coordination. In summary, your somatic nervous system, which is part of the PNS and controls voluntary movements, is heavily involved in this process.