For multicellular organisms to form and operate, cellular behavior must be vastly more complex than what is seen on the single-cell level. Cells must not only regulate their own growth and behavior, they must also communicate and interact with their neighbors to ensure the correct behavior of the entire organism. Intercellular communication occurs in a variety of ways, ranging from hormonal communication on the level of the entire body to localized interactions between individual cells. Our discussion in this chapter is limited to cellular communication processes that occur between cells or over a region of a small number of cells. Other forms of communication and control, such as hormone feedback systems, will be studied in other chapters.
There are two primary ways that cells communicate with neighbors. Many cells (muscle and cardiac cells for example) are connected to their immediate neighbors by gap junctions in the cell membrane that form a relatively nonselective, low-resistance, pore through which electrical current or chemical species can flow. Hence, a gap junction is also called an electrical synapse. The second means of communication is through a chemical synapse, in which the message is mediated by the release of a chemical from one cell and detected by receptors on its neighbor. Electrically active cells such as neurons typically communicate via chemical synapses, which are thus a crucial feature of the nervous system. Because chemical synapses form the basis for neuronal communication they have been studied in considerably more detail than electrical synapses.
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