L4 - Physiology PDF Notes - Because the cell is the...

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Because the cell is the functional unit of biological systems, we can examine signaling systems that operate within cells ( intracellular signaling ), and those that operate between cells ( extracellular or intercellular signaling ). In multicellular organisms, intercellular signaling via released chemicals, referred to as chemical messenger molecules , is necessary to maintain the coordinated activity of various tissues and organ-systems. The cells in the immediate surrounding would have receptors for the signaling chemical and would respond in an appropriate manner. This type of chemical signaling is now referred to as paracrine signaling. Paracrine signaling refers to chemical messenger molecules that are secreted and travel only a short distance by diffusion to effect cells in the immediate environment of the cell that secreted the messenger chemical. A special form of paracrine signaling is signaling in an autocrine fashion. Autocrine refers to a situation in which the chemical messenger that is released by one cell binds to receptors on the surface of the very cell that secreted the messenger molecule. In most animals, there are three extracellular communication systems that play the role of maintaining organismal homeostasis. These are the nervous system , the endocrine system , and the immune system .
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