Cells use a variety of signaling mechanisms to coordinate cellular processes and communicate with other cells. A signaling pathway is initiated when a signaling molecule called a ligand binds to its target receptor. Paracrine signaling occurs when a secreted ligand targets the neighboring cells. Whereas, autocrine signaling occurs when the ligand targets the same cell that released it. In a juxtacrine or contact-dependent signaling pathway, a ligand remains bound to the cell surface and interacts directly with the receptors on adjacent cells. Small soluble ligands may also directly flow between cells via gap junctions in the plasma membrane. Multicellular organisms coordinate cellular behavior over a long distances. In endocrine signaling, hormones travel to different body parts through the bloodstream. Synaptic signaling between neurons is a specialized form of long-range signaling. At the synaptic junction, the axon terminal of a neuron secretes ligands called neurotransmitters to communicate with target cells. Additionally, quorum sensing is carried out by some bacterial communities to react together by sensing a critical concentration of signaling ligands.