Functionally, these neurons are either motor (conducting impulses that will cause activity such as the contraction of muscles) or association (conducting impulses and permitting 'communication' between neurons within the central nervous system).
The scant research that has been done on the causes of pica suggests that the disorder is a specific appetite caused by mineral deficiency in many cases, such as iron deficiency, which sometimes is a result of celiac disease or hookworm infection.
For the uninitiated, pica (pronounced PY-kah) is an appetite for non-food items. This tendency is relatively common in children and adults with autism or other developmental disabilities. They may try to eat all sorts of things. The items I most commonly hear about are paper, soap, pebbles, thread and bits of clothing.