Cell signalling. Computer artwork showing four different cell signalling pathways. Signalling molecules (blue) travel between cells, allowing them to respond to changes in their environment and to coordinate this response. They can trigger a number of reactions, including changes in metabolism and gene expression. The different pathways are: at top left, cell communication over short distances (paracrine signalling), at upper right, communication between cells in physical contact (juxtacrine signalling), at bottom right, communication via nerve cells (paracrine signalling) and at bottom left, communication over long distances via the bloodstream (endocrine signalling).