Endocrine system Flashcards
Neuroendocrine system
- general funciton: communication, integration, and control
- secreting cells send hormone molecules by way of the blood to specific target cells
- hormones are carried to almost every point in the body and can regulate most cells
- endocrine glands are “ductless glands” composed of glandular epithelium
Endocrine glands
Responsible for secreting hormones directly into the blood
- ductless glands
- widely scattered throughout the body
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Hormones
Tropic - sex - anabolic - steroid - non steroid
Steroid hormones
- synthesized from cholesterol
- lipid soluble
Nonsteroid hormones
- synthesized primarily from amino acids
- protein hormones
- glycoproteins hormones
- peptide hormones
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General principles of hormone action
- hormones signal a cell by inning to the target cell’s specific receptors in a “lock and key” mechanisms
- different hormone receptor interactions produce different regulatory changes within the target cell through chemical reactions
- combine hormone actions: synergism, permissiveness, antagonism
How hormones work
- diffuse into the blood and are transported to almost every point in the body
- a given hormone affects only specific target cells
- effects work more slowly and last longer than those of neurotransmitters
Primary effect
Directly regulate target cells
Secondary effects
Influence or modulate other regulatory mechanism in target cells
- endocrine glands produce more hormone molecules than are actually needed
- unused hormones are quickly excreted by the kidneys or broken down by metabolic processes
Mechanism of steroid hormone action
- steroid hormones are lipid soluble, and their receptors are normally found in the target cell’s cytosol
- after a steroid hormone molecule has diffused into the target cell, it binds to a receptor molecule to form a hormone receptor complex
- mobile-receptor model: hormone passes into the nucleus, where it binds to a mobile receptor and activates a certain gene sequence to begin the transcription
Mechanisms of nonsteroid action
Most operate using second messenger model
- first messenger: non steroidal hormone delivers chemical message to fixed receptor on target cell’s plasma membrane. Message is then passed into the cell
- second messenger: picks up message passed into cell by first messenger. Triggers appropriate cellular changes
Second messenger mechanism
- amplifies effects of the hormone.
- doesn’t take a lot of nonsteroid hormone to produce a huge effect
- faster than steroid mechanism
- produce effects in seconds to minutes (vs 45 min-days with steroid hormones)
Regulation of hormone secretion
Most hormonal regulatory systems are negative feedback loops. Tend to reverse any deviation of the internal environment away from its stable point
- a few are positive feedback loops
- responses that result from the operation of feedback loop within the endocrine system are called endocrine reflexes
- regulated by:
1. Signals from the nervous system
2. Chemical changes in the blood
3. Other hormones
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Mechanisms of hormone secretion 1
An endocrine gland is sensitive to the physiological changes produced by its target cells
- parathyroid hormone (PTH) produces responses in its target cells (bone, kidneys, intestinal wall) that increase Ca++ concentration in blood. When Ca++ exceeds the set point value, parathyroid cells sense it and reflexively reduce their output of PTH
Mechanisms of hormone secretion 2
Endocrine gland secretion may also be regulated by a hormone produced by another gland
- the pituitary gland produces thyroid-stimulating hormone (TSH), which stimulated the thyroid gland to release it hormones
Mechanisms of hormone secretion 3
Endocrine gland secretions may be influenced but input from the nervous system
- secretion by the posterior pituitary is not regulated by hormonal input but by direct nervous input from the hypothalamus
Regulation of target cell sensitivity
- sensitivity of target cell to any particular hormone depends in part on the number of receptors it has
- the more receptors there are, the more sensitive the target cell
- hormone receptors are constantly broken down by the cell and replaced with newly synthesized receptors
- if synthesis of new receptors occurs faster than degradation of old receptors, the target cell will have more receptors and thus be more sensitive to the hormone
- up and down regulation
Up regulation
Increased umber of hormone receptors increases sensitivity
Down regulation
Decreased number of hormone receptors decreases sensitivity
Specific endocrine loops
- Hypothalamus and pituitary gland
- thyroid gland
- pancreatic islets
Hypothalamus and pituitary gland
- Hypothalamus
- major link between the nervous and endocrine systems
- cells in the hypothalamus synthesize at least 9 different hormones - Pituitary gland
- secretes 7 different hormones
- driven by the hypothalamus - Hypothalamus and pituitary together regulate virtually all aspects of growth, development, metabolism, and homeostasis
Insulin like growth factors
Influence carbohydrate metabolism by decreasing glucose uptake, which decreases use of glucose for ATP production in cells
- spares glucose so its available to neurons for ATP production when glucose is low
- IGFs and growth hormone may stimulate liver cells to release glucose into the blood
Hypoglycemia
- stimulates hypothalamus to secrete growth hormone- releasing hormone (GHRH)
- GHRH goes to the pituitary gland and stimulates release of GH
- GH stimulates secretion of IGFs = speed breakdown of liver glycogen into glucose
- blood glucose increases above normal levels and inhibits release of GHRH
Hyperglycemia
- stimulates hypothalamus secrete growth hormone inhibiting hormone GHIH
- GHIH goes to the pituitary gland and inhibits secretion of GH
- low levels of GH and IGFs slows breakdown of glycogen in the liver and blood glucose levels decrease
- decreased blood glucose levels below normal inhibits release of GHIH
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Thyroid gland
- located just inferior to the larynx
- only endocrine gland that stores its secretory product in large quantities (100 day supply)
- actions of thyroid hormones:
1. increase metabolic rate - metabolism of carbs, lipids, and proteins increase
2. stimulate synthesis of additional sodium-potassium pumps
3. stimulate protein synthesis and increase the use of glucose and fatty acids for ATP production
4. with GH and insulin, accelerate body growth of nervous and skeletal systems
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Pancreatic islets
Pancreas is an endocrine gland and an exocrine gland (exocrine functions are digestive)
- its at the first part of the small intestine
- endocrine tissue of pancreas is called the pancreatic islets (artist formerly known as islets of langerhans)
Pancreatic islets: hypoglycemia
Glucagon and insulin secretion control
- low blood sugar stimulate secretion of glucagon from alpha cells of the islets
- glucagon acts on liver cells to accelerate conversion of glycogen to glucose and promotes formation of glucose from lactic acid and certain amino acids (gluconeiogenesis)
- hepatocytes (liver cells) then release glucose into the blood = rise in blood glucose levels
Pancreatic islets: hyperglycemia
- stimulates secretion of insulin by beta cells of the pancreatic islets to lower blood glucose levels
- insulin acts on the body to:
- acccelerate facilitated diffusion of glucose into cells
- sped conversion of glucose into glycogen (glycogensis)
- increase uptake of amino acids by cells
- increase protein synthesis
- speed up synthesis of fatty acids (lipogenesis)
- slow conversion of glycogen to glucose
- slow formation of glucose from lactic acid and amino acids
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Exercise and glucagon
- increased activity of the sympathetic nervous system stimulates glucagon secretion
- consistent positive effects on carbohydrate metabolism and insulin sensitivity
- improves skeletal muscle glucose transport and whole body glucose homeostasis
