Soon, smiling becomes automatic, just like breathing. In a similar way, the baby learns to reach, grasp, speak and walk. More and more complex actions become possible as the baby builds the neural pathways that travel between the muscles and the brain via the nervous system. Handwriting is an extremely complex motor task, which is not usually learned until the child is 5 or 6 years old and has mastered simpler skills. The motor system controls the movement and posture needed for handwriting by contraction and relaxation of muscles.
Messages go to and from the muscles and brain via the nervous system.
The brain makes the adjustments needed to give a maximal outcome. Eventually a motor program is formed. This is a set of muscle commands that can be carried out with the correct timing and sequence automatically, without feedback, to give the best possible result. Handwriting is the result of such stored motor knowledge. Handwriting is distal, meaning that it occurs at the extremities and involves fine motor activity as opposed to a skill like walking which is proximal - a large, or gross motor skill.