Medical brain notes 44

  Coordination of Body Functions by Chemical Messengers The multiple activities of the cells, tissues, and organs of the body are coordinated by the interplay of several types of chemical messenger systems: 1.                Neurotransmitters  are released by axon terminals of neurons into thesynaptic junctions and act locally to control nerve cell functions. 2.                Endocrine hormones  are released by glands or specialized cells into thecirculating blood and influence the function of cells at another location in the body. 3.                Neuroendocrine hormones  are secreted by neurons into the circulatingblood and influence the function of cells at another location in the body. 4.            ...

  Chemical Structure and Synthesis of Hormones There are three general classes of hormones: 1.   Proteins and polypeptides,  including hormonessecreted by the anterior and posterior pituitary gland, the pancreas (insulin and glucagon), the parathyroid gland (parathyroid hormone), and many others (see Table 74–1).   2.   Steroids  secreted by the adrenal cortex (cortisoland aldosterone), the ovaries (estrogen and progesterone), the testes (testosterone), and the placenta (estrogen and progesterone).   3.   Derivatives of the amino acid tyrosine,  secretedby the thyroid (thyroxine and triiodothyronine) and the adrenal medullae (epinephrine and norepinephrine). There are no known polysaccharides or nucleic acid hormones.   Polypeptide and Protein Hormones Are Stored in Secretory Vesi-cles Until Needed.  Most of the hormones in the bodyare polypeptides and proteins. These hormones range in size from small peptides with as few as 3 amino...

  Hormone Secretion, Transport, and Clearance from the Blood Onset of Hormone Secretion After a Stimulus, and Duration of Action of Different Hormones.  Some hormones, such asnorepinephrine and epinephrine, are secreted within seconds after the gland is stimulated, and they may develop full action within another few seconds to minutes; the actions of other hormones, such as thy-roxine or growth hormone, may require months for full effect. Thus, each of the different hormones has its own characteristic onset and duration of action—each tailored to perform its specific control function. Concentrations of Hormones in the Circulating Blood, and Hor-monal Secretion Rates.   The concentrations of hormonesrequired to control most metabolic and endocrine functions are incredibly small. Their concentrations in the blood range from as little as 1 picogram (which is one millionth of one millionth of a gram) in each mil-liliter of blood up to at most a few micrograms (a few millionth...

  Mechanisms of Action of Hormones Hormone Receptors and Their Activation The first step of a hormone’s action is to bind to specific  receptors  at the target cell. Cells that lack receptors for the hormones do not respond. Receptors for some hormones are located on the target cell membrane, whereas other hormone receptors are located in the cytoplasm or the nucleus. When the hormone combines with its receptor, this usually initi-ates a cascade of reactions in the cell, with each stage becoming more powerfully activated so that even small concentrations of the hormone can have a large effect. Hormonal receptors are large proteins, and each cell that is to be stimulated usually has some 2000 to 100,000 receptors. Also, each receptor is usually highly specific for a single hormone; this determines the type of hormone that will act on a particular tissue. The target tissues that are affected by a hormone are those that contain its specific receptors. The locations for the d...

  Intracellular Signaling After Hormone Receptor Activation Almost without exception, a hormone affects its target tissues by first forming a hormone-receptor complex. This alters the function of the receptor itself, and the activated receptor initiates the hormonal effects. To explain this, let us give a few examples of the different types of interactions. Ion Channel–Linked Receptors.  Virtually all the neuro-transmitter substances, such as acetylcholine and norepinephrine, combine with receptors in the postsy-naptic membrane. This almost always causes a change in the structure of the receptor, usually opening or closing a channel for one or more ions. Some of these  ion channel–linked receptors  open (or close) channelsfor sodium ions, others for potassium ions, others for calcium ions, and so forth. The altered movement of these ions through the channels causes the subsequent effects on the postsynaptic cells. Although a few hormones may exert some of their actio...

  Second Messenger Mechanisms for Mediating Intracellular Hormonal Functions We noted earlier that one of the means by which hormones exert intracellular actions is to stimulate for-mation of the second messenger cAMP inside the cell membrane. The cAMP then causes subsequent intra-cellular effects of the hormone. Thus, the only direct effect that the hormone has on the cell is to activate a single type of membrane receptor. The second mes-senger does the rest.  cAMP is not the only second messenger used by the different hormones. Two other especially important ones are (1) calcium ions and associated  calmodulin  and (2) products of membrane phospholipid breakdown. Adenylyl Cyclase–cAMP Second Messenger System Table 74–2 shows a few of the many hormones that use the adenylyl cyclase–cAMP mechanism to stimulate their target tissues, and Figure 74–7 shows the adeny-lyl cyclase–cAMP second messenger system itself. Binding of the hormones with the receptor allows couplin...