CELL COMMUNICATION

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. 4-10). With paracrine signaling, enzymes rapidly metabolize the chemical mediators and therefore 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 neurotrans-mitters 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 neuro-transmitter, a paracrine mediator, and a hormone secreted by neurons into the bloodstream.