Cells in multicellular organisms need to communicate with one another to coordinate their function and control their growth. The human body has several means of transmitting information between cells. These mechanisms include direct communication between adjacent cells through gap junctions, autocrine and paracrine signaling, and endocrine or synaptic signaling. Autocrine signaling occurs when a cell releases a chemical into the extracellular fluid that affects its own activity (Fig. 4.9).
With paracrine signaling, enzymes rapidly metabolize the chemical mediators, and therefore they act mainly on nearby cells. Endocrine signaling relies on hormones carried in the bloodstream to cells throughout the body. Synaptic signaling occurs in the nervous system, where neurotransmitters act only on adjacent nerve cells through special contact areas called synapses. In some parts of the body, the same chemical messenger can function as a neurotransmitter, a paracrine mediator, and a hormone secreted by neurons into the bloodstream.