Thursday, January 19, 2012

Intercellular Chemical Messengers

Essential to reflexes and local homeostatic responses, and therefore to homeostasis, is the ability of cells to communicate with one another. In the majority of cases, this communication between cells—intercellular communication— is performed by chemical messengers. There are three categories of such messengers: hormones, neurotransmitters, and paracrine agents. A hormone functions as a chemical messenger that enables the hormone-secreting cell to communicate with cells acted upon by the hormone—its target cells—with the blood acting as the delivery system. Most nerve cells (neurons) communicate with each other or with effector cells by means of chemical messengers called neurotransmitters. Thus, one nerve cell alters the activity of another by releasing from its ending a neurotransmitter that diffuses through the extracellular fluid separating the two nerve cells and acts upon the second cell. Similarly, neurotransmitters released from nerve cells into the extracellular fluid in the immediate vicinity of effector cells constitute the controlling input to the effector cells. Chemical messengers participate not only in reflexes but also in local responses. Chemical messengers involved in local communication between cells are known as paracrine agents.