Perception and integrated behavior do not arise from single genes or cells, but rather they emerge from the connectivity patterns amongst many neurons assembled into physiological (often anatomical as well) networks. One major question we grapple with is understanding how neurons self assemble into such non-random, functional circuits?
There seems to be significant bidirectional communication between targets and neurons. In other words, as a neuron I know who I am partially because the target tells me. It is a major challenge to identify the substance and context of these cellular and molecular conversations.
A "Simple" Motor System
Annelids have closed, pressurized circulatory systems and generate circulation of blood by rhythmic contraction of "hearts." The leech hearts are a bilateral pair of tubes that contract in coordinated waves that propel blood, and combined with side to side pressure differentials and phased contractions of lateral vessels with associated sphincters and valves (see section of motor output) these complex coordinated contractions perfuse tissues on demand and provide effective gas exchange at the skin. Leech hearts are innervated by a pair of motor neurons called heart excitors (HEs).
The rhythmic activity of HEs is patterned by other neurons within the CNS. The mature HEs have no muscle target other than the heart tubes (HT).
During embryonic life the HEs extend multiple arbors into the body wall muscle BEFORE the heart muscle is even present. The HE seems to not quite know who it is or exactly where to go.
BUT......As the heart emerges HE growth cones reorient and the HE arborizes upon the heart. At the same time the HE withdraws all of the projections elsewhere. It is contact with the heart muscle that is required for this anatomical remodeling and subsequent pruning.
In the absence of the heart muscle, HE motor neurons continue to elaborate in the bodywall muscle. HE deprived of peripheral heart contact also cannot be recognized by the interneurons that normally entrain the rhythm!
The muscle also provides synaptogenic signals, for more on that - click this sentence.