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The USA physical geography

 

Activity of the endocrine glands is regulated by signal (input) and by deviation (quantity). We have already examined the mechanism of regulation by quantity when we discussed regulation of the activity of hormone-producing cells. Now let us look at the 2nd type of regulation Ц by УinputФ. Regulation of endocrine glands by input (entry) is realized through neural and humoral mechanisms. An illustration of the neural mechanism is activation of the adrenal medulla in case of stimulation of sympathetic NS with participation of dorsal hypothalamic nuclei of the diencephalon. The humoral mechanism of activation of pituitary-dependent endocrine glands also starts in the hypothalamic region where appropriate releasing hormones (liberins) are secreted. These hormones activate cells of the anterior pituitary (adenohypophysis) to produce tropic hormones, which in turn activate pituitary-dependent endocrine glands. This route of activation is effective in relation to the adrenal cortex.

Action on an organism of a strong stimulus activates not only sympatho-adrenal complex, but also the neurosecretory activity of cells of the hypothalamus, and, particularly, promotes secretion of corticoliberin hormone. This hormone is carried with the bloodstream to the anterior pituitary where it activates synthesis and secretion of adrenocorticotropic hormones. The latter are transported throughout the body and УfindФ target cells in the adrenal cortex, which respond to this by activation of synthesis and secretion of corticosteroids, in particular, of hydrocortison.

Both mechanisms of regulation of the activity of endocrine glands are complementary and act at different stages of adaptation of the body to varying environment.

In each cell there usually function different kinds of receptors to one hormone (for example, both α- and β-adrenoreceptors). Besides, a cell is usually sensitive to 7-10 different endocrine regulators (neurotransmitters, hormones, prostaglandins, growth factors). Each of these regulators is characterized by specific duration and amplitude of regulatory signal, and each regulator is characterized by a certain proportion of activities of second messenger generation systems in the cell or by certain variations in the membrane potential. At the level of effector systems of the cell there may occur both potentiation and mutual extinction of different regulator signals.

Besides, in each cell there also function special biochemical mechanisms that regulate sensitivity of the cell to a hormone. Let us illustrate these mechanisms on an example of a receptor coupled with G-proteins. The level of hormones that act through this system of transmembrane signalization (which, besides substances mentioned above, include prostaglandins, pituitary hormones, angiotensin II, bradykinin, vasopressin, oxytocin, histamine, dopamine, enkephalin, endorphin, serotonin, endothelin, cholecystokinin, gastrin, parathyroid hormone) usually rises for several minutes. This period is sufficient for formation of the required quantity of the second messengers (cyclic AMP, Ca2+, diacylglycerole), which activate the respective protein kinases with subsequent phosphorylation of proteins. But if the level of hormones stays high for dozens of minutes or several hours (due to hyperfunction of the endocrine gland or to pharmacological intervention), the desensitization of the respective receptor results. At first the receptor becomes phosphorylated by protein kinase present in the plasma membrane practically of all cells, which decreases their affinity to this hormone 2-5-fold. This protein kinase may phosphorylate only hormone+receptor complex; therefore the longer the hormone remains bound to the receptor, the higher the probability for this receptor to be phosphorylated. If such phosphorylation is unable to suppress the hormone signal, then 15-30 minutes later the receptor becomes phosphorylated by protein kinase that is activated by the respective second messenger (for example, by cAMP-dependent protein kinase in case of β-adrenergic receptors that activate adenylate cyclase; or by protein kinase C in case of α1-adrenergic or M1- and M3-cholinergic receptors that activate phospholipase C). Phosphorylation of receptors by protein kinases dependent on second messengers, deranges coupling with G-proteins that weakens activating or inhibiting influence of hormones acting through receptors on adenylate cyclase or K+ channels. If the level of the hormone remains high within hours, and the above mentioned desensitization mechanisms cannot suppress the regulatory signal, the hormone+receptor complexes undergo endocytosis, with receptorosomes appearing inside cells. If the level of hormone decreases within the first 2-3 hours, they may again build into the plasma membrane. If the level does not decrease, they fuse with the lysosomes, after which receptors become disrupted. It is clear that restoration of the sensitivity of the cell to this hormone will require new synthesis of hormones.



The USA physical geography

 

I Physiographic divisions. Relief.

II Major Rivers and Lakes.

III Climate.

IV The main industries and cities.

 

I Physiographic divisions. Relief.

The geography of the United States varies across their immense area. The conterminous United States may be divided into eight broad physiographic divisions:

1. from east to west, the Atlantic-Gulf Coastal Plain Ц the coastal regions of the eastern and southern parts includes the continental shelf, the Atlantic Coast and the Gulf Coast;

2. the Appalachian Highlands - lying on the eastern side of the United States, it includes the Appalachian Mountains (јппалачи), the Watchung Mountains, the Adirondacks and New England province originally containing the Great Eastern Forest. The Appalachian Mountains, ancient mountains, extend southwest from SE Canada to the Gulf Coastal Plain in Alabama. In E New England, the Appalachians extend in a few places to the Atlantic Ocean, contributing to a rocky, irregular coastline. The Appalachians and the Adirondack Mountains (јдирондак) of New York (which are geologically related to the Canadian Shield) include all the chief highlands of E United States; Mt. Mitchell (6,684 ft/2,037 m high), in the Black Mts. of North Carolina, is the highest point of E North America.

3. the Interior Plains - part of the interior continental United States, it includes much of what is called the Great Plains;

4. the Interior Highlands - also part of the interior continental United States, this division includes the Ozark Plateau (плато ќзарк) and the Ouachita Mountains (”ошито);

5. the Rocky Mountain System - one branch of the Cordilleran system lying far inland in the western states. This geologically young and complex system extends into NW United States from Canada and runs S into New Mexico. There are numerous high peaks in the Rockies (—калистые горы); the highest is Mt. Elbert (14,433 ft/4,399 m). The Rocky Mts. are divided into four sections - the Northern Rockies, the Middle Rockies, the Wyoming (Great Divide) Basin, and the Southern Rockies.

6. the Intermountain (ћежгорный) Region - also divided into the Columbia Plateau, the Colorado Plateau and the Basin and Range Province, it is a system of plateaus, basins, ranges and gorges between the Rocky and Pacific Mountain Systems. It is the setting for the Grand Canyon, the Great Basin and Death Valley. The lowest point in North America, in Death Valley (282 ft/86 m below sea level), is there. The largest basin in the region is the Great Basin, an area of interior drainage (the Humboldt River is the largest stream) and of numerous salt lakes, including the Great Salt Lake.

7. the Pacific Mountain System - the coastal mountain ranges and features in the west coast of the United States. The Cascade Range, with its numerous volcanic peaks extends S from SW Canada into N California, and from there is continued south by the Sierra Nevada, a great fault block. Mt. Whitney (14,495 ft/4,418 m), in the Sierra Nevada, is the highest peak in the conterminous United States.

8. the Laurentian Uplands (Ћаврентийска€ возвышенность)- a part of the Canadian Shield, that extends into the United States from Canada in the Great Lakes region. It is an area of little local relief, with an irregular drainage system and many lakes, as well as some of the oldest exposed rocks in the United States.

 

Alaska may be divided into four physiographic regions; they are, from north to south, the Arctic Lowlands, the coastal plain of the Arctic Ocean; the Rocky Mountain System, of which the Brooks Range is the northernmost section; the Central Basins and Highlands Region, which is dominated by the Yukon River basin; and the Pacific Mountain System, which parallels Alaska's southern coast and which rises to Mt. McKinley (Denali; 20,320 ft/6,194 m), the highest peak of North America. The islands of SE Alaska and those of the Aleutian Islands chain are partially submerged portions of the Pacific Mountain System and are frequently subjected to volcanic activity and earthquakes. These islands, like those of Hawaii, are the tops of volcanoes that rise from the floor of the Pacific Ocean. Mauna Kea and Mauna Loa on Hawaii are active volcanoes; the other Hawaiian islands are extinct volcanoes.

Elevation extremes:

Lowest point: Death Valley, Inyo County, California -282 ft (−86 m) below sea level.

Highest point: Mount McKinley, Denali Borough, Alaska +20,320 ft (6,194 m) above sea level.


Date: 2015-01-02; view: 262


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