In neuronal signaling, the chemical messenger is a neurotransmitter (acetylcholine, for example). Electrical signals (nerve impulses) originate in the cell body of a neuron and travel very rapidly over long distances to the axon tip, where neurotransmitters are released and diffuse to the target cell. The target cell (another neuron, a myocyte, or a secretory cell) is only a fraction of ___ micrometer or a few micrometers away from the site of neurotransmitter release.