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University of Toronto Scarborough
Biological Sciences

PRINCIPLES OF ANIMAL PHYSIOLOGY 3 | CELL SIGNALING AND ENDOCRINE REGULATION  Two of the most familiar types of cellular communication involve the nervous system and endocrine system  cAMP - cyclic adenosine monophosphate  The fundamental features of cellular communication of all living things are: o The production of a signal in one cell o The transport of that signal to a target cell o The transduction of that signal into a response in the target cell  The endocrine system is involved in controlling and regulating almost every physiological process including growth, development, metabolism, and ion and water balance █ Overview  Signal transduction pathways cause a response within the target cell  Chemical messengers can travel from a signaling cell to nearby target cells by diffusion in paracrine communication. These messengers can even affect the signaling cell in autocrine communication  Long distance cell to cell communication involves the endocrine and nervous system o Endocrine messengers are called hormones o The nervous system uses chemical messengers called neurotransmitters █ The Biological Basis of Cell Signaling General Features of Cell Signaling  Cells can overcome the problem of moving a hydrophilic chemical messenger through the lipid environment of the membrane by communicating via gap junctions o Gap junctions create a aqueous pore between two neighbouring cells and composed of connexins (in vertebrates) or innexins (in invertebrates) o Direct communication involves movement of ions between cells o Gap junctions can be opened and closed to regulate communication of substances between cells o Indirect cell signaling involves three steps:  Release of a chemical messenger from the signaling cell into the extracellular environment  Transport of the chemical messenger through the extracellular environment  Communication of the signal to the target cell via receptor binding  TABLE 3.1  The most important distinction between different systems for cellular communication is the distance across which the chemical messenger must travel  A synapse is a region where the signaling cell and the target cell are very close together  In exocrine communication, the chemical pheromone is released by one individual and travels through the external environment to exert its effects on a different individual  There are 6 main classes of chemicals that are known to participate in cellular signaling in animals: peptides, steroids, amines, lipids, purines, and gases  TABLE 3.2 Peptide Messengers  Amino acids typically act as neurotransmitters while peptides and proteins may be autocrines, paracrines, neurotransmitters, neurohormones, hormones, or pheromones  Peptide and proteins messengers are synthesized on the RER  Cytokines are only synthesized on demand  Peptide hormones are often synthesized as large inactive polypeptides called preprohormones o Contain multiple copies of a peptide hormone/hormones and a signal sequence that targets the polypeptide for secretion o After cleaving, it is called the prohormone  Hydrophilic signaling molecules cannot pass through the membrane of target cells, but bind instead to transmembrane receptors o The extracellular portion of the transmembrane receptor is called the ligand- binding domain o A ligand is any small molecule that binds specifically to a protein Steroid Messengers  Steroids are derived from cholesterol  There are three important classes of steroids: o Mineralocorticoids  Involved in regulating sodium uptake by the kidney, and are important for fluid and electrolyte balance o Glucocorticoids (cortisol, cortisone, and corticosterone)
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