The endocrine system acts by releasing hormones that in turn trigger actions in specific target cells . Receptors on target cell membranes bind only to one type of hormone. The binding hormone changes the shape of the receptor causing the response to the hormone. There are two mechanisms of hormone action on all target cells. Five different second messenger chemicals, including cyclic AMP have been identified.
Second messengers activate other intracellular chemicals to produce the target cell response.
Images from W.H. Freeman and Sinauer Associates, used by permission. The second mechanism involves steroid hormones, which pass through the plasma membrane and act in a two step process. Steroid hormones bind, once inside the cell, to the nuclear membrane receptors, producing an activated hormone-receptor complex. The activated hormone-receptor complex binds to DNA and activates specific genes, increasing production of proteins. Image from W.H. Freeman and Sinauer Associates, used by permission.