Signals can be of many kinds light, sound, smell, touch etc. many of which are due to chemicals

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


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.


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


Types of Secreted Signaling Molecules

• Secreted chemical signals include

– Hormones

– Local regulators

– Neurotransmitters

– Neurohormones

– Pheromones


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


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


• 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


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


• 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


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


• 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)


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


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


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


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


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


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


• 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.


• 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


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)


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


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


Figure 42.6 Effects of Excess Growth Hormone

Releasing and Release inhibiting Neurohormones of the Hypothalumus


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


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 )

–


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.


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.


• 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


• 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.


Hypothalamus

Anteriorpituitary

TSH

Thyroid

T3 T4+


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.


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


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


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


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.



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.



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.



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.


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.



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.


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


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


• Glucagon increases blood glucose levels by

– Stimulating conversion of glycogen to glucose in the liver

– Stimulating breakdown of fat and protein into glucose



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.



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.


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.


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


• 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



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.


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


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


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



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.


Figure 42.11 The Corticosteroid Hormones are Built from Cholesterol



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.



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.



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.


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.



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.



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.



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.



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.


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



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.


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.



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.


Figure 42.16 Dose-Response Curves Quantify Response to a Hormone



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



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


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


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

Documents

Signals can be of many kinds light, sound, smell, touch etc. many of which are due to chemicals