signals can be of many kinds light, sound, smell, touch etc. many of which are due to chemicals
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Two systems coordinate communication throughout the body: the endocrine system and the nervous system The endocrine system secretes hormones that coordinate slower but longer-acting responses including reproduction, development, energy metabolism, growth, and behavior - PowerPoint PPT PresentationTRANSCRIPT
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
• Two systems coordinate communication throughout the body: the endocrine system and the nervous system
• The endocrine system secretes hormones that coordinate slower but longer-acting responses including reproduction, development, energy metabolism, growth, and behavior
• The nervous system conveys high-speed electrical signals along specialized cells called neurons; these signals regulate other cells
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In multicellular animals, nerve impulses provide electric signals; hormones provide chemical signals.
Hormones are secreted by cells, diffuse into the extracellular fluid, and often are distributed by the circulatory system. Hormones reach all parts of the body, but only target cells are equipped to respond
Hormones work much more slowly than nerve impulse transmission and are not useful for controlling rapid actions.
Hormones coordinate longer-term developmental processes such as reproductive cycles.
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Concept 45.1: Hormones and other signaling molecules bind to target receptors, triggering specific response pathways
• Signals can be of many kinds light, sound, smell, touch etc. many of which are due to chemicals.
• Chemical signals bind to receptor proteins on target cells
• Only target cells with the appropriate receptor respond to the signal
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Types of Secreted Signaling Molecules
• Secreted chemical signals include
– Hormones
– Local regulators
– Neurotransmitters
– Neurohormones
– Pheromones
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Local Regulators
• Local regulators are chemical signals that travel over short distances by diffusion
• Local regulators help regulate blood pressure, nervous system function, and reproduction
• Local regulators are divided into two types
– Paracrine signals act on cells near the secreting cell
– Autocrine signals act on the secreting cell itself
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Signaling by Local Regulators
• In paracrine signaling, nonhormonal chemical signals called local regulators elicit responses in nearby target cells
• Types of local regulators:
– Cytokines and growth factors
– Nitric oxide (NO)
– Prostaglandins
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• Long distance signaling involves the chemical being taken by circulation to the target tissues. Usually hormones are released from a special cell or organ called an endocrine cell/organ, travel through the bloodstream, and interact with the receptor or a target cell to cause a physiological response.
Fig. 45-2
Bloodvessel Response
Response
Response
Response
(a) Endocrine signaling
(b) Paracrine signaling
(c) Autocrine signaling
(d) Synaptic signaling
Neuron
Neurosecretorycell
(e) Neuroendocrine signaling
Bloodvessel
Synapse
Response
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Chemical Classes of Hormones
• Three major classes of molecules function as hormones in vertebrates:
– Polypeptides (proteins and peptides)
– Amines derived from amino acids
– Steroid hormones
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• Lipid-soluble hormones (steroid hormones) pass easily through cell membranes, while water-soluble hormones (polypeptides and amines) do not
• The solubility of a hormone correlates with the location of receptors inside or on the surface of target cells
Fig. 45-3
Water-soluble Lipid-soluble
Steroid:Cortisol
Polypeptide:Insulin
Amine:Epinephrine
Amine:Thyroxine
0.8 nm
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Cellular Response Pathways
• Water and lipid soluble hormones differ in their paths through a body
• Water-soluble hormones are secreted by exocytosis, travel freely in the bloodstream, and bind to cell-surface receptors
• Lipid-soluble hormones diffuse across cell membranes, travel in the bloodstream bound to transport proteins, and diffuse through the membrane of target cells
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• Signaling by any of these hormones involves three key events:
– Reception
– Signal transduction
– Response
Fig. 45-5-1
NUCLEUS
Signalreceptor
(a) (b)
TARGETCELL
Signal receptor
Transportprotein
Water-solublehormone
Fat-solublehormone
Fig. 45-5-2
Signalreceptor
TARGETCELL
Signal receptor
Transportprotein
Water-solublehormone
Fat-solublehormone
Generegulation
Cytoplasmicresponse
Generegulation
Cytoplasmicresponse
OR
(a) NUCLEUS (b)
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Pathway for Water-Soluble Hormones
• Binding of a hormone to its receptor initiates a signal transduction pathway leading to responses in the cytoplasm, enzyme activation, or a change in gene expression
Animation: Water-Soluble HormoneAnimation: Water-Soluble Hormone
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Multiple Effects of Hormones
• The same hormone may have different effects on target cells that have
– Different receptors for the hormone
– Different signal transduction pathways
– Different proteins for carrying out the response
• A hormone can also have different effects in different species
Fig. 45-8-1
Glycogendeposits
receptor
Vesseldilates.
Epinephrine
(a) Liver cell
Epinephrine
receptor
Glycogenbreaks downand glucoseis released.
(b) Skeletal muscle blood vessel
Same receptors but differentintracellular proteins (not shown)
Fig. 45-8-2
Glycogendeposits
receptor
Vesseldilates.
Epinephrine
(a) Liver cell
Epinephrine
receptor
Glycogenbreaks downand glucoseis released.
(b) Skeletal muscle blood vessel
Same receptors but differentintracellular proteins (not shown)
Epinephrine
receptor
Different receptors
Epinephrine
receptor
Vesselconstricts.
(c) Intestinal blood vessel
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Hormones
• Endocrine signals (hormones) are secreted into extracellular fluids and travel via the bloodstream
• Endocrine glands are ductless and secrete hormones directly into surrounding fluid
• Hormones mediate responses to environmental stimuli and regulate growth, development, and reproduction
Exocrine glands have ducts and secrete substances onto body surfaces or into body cavities (for example, tear ducts)
Fig. 45-10Major endocrine glands:
Adrenalglands
Hypothalamus
Pineal gland
Pituitary gland
Thyroid gland
Parathyroid glands
Pancreas
Kidney
Ovaries
Testes
Organs containingendocrine cells:
Thymus
Heart
Liver
Stomach
Kidney
Smallintestine
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Concept 45.2: Negative feedback and antagonistic hormone pairs are common features of the endocrine system
• Hormones are assembled into regulatory pathways
• A negative feedback loop inhibits a response by reducing the initial stimulus
• Negative feedback regulates many hormonal pathways involved in homeostasis
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Coordination of Endocrine and Nervous Systems in Invertebrates
• In insects, molting and development are controlled by a combination of hormones:
– A brain hormone stimulates release of ecdysone from the prothoracic glands
– Juvenile hormone promotes retention of larval characteristics
– Ecdysone promotes molting (in the presence of juvenile hormone) and development (in the absence of juvenile hormone) of adult characteristics
Fig. 45-13-3
Ecdysone
Brain
PTTH
EARLYLARVA
Neurosecretory cells
Corpus cardiacum
Corpus allatum
LATERLARVA PUPA ADULT
LowJH
Juvenilehormone(JH)
Prothoracicgland
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Coordination of Endocrine and Nervous Systems in Vertebrates
• The hypothalamus receives information from the nervous system and initiates responses through the endocrine system
• Attached to the hypothalamus is the pituitary gland composed of the posterior pituitary and anterior pituitary
The pituitary gland of mammals is a link between the nervous system and many endocrine glands and plays a crucial role in the endocrine system.
Fig. 45-14
Spinal cord
Posteriorpituitary
Cerebellum
Pinealgland
Anteriorpituitary
Hypothalamus
Pituitarygland
Hypothalamus
Thalamus
Cerebrum
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• The posterior pituitary stores and secretes hormones that are made in the hypothalamus:these hormones are called neuroendocrine hormones
• The anterior pituitary makes and releases hormones under regulation of the hypothalamus
Fig. 45-15
Posteriorpituitary
Anteriorpituitary
Neurosecretorycells of thehypothalamus
Hypothalamus
Axon
HORMONE OxytocinADH
Kidney tubulesTARGET Mammary glands,uterine muscles
ADHThe function of antidiuretic hormone (ADH) is to
increase water conservation by the kidney. If there is a high level of ADH secretion, the
kidneys resorb water. If there is a low level of ADH secretion, the
kidneys release water in dilute urine.ADH release by the posterior pituitary increases if
blood pressure falls or blood becomes too salty.ADH causes peripheral blood vessel constriction
to help elevate blood pressure and is also called vasopressin.
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• Oxytocin induces uterine contractions and the release of milk
• Suckling sends a message to the hypothalamus via the nervous system to release oxytocin, which further stimulates the milk glands
• This is an example of positive feedback, where the stimulus leads to an even greater response
Fig. 45-17
Hypothalamicreleasing andinhibitinghormones
Neurosecretory cellsof the hypothalamus
HORMONE
TARGET
Posterior pituitary
Portal vessels
Endocrine cells ofthe anterior pituitary
Pituitary hormones
Tropic effects only:FSHLHTSHACTH
Nontropic effects only:ProlactinMSH
Nontropic and tropic effects:GH
Testes orovaries
Thyroid
FSH and LH TSH
Adrenalcortex
Mammaryglands
ACTH Prolactin MSH GH
Melanocytes Liver, bones,other tissues
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Tropic Hormones
• A tropic hormone regulates the function of endocrine cells or glands
• The four strictly tropic hormones are
– Thyroid-stimulating hormone (TSH)
– Follicle-stimulating hormone (FSH)
– Luteinizing hormone (LH)
– Adrenocorticotropic hormone (ACTH)
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Nontropic Hormones
• Nontropic hormones target nonendocrine tissues
• Nontropic hormones produced by the anterior pituitary are
– Prolactin (PRL)
– Melanocyte-stimulating hormone (MSH)
• Prolactin stimulates lactation in mammals but has diverse effects in different vertebrates
• MSH influences skin pigmentation in some vertebrates and fat metabolism in mammals
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Growth Hormone
• Growth hormone (GH) is secreted by the anterior pituitary gland and has tropic and nontropic actions
• It promotes growth directly and has diverse metabolic effects
• It stimulates production of growth factors
• An excess of GH can cause gigantism, while a lack of GH can cause dwarfism
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Figure 42.6 Effects of Excess Growth Hormone
Releasing and Release inhibiting Neurohormones of the Hypothalumus
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Anterior Pituitary Hormones
• Hormone production in the anterior pituitary is controlled by releasing and inhibiting hormones from the hypothalamus
• For example, the production of thyrotropin releasing hormone (TRH) in the hypothalamus stimulates secretion of the thyroid stimulating hormone (TSH) from the anterior pituitary
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Hormone Cascade Pathways
• A hormone can stimulate the release of a series of other hormones, the last of which activates a nonendocrine target cell; this is called a hormone cascade pathway
• The release of thyroid hormone results from a hormone cascade pathway involving the hypothalamus, anterior pituitary, and thyroid gland
• Hormone cascade pathways are usually regulated by negative feedback
Fig. 45-18-3
Cold
Pathway
Stimulus
Hypothalamus secretesthyrotropin-releasinghormone (TRH )
Neg
ativ
e fe
edb
ack
Example
Sensoryneuron
Neurosecretorycell
Bloodvessel
Anterior pituitary secretes thyroid-stimulatinghormone (TSHor thyrotropin )
Targetcells
Response
Body tissues
Increased cellularmetabolism
–
Thyroid gland secretes thyroid hormone (T3 and T4 )
–
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Thyroid Hormone: Control of Metabolism and Development
• The thyroid gland consists of two lobes on the ventral surface of the trachea
• It produces two iodine-containing hormones: triiodothyronine (T3) and thyroxine (T4)
Two forms of thyroxine, T3 and T4, are made from tyrosine. T3
(triiodothyronine) has three iodine atoms. T4 has four iodine atoms.
More T4 is produced in thyroid, but it can be converted to T3 by an enzyme in the blood. T3 is the more active form of the hormone.
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Thyroxine has many roles in regulating metabolism.
It stimulates the transcription of many genes in nearly all cells in the body. These include genes for enzymes of energy pathways, transport proteins, and structural proteins.
It elevates metabolic rates in most cells and tissues.
It promotes the use of carbohydrates over fats for fuel.
It promotes amino acid uptake and protein synthesis and so is critical for metabolism, growth and development. Insufficient thyroxine may result in cretinism.
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• Hyperthyroidism, excessive secretion of thyroid hormones, causes high body temperature, weight loss, irritability, and high blood pressure
• Hypothyroidism, low secretion of thyroid hormones, causes weight gain, lethargy, and intolerance to cold
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• A goiter is an enlarged thyroid gland associated with either very low (hypothyroidism) or very high (hyperthyroidism) levels of thyroxine.
Hyperthyroid goiter results when the negative feedback mechanism fails even though blood levels of thyroxine are high.
A common cause is an autoimmune disease in which an antibody to the TSH receptor is produced. This antibody binds the TSH receptor, causing the thyroid cells to release excess thyroxine. (Graves’ disease)
The thyroid remains maximally active and grows larger, causing symptoms associated with high metabolic rates.
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Hypothalamus
Anteriorpituitary
TSH
Thyroid
T3 T4+
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Hypothyroid goiter results when there is insufficient thyroxine to turn off TSH production.
The most common cause is a deficiency of dietary iodine.
With high TSH levels, the thyroid gland continues to produce nonfunctional thyroxine and becomes very large.
The body symptoms of this condition are low metabolism, cold intolerance, and physical and mental sluggishness.
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Parathyroid Hormone and Vitamin D: Control of Blood Calcium
• Two antagonistic hormones regulate the homeostasis of calcium (Ca2+) in the blood of mammals
– Parathyroid hormone (PTH) is released by the parathyroid glands
– Calcitonin is released by the thyroid gland
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Vertebrate Endocrine Systems- Calcium regulation
Calcium levels in the blood must be regulated within a narrow range. Small changes in blood calcium levels have serious effects.
Most calcium in the body is in the bones (99%). About 1% is in the cells, and only 0.1% is in the extracellular fluids.
Blood calcium levels are regulated by:
Deposition and absorption of bone
Excretion of calcium by the kidneys
Absorption of calcium from the digestive tract
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CalcitoninThyroid glandreleasescalcitonin.
StimulatesCa2+ depositionin bones
ReducesCa2+ uptakein kidneys
STIMULUS:Rising bloodCa2+ level
Blood Ca2+
level declinesto set point
Homeostasis:Blood Ca2+ level
(about 10 mg/100 mL)
Blood Ca2+
level risesto set point
STIMULUS:Falling bloodCa2+ level
StimulatesCa2+ releasefrom bones
Parathyroidgland
IncreasesCa2+ uptakein intestines
Activevitamin D
Stimulates Ca2+
uptake in kidneys
PTH
Hormonal control of calcium homeostasis in mammals
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Vertebrate Endocrine Systems
Calcitonin, released by the thyroid gland, acts to lower calcium levels in the blood.
Bone is constantly remodeled by absorption of old bone and production of new bone.
Osteoclasts break down bone and release calcium.
Osteoblasts use circulating calcium to build new bone.
Calcitonin decreases osteoclast activity and stimulates the osteoblasts to take up calcium for bone growth.
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Vertebrate Endocrine Systems
Blood calcium decrease triggers release of parathyroid hormone (PTH), from the parathyroid glands which are embedded in the posterior surface of the thyroid gland.
This in turn causes the osteoclasts to dissolve bone and release calcium.
Parathyroid hormone also promotes calcium resorption by the kidney to prevent loss in the urine.
It also promotes vitamin D activation, which stimulates the gut to absorb calcium from food.
Parathyroid hormone and calcitonin act antagonistically to regulate blood calcium levels.
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Vertebrate Endocrine Systems
In the kidneys, vitamin D acts with PTH to decrease calcium loss in urine.
In bone vitamin D acts like PTH to stimulate bone turnover and the liberation of calcium
The overall action of vitamin D is to raise blood calcium levels, which promotes bone deposition.
Vitamin D also acts in a negative feedback loop to inhibit transcription of the PTH gene in the parathyroid glands.
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Vertebrate Endocrine Systems
Bone minerals have both calcium and phosphate.
When PTH stimulates the release of calcium from bone, phosphate is also released.
Normal levels of calcium and phosphate in the blood are close to the concentration that could cause them to precipitate as calcium phosphate salts.
Calcium phosphate salts are involved in the formation of kidney stones and hardening of artery walls.
PTH acts on the kidneys to increase the elimination of phosphate to reduce the possibility of calcium phosphate salt precipitation.
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Vertebrate Endocrine Systems –Glucose Regulation
Insulin and glucagon are antagonistic hormones that help maintain glucose homeostasis
Insulin is produced in the pancreas in cell clusters called islets of Langerhans.
Several cell types have been identified in the islets:
Beta () cells produce and secrete insulin.
Alpha () cells produce and secrete glucagon (antagonist of insulin).
Delta cells produce somatostatin.
The remainder of the pancreas acts as an exocrine gland with digestive functions.
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Vertebrate Endocrine Systems
After a meal, blood glucose levels rise and stimulate the cells to release insulin.
Insulin stimulates cells to use glucose and to convert it to glycogen and fat.
When blood glucose levels fall, the pancreas stops releasing insulin, and cells switch to using glycogen and fat for energy.
If blood glucose falls too low, the cells release glucagon which stimulates the liver to convert glycogen back to glucose.
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Maintenance of glucose homeostasis
Beta cells ofpancreas are stimulatedto release insulininto the blood.
Insulin
Liver takesup glucoseand stores itas glycogen.
Body cellstake up moreglucose.
Blood glucose leveldeclines to set point;stimulus for insulinrelease diminishes.
STIMULUS:Rising blood glucose
level (for instance, aftereating a carbohydrate-
rich meal)
Homeostasis:Blood glucose level
(about 90 mg/100 mL)
Blood glucose levelrises to set point;
stimulus for glucagonrelease diminishes.
STIMULUS:Dropping blood glucoselevel (for instance, after
skipping a meal)
Alpha cells of pancreasare stimulated to releaseglucagon into the blood.
Liver breaksdown glycogenand releasesglucose intoblood.
Glucagon
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Target Tissues for Insulin and Glucagon
• Insulin reduces blood glucose levels by
– Promoting the cellular uptake of glucose
– Slowing glycogen breakdown in the liver
– Promoting fat storage
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• Glucagon increases blood glucose levels by
– Stimulating conversion of glycogen to glucose in the liver
– Stimulating breakdown of fat and protein into glucose
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Vertebrate Endocrine Systems
Diabetes mellitus is a disease caused by a lack of the protein hormone insulin (Type I) or a lack of insulin receptors on target cells (Type II).
Insulin binds to receptors on the cell membrane and allows glucose uptake.
Without insulin or the receptors, glucose accumulates in the blood until it is lost in urine.
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Vertebrate Endocrine Systems
High glucose levels in the blood cause water to move from the cells into the blood by osmosis.
The kidneys then increase urine output to get rid of the fluid excess.
Cells not taking up glucose use fat and protein for fuel, resulting in the body’s wasting away and tissue and organ damage.
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Adrenal Hormones: Response to Stress
• The adrenal glands are adjacent to the kidneys
• Each adrenal gland actually consists of two glands: the adrenal medulla (inner portion) and adrenal cortex (outer portion)
The medulla produces epinephrine and norepinephrine.
The medulla develops from the nervous system and remains under its control.
The cortex is under hormonal control, mainly by adrenocorticotropin (ACTH) from the anterior pituitary.
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Catecholamines from the Adrenal Medulla
• The adrenal medulla secretes epinephrine (adrenaline) and norepinephrine (noradrenaline)
• These hormones are members of a class of compounds called catecholamines
• They are secreted in response to stress-activated impulses from the nervous system
• They mediate various fight-or-flight responses
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• Epinephrine and norepinephrine
– Trigger the release of glucose and fatty acids into the blood
– Increase oxygen delivery to body cells
– Direct blood toward heart, brain, and skeletal muscles, and away from skin, digestive system, and kidneys
• The release of epinephrine and norepinephrine occurs in response to nerve signals from the hypothalamus
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Vertebrate Endocrine Systems
Epinephrine and norepinephrine are amine hormones. They bind to two types of receptors in target cells: -adrenergic and -adrenergic
Norepinephrine acts mostly on the alpha type, so drugs called beta blockers, which inactivate only -adrenergic receptors, can be used to reduce fight-or-flight responses to epinephrine.
The beta blockers leave the alpha sites open to norepinephrine and its regulatory functions.
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Fig. 45-21
Stress
Adrenalgland
Nervecell
Nervesignals
Releasinghormone
Hypothalamus
Anterior pituitary
Blood vessel
ACTH
Adrenal cortex
Spinal cord
Adrenal medulla
Kidney
(a) Short-term stress response (b) Long-term stress response
Effects of epinephrine and norepinephrine:
2. Increased blood pressure
3. Increased breathing rate
4. Increased metabolic rate
1. Glycogen broken down to glucose; increased blood glucose
5. Change in blood flow patterns, leading to increased alertness and decreased digestive, excretory, and reproductive system activity
Effects ofmineralocorticoids:
Effects ofglucocorticoids:
1. Retention of sodium ions and water by kidneys
2. Increased blood volume and blood pressure
2. Possible suppression of immune system
1. Proteins and fats broken down and converted to glucose, leading to increased blood glucose
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Fig. 45-21b
(a) Short-term stress response
Effects of epinephrine and norepinephrine:
2. Increased blood pressure
3. Increased breathing rate
4. Increased metabolic rate
1. Glycogen broken down to glucose; increased blood glucose
5. Change in blood flow patterns, leading to increased alertness and decreased digestive, excretory, and reproductive system activity
Adrenalgland
Adrenal medulla
Kidney
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Fig. 45-21c
(b) Long-term stress response
Effects ofmineralocorticoids:
Effects ofglucocorticoids:
1. Retention of sodium ions and water by kidneys
2. Increased blood volume and blood pressure
2. Possible suppression of immune system
1. Proteins and fats broken down and converted to glucose, leading to increased blood glucose
Adrenalgland
Kidney
Adrenal cortex
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Vertebrate Endocrine Systems
Adrenal cortex cells use cholesterol to produce three classes of steroid hormones called corticosteroids:
Glucocorticoids influence blood glucose concentrations and other aspects of fuel molecule metabolism.
Mineralocorticoids influence extracellular ionic balance.
Sex steroids stimulate sexual development and reproductive activity. These are secreted in only minimal amounts by the adrenal cortex.
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Figure 42.11 The Corticosteroid Hormones are Built from Cholesterol
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Vertebrate Endocrine Systems
The main mineralocorticoid, aldosterone, stimulates the kidney to conserve sodium and excrete potassium.
The main glucocorticoid, cortisol, mediates the body’s response to stress.
The fight-or-flight response ensures that muscles have adequate oxygen and glucose for immediate response.
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Vertebrate Endocrine Systems
Shortly after a frightening stimulus, blood cortisol rises.
Cortisol stimulates cells that are not critical to the emergency to decrease their use of glucose.
It also blocks the immune system reactions, which temporarily are less critical.
Cortisol can therefore be used to reduce inflammation and allergy.
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Vertebrate Endocrine Systems
Cortisol release is controlled by ACTH from the anterior pituitary which, in turn is controlled by adrenocorticotropin-releasing hormone from the hypothalamus.
The cortisol response is much slower than the epinephrine response.
Turning off the cortisol response is also critical to avoid the consequences of long-term stress.
Cortisol has negative feedback effect on brain cells that decreases the release of adrenocorticotropin-releasing hormone.
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Vertebrate Endocrine Systems- Reproduction
The gonads (testes and ovaries) produce steroid hormones synthesized from cholesterol.
Androgens are male steroids, the dominant one being testosterone.
Estrogens and progesterone are female steroids, the dominant estrogen being estradiol.
Sex steroids determine whether a fetus develops into a male or female.
After birth, sex steroids control maturation of sex organs and secondary sex characteristics such as breasts and facial hair.
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Vertebrate Endocrine Systems
Until the seventh week of an embryo’s development, either sex may develop.
In mammals, the Y chromosome causes the gonads to start producing androgens in the seven-week-old embryo, and the male reproductive system develops.
If androgens are not released, the female reproductive system develops.
In birds, the opposite rules apply: male features are produced unless estrogens are present to trigger female development.
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Vertebrate Endocrine Systems
Sex steroid production increases rapidly at puberty, or sexual maturation, in humans.
Control of sex steroids (both male and female) is under the anterior pituitary tropic hormones called luteinizing hormone (LH) and follicle-stimulating hormone (FSH).
These gonadotropins are controlled by the hypothalamic gonadotropin-releasing hormone (GnRH).
Before puberty, the hypothalamus produces low levels of GnRH.
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Vertebrate Endocrine Systems
At puberty GnRH release increases, stimulating gonadotropin production and, hence, sex steroid production.
In females, increased LH and FSH at puberty induce the ovaries to begin female sex hormone production to initiate sexually mature body traits.
In males, increased LH stimulates cells in the testes to make androgens which induce changes associated with adolescence.
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Vertebrate Endocrine Systems
Synthetic androgens (anabolic steroids) can exaggerate body strength and muscle development.
Negative side effects in females include more masculine body features, such as shrinking the uterus and causing an irregular menstrual cycle.
In males, the negative side effects include shrinking of the testes, enlarged breasts, and sterility.
Continued use of anabolic steroids may increase risk of heart disease, some cancers, kidney damage, and personality disorders.
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Melatonin and Biorhythms
• The pineal gland, located in the brain, secretes melatonin
• Light/dark cycles control release of melatonin
• Primary functions of melatonin appear to relate to biological rhythms associated with reproduction
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Vertebrate Endocrine Systems
Many other body organs, such as the gut and the heart, produce hormones.
When blood pressure stretches the heart wall, its cells release atrial natriuretic hormone.
This hormone increases sodium ion and water excretion by the kidney, lowering blood pressure and blood volume.
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Hormone Actions:The Role of Signal Transduction Pathways
Hormones are released in very small amounts, yet they cause large and very specific responses in target organs and tissues.
Strength of hormone action results from signal transduction cascades that amplify the original signal.
Selective action is keyed to appropriate receptors of cells responding to hormones.
Specific receptors can also be linked to different response mechanisms, as is the case with receptors for epinephrine and norepinephrine.
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Hormone Actions:The Role of Signal Transduction Pathways
The abundance of hormone receptors can be under feedback control.
Continuous high levels of a hormone can decrease the number of its receptors, a process called downregulation.
High levels of insulin in type II diabetes mellitus result in a loss of insulin receptors.
Upregulation of receptors is a positive feedback mechanism and is less common than downregulation.
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Figure 42.16 Dose-Response Curves Quantify Response to a Hormone
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Hormone Actions:The Role of Signal Transduction Pathways
Anything that changes the responsiveness of a system to a hormone is reflected in the dose–response curve. These may include:
The number of receptors in the responding cells
Changes in signaling pathways
Changes in rate-limiting enzymes
The availability of substrates or cofactors
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Hormone Actions:The Role of Signal Transduction Pathways
Hormone responses may also vary in their time course.
The length of time for the concentration of a hormone to drop to one-half of the maximum is called its half-life.
Epinephrine’s half-life in the blood is only 1–3 minutes. Cortisol or thyroxine half-lives are on the order of days.
Half-life is partially determined by degradation and elimination processes.
Most hormones are broken down in the liver, removed from the blood by the kidney, and excreted in the urine.
Table 45-1b
Table 45-1c
Table 45-1d
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
You should now be able to:
1. Distinguish between the following pairs of terms: hormones and local regulators, paracrine and autocrine signals
2. Describe the evidence that steroid hormones have intracellular receptors, while water-soluble hormones have cell-surface receptors
3. Explain how the body regulates general metabolism, Glucose and Calcium homeostatis.
4. How does the body responds to long tern stress?
5. Distinguish between hypothyroid and hyperthyroid goitre
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
1. Explain how the hypothalamus and the pituitary glands interact and how they coordinate the endocrine system
2. Explain the role of tropic hormones in coordinating endocrine signaling throughout the body
3. List and describe the functions of hormones released by the following: anterior and posterior pituitary lobes, thyroid glands, parathyroid glands, adrenal medulla, adrenal cortex, gonads, pineal gland.
4. Explain how these hormones are regulated.
5. Explain the general mechanisms of hormone action.
6. Distinguish signaling by the nervous system vs. the endocrine system