Chapter 45Hormones: The Body’s Long-

Distance Regulators

• Animal hormones are chemical signals that are secreted into the circulatory system and communicate regulatory messages within the body

• Hormones reach all parts of the body, but only target cells are equipped to respond. Why?

Metamorphosis is regulated by hormones Metamorphosis= the

hormonal reactivation of development

• Remodeling• Replacement (respecification)• New structures• Death

Larva function: dispersal (sea urchin) or growth (amphibians)

Eye migration and associated neuronal changes during metamorphosis of Xenopus laevis

Axons cross midline in early Axons do not cross in late

Eyes move from prey mode to predator mode (in front for bifocal vision)

(frogs & toads)

death

remodeling

respecification

growth

If thyroid removed or destroyed remain a tadpole 4-ever.

T4

T3

In the wild remains in juvenile stage because thyroid does not release thyrotropin

Give the hormone in the lab and get adult form that is never seen in the wild.

Here neoteny = retention of juvenile form with adult gametes

The endocrine system and the nervous system act individually and together in

regulating an animal’s physiology

• Animals have two systems of internal communication and regulation: the nervous system and the endocrine system

• The nervous system conveys high-speed electrical signals along specialized cells called neurons

• The endocrine system secretes hormones that coordinate slower but longer-acting responses

Overlap Between Endocrine and Nervous Regulation

• The endocrine and nervous systems function together in maintaining homeostasis, development, and reproduction

• Specialized nerve cells known as neurosecretory cells release neurohormones into the blood

• Both endocrine hormones and neurohormones function as long-distance regulators of many physiological processes

Control Pathways and Feedback Loops

• There are three types of hormonal control pathways: simple endocrine, simple neurohormone, and simple neuroendocrine

• A common feature is a feedback loop connecting the response to the initial stimulus

• Negative feedback regulates many hormonal pathways involved in homeostasis

LE 45-2a

Targeteffectors

Response

Simple endocrine pathway

Glycogenbreakdown,glucose releaseinto blood

Liver

Bloodvessel

Pancreassecretesglucagon ( )Endocrine

cell

Low bloodglucose

Receptorprotein

Stimulus

Pathway Example

LE 45-2b

Targeteffectors

Response

Simple neurohormone pathway

Stimulus

Pathway Example

Suckling

Milk release

Smooth musclein breast

Neurosecretorycell

Bloodvessel

Posterior pituitarysecretes oxytocin( )

Hypothalamus/posterior pituitary

Sensoryneuron

LE 45-2c

Targeteffectors

Response

Simple neuroendocrine pathway

Stimulus

Pathway Example

Milk production

Bloodvessel

Hypothalamus

Sensoryneuron

Mammary glands

Endocrinecell

Bloodvessel

Anteriorpituitarysecretesprolactin ( )

Hypothalamussecretes prolactin-releasinghormone ( )

Neurosecretorycell

Hypothalamicneurohormonereleased inresponse to neural andhormonalsignals

Hormones and other chemical signals bind to target cell receptors, initiating pathways that

culminate in specific cell responses

• Hormones convey information via the bloodstream to target cells throughout the body

• Three major classes of molecules function as hormones in vertebrates:– Proteins and peptides– Amines derived from amino acids– Steroids

• Signaling by any of these hormones involves three key events:– Reception– Signal transduction– Response

Cell-Surface Receptors for Water-Soluble Hormones

• The receptors for most water-soluble hormones are embedded in the plasma membrane, projecting outward from the cell surface

LE 45-3SECRETORYCELL

Hormonemolecule

Signal receptor

VIABLOOD

VIABLOOD

TARGETCELL TARGET

CELLSignaltransductionpathway

OR

Cytoplasmicresponse

DNA

NUCLEUS

Nuclearresponse

Receptor in plasma membrane Receptor in cell nucleus

DNA

NUCLEUS

mRNA

Synthesis ofspecific proteins

Signaltransductionand response

Signalreceptor

Hormonemolecule

SECRETORYCELL

• Binding of a hormone to its receptor initiates a signal transduction pathway leading to responses in the cytoplasm or a change in gene expression

• 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

LE 45-4

Different receptors different cell responses

Epinephrine

receptor

Epinephrine

receptor

Epinephrine

receptor

Vesselconstricts

Vesseldilates

Intestinal bloodvessel

Skeletal muscleblood vessel

Liver cell

Different intracellular proteins different cell responses

Glycogendeposits

Glycogenbreaks downand glucoseis releasedfrom cell

The hormone epinephrine has multiple effects in mediating the body’s response to short-term stress

Intracellular Receptors for Lipid-Soluble Hormones

• Steroids, thyroid hormones, and the hormonal form of vitamin D enter target cells and bind to protein receptors in the cytoplasm or nucleus

• Protein-receptor complexes then act as transcription factors in the nucleus, regulating transcription of specific genes

Paracrine Signaling by Local Regulators• In paracrine signaling, nonhormonal chemical

signals called local regulators elicit responses in nearby target cells

• Types of local regulators:– Neurotransmitters– Cytokines and growth factors– Nitric oxide– Prostaglandins

• Prostaglandins help regulate aggregation of platelets, an early step in formation of blood clots

• The hypothalamus and the pituitary gland control much of the endocrine system

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Testis(male)

Ovary(female)

Adrenal glands

Pancreas

Parathyroid glandsThyroid gland

Pituitary gland

Pineal gland

Hypothalamus

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Relation Between the Hypothalamus and Pituitary Gland

• The hypothalamus, a region of the lower brain, contains neurosecretory cells

• The posterior pituitary, or neurohypophysis, is an extension of the hypothalamus

• Hormonal secretions from neurosecretory cells are stored in or regulate the pituitary gland

LE 45-7

Mammary glands,uterine muscles

Hypothalamus

Kidney tubules

OxytocinHORMONE

TARGET

ADH

Posteriorpituitary

Neurosecretorycells of thehypothalamus

Axon

Anteriorpituitary

• Other hypothalamic cells produce tropic hormones, hormones that regulate endocrine organs

• Tropic hormones are secreted into the blood and transported to the anterior pituitary, or adenohypophysis

• The tropic hormones of the hypothalamus control release of hormones from the anterior pituitary

LE 45-8

Neurosecretory cells of the hypothalamus

Endocrine cells of theanterior pituitary

Portal vessels

Pituitary hormones(blue dots)

Pain receptorsin the brain

Endorphin Growth hormone

BonesLiver

MSH

Melanocytes

Prolactin

Mammaryglands

ACTH

Adrenalcortex

TSH

ThyroidTestes orovaries

FSH and LH

TARGET

HORMONE

Hypothalamicreleasinghormones(red dots)

Tropic Effects OnlyFSH, follicle-stimulating hormoneLH, luteinizing hormoneTSH, thyroid-stimulating hormoneACTH, adrenocorticotropic hormone

Nontropic Effects OnlyProlactinMSH, melanocyte-stimulating hormoneEndorphin

Nontropic and Tropic EffectsGrowth hormone

Posterior Pituitary Hormones• The two hormones released from the posterior

pituitary act directly on nonendocrine tissues• Oxytocin induces uterine contractions and milk

ejection• Antidiuretic hormone (ADH) enhances water

reabsorption in the kidneys

Anterior Pituitary Hormones

• The anterior pituitary produces both tropic and nontropic hormones

Tropic Hormones

• The four strictly tropic hormones are– Follicle-stimulating hormone (FSH)– Luteinizing hormone (LH)– Thyroid-stimulating hormone (TSH)– Adrenocorticotropic hormone (ACTH)

• Each tropic hormone acts on its target endocrine tissue to stimulate release of hormone(s) with direct metabolic or developmental effects

Nontropic Hormones

• Nontropic hormones produced by the anterior pituitary:– Prolactin: stimulates lactation in mammals but has

diverse effects in different vertebrates

– Melanocyte-stimulating hormone (MSH) influences skin pigmentation in some vertebrates and fat metabolism in mammals

– -endorphin: inhibits pain

Growth Hormone

• Growth hormone (GH) has tropic and nontropic actions

• It promotes growth directly and has diverse metabolic effects

• It stimulates production of growth factors

Nonpituitary hormones help regulate metabolism, homeostasis,

development, and behavior

Thyroid Hormones

• 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)

Hypothalamus

TRH

Anteriorpituitary

TSH

Thyroid

T3 T4

The hypothalamus and anterior pituitary control secretion of thyroid hormones through two negative feedback loops

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Parathyroid Hormone and Calcitonin: Control of Blood Calcium

• Two antagonistic hormones, parathyroid hormone (PTH) and calcitonin, play the major role in calcium (Ca2+) homeostasis in mammals

• The thyroid gland produces calcitonin.

• Calcitonin stimulates Ca2+ deposition in bones and secretion by kidneys, lowering blood Ca2+ levels

• PTH, secreted by the parathyroid glands, has the opposite effects on the bones and kidneys, and therefore raises Ca2+ levels

• PTH also has an indirect effect, stimulating the kidneys to activate vitamin D, which promotes intestinal uptake of Ca2+ from food

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STIMULUS:Rising blood

Ca2+ level

Thyroid glandreleasescalcitonin.

Calcitonin

StimulatesCa2+ depositionin bones

ReducesCa2+ uptakein kidneys

Blood Ca2+

level declinesto set point

Homoeostasis:Blood Ca2+ level

(about 10 mg/100 mL)

STIMULUS:Falling blood

Ca2+ level

Blood Ca2+

level risesto set point

StimulatesCa2+ releasefrom bones

PTH

Parathyroidgland

Stimulates Ca2+ uptake in kidneys

Activevitamin D

IncreasesCa2+ uptakein intestines

Insulin and Glucagon: Control of Blood Glucose

• The pancreas secretes insulin and glucagon, antagonistic hormones that help maintain glucose homeostasis

• Glucagon is produced by alpha cells• Insulin is produced by beta cells

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

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Beta cells ofpancreasrelease insulininto the blood.

Insulin

Liver takesup glucoseand stores itas glycogen.

STIMULUS:Rising blood glucose

level (for instance, aftereating a carbohydrate-

rich meal)

Blood glucose leveldeclines to set point;stimulus for insulinrelease diminishes.

Homeostasis:Blood glucose level

(about 90 mg/100 mL)

STIMULUS:Dropping blood glucoselevel (for instance, after

skipping a meal)

Blood glucose levelrises to set point;

stimulus for glucagonrelease diminishes.

Liver breaksdown glycogenand releasesglucose into theblood.

Body cellstake up moreglucose.

Alpha cells of pancreasrelease glucagon into the blood.

Glucagon

Diabetes Mellitus• Diabetes mellitus is perhaps the best-known

endocrine disorder• It is caused by a deficiency of insulin or a

decreased response to insulin in target tissues• It is marked by elevated blood glucose levels• Type I diabetes mellitus (insulin-dependent) is an

autoimmune disorder in which the immune system destroys pancreatic beta cells

• Type II diabetes mellitus (non-insulin-dependent) involves insulin deficiency or reduced response of target cells due to change in insulin receptors (old age, sigh)

Adrenal Hormones: Response to Stress• The adrenal glands are adjacent to the kidneys• Each adrenal gland actually consists of two

glands: the adrenal medulla and adrenal cortex

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

Stress Hormones from the Adrenal Cortex

• Hormones from the adrenal cortex also function in response to stress

• They fall into three classes of steroid hormones:– Glucocorticoids, such as cortisol, influence glucose

metabolism and the immune system– Mineralocorticoids, such as aldosterone, affect salt

and water balance– Sex hormones are produced in small amounts

LE 45-13

Stress

Hypothalamus

Anterior pituitaryBlood vessel

ACTH

ACTH

Releasinghormone

Nervecell

Nerve cell

Long-term stress response

Effects ofmineralocorticoids:

1. Retention of sodium ions and water by kidneys2. Increased blood volume and blood pressure

Effects ofglucocorticoids:

1. Proteins and fats broken down and converted to glucose, leading to increased blood glucose2. Immune system may be suppressed

Short-term stress response

Effects of epinephrine and norepinephrine:

1. Glycogen broken down to glucose; increased blood glucose2. Increased blood pressure3. Increased breathing rate4. Increased metabolic rate5. Change in blood flow patterns, leading to increased alertness and decreased digestive and kidney activity

NervesignalsSpinal cord

(cross section)

AdrenalglandKidney