Biology 30 Endocrine System

8/3/2019 Biology 30 Endocrine System

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Hormonal Regulation and Homeostasis


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Overview: The Bodys Long-Distance

Regulators

` Animal hormones are chemical signals that are secretedinto the circulatory system and communicate regulatorymessages within the body

` Hormones reach all parts of the body, but only targetcells are equipped to respond


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The endocrine system and the nervous system act individually and

together in regulating an animals physiology

y Animals have two systems of internal communicationand regulation: the nervous system and the endocrinesystem


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` The nervous system conveys high-speed electricalsignals along specialized cells called neurons

` The endocrine system secretes hormones that

coordinate slower but longer-acting responses


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Overlap Between Endocrine and

Nervous Regulation

y The endocrine and nervous systems function together inmaintaining homeostasis, development, andreproduction

y Specialized nerve cells known as neurosecretory cellsrelease neurohormones into the blood

y Both endocrine hormones and neurohormones functionas long-distance regulators of many physiologicalprocesses


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Control Pathways and Feedback

Loops

y There are three types of hormonal control pathways:

simple endocrine, simple neurohormone, and simpleneuroendocrine

y Acommon feature is a feedback loop connecting theresponse to the initial stimulus

y

Negative feedback regulates many hormonal pathwaysinvolved in homeostasis


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Targeteffectors

Response

Simple endocrine pathway

Glycogenbreakdown,glucose releaseinto blood

Liver

Blood

vessel

Pancreassecretesglucagon ( )

Endocrinecell

Low bloodglucose

Receptorprotein

Stimulus

Pathway Example


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Targeteffectors

Response

Simple neurohormone pathway

Stimulus

Pathway Example

Suckling

Milk release

Smooth musclein breast

Neurosecretorycell

Bloodvessel

Posterior pituitarysecretes oxytocin( )

Hypothalamus/posterior pituitary

Sensoryneuron


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Targeteffectors

Response

Simple neuroendocrine pathway

Stimulus

Pathway Example

Milk production

Bloodvessel

Hypothalamus

Sensoryneuron

Mammary glands

Endocrinecell

Bloodvessel

Anteriorpituitarysecretesprolactin ( )

Hypothalamussecretes prolactin-releasinghormone ( )

Neurosecretorycell

Hypothalamicneurohormonereleased inresponse toneural and

hormonalsignals


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Hormones and other chemical signals bind to target cell

receptors, initiating pathways that culminate in specific cell

responses.

y Hormones convey information via the bloodstream to

target cells throughout the body

y Three major classes of molecules function as hormonesin vertebrates:

y Proteins and peptidesy Amines derived from amino acids

y Steroids


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y Signaling by any of these hormones involves three keyevents:

y Reception

y Signal transduction

y Response


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SECRETORYCELL

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

Signal

transductionand response

Signalreceptor

Hormonemolecule

SECRETORYCELL


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y Binding of a hormone to its receptor initiates a signal

transduction pathway leading to responses in thecytoplasm or a change in gene expression

y The same hormone may have different effects on target

cells that havey Different receptors for the hormone

y Different signal transduction pathways

y Different proteins for carrying out the response


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` The hormone epinephrine has multiple effects inmediating the bodys response to short-term stress


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Different receptors different cell responses

Epinephrine

E receptor

Epinephrine

F receptor

Epinephrine

F receptor

Vesselconstricts

Vesseldilates

Intestinal bloodvessel

Skeletal muscleblood vessel

Liver cell

Different intracellular proteins different cell responses

Glycogendeposits

Glycogen

breaks down

and glucose

is released

from cell


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Intracellular Receptors for Lipid-Soluble Hormones

y Steroids, thyroid hormones, and the hormonal form of

vitamin D enter target cells and bind to protein receptorsin the cytoplasm or nucleus

y Protein-receptor complexes then act as transcription

factors in the nucleus, regulating transcription ofspecific genes


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Paracrine Signaling by Local Regulators

y In paracrine signaling, nonhormonal chemical signalscalled local regulators elicit responses in nearby targetcells

y Types of local regulators:

y Neurotransmitters

y Cytokines and growth factors

y Nitric oxide

y Prostaglandins


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` Prostaglandins help regulate aggregation of platelets, an

early step in formation of blood clots


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Testis(male)

Ovary(female)

Adrenal glands

Pancreas

Parathyroid glands

Thyroid gland

Pituitary gland

Pineal gland

Hypothalamus


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The hypothalamus and pituitary integrate many functions of the

vertebrate endocrine system

y The hypothalamus and the pituitary gland control muchof the endocrine system


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Relation Between the Hypothalamus and

Pituitary Gland

The hypothalamus, a region

of the lower brain, contains

neurosecretory cells

The posterior pituitary is anextension of the

hypothalamus

Hormonal secretions fromneurosecretory cells are

stored in or regulate the

pituitary gland


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Mammary glands,

uterine muscles

Hypothalamus

Kidney tubules

OxytocinHORMONE

TARGET

ADH

Posteriorpituitary

Neurosecretorycells of thehypothalamus

Axon

Anteriorpituitary


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y

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

y Tropic hormones are secreted into the blood and

transported to the anterior pituitary

y The tropic hormones of the hypothalamus controlrelease of hormones from the anterior pituitary


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Neurosecretory cellsof 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 hormone

LH, luteinizing hormoneTSH, thyroid-stimulating hormone

ACTH, adrenocorticotropic hormone

Nontropic Effects OnlyProlactin

MSH, melanocyte-stimulating hormoneEndorphin

Nontropic and Tropic Effects

Growth hormone


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Posterior Pituitary Hormones

y The two hormones released from the posterior pituitaryact directly on nonendocrine tissues

y Oxytocin induces uterine contractions and milk ejection

y Antidiuretic hormone (ADH) enhances waterreabsorption in the kidneys


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Anterior Pituitary Hormones

y The anterior pituitary produces both tropic andnontropic hormones


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Tropic Hormones

yThe four strictly tropic hormones arey Follicle-stimulating hormone (FSH)

y Luteinizing hormone (LH)

y Thyroid-stimulating hormone (TSH)

y Adrenocorticotropic hormone (ACTH)

Each tropic hormone acts on its target endocrine tissueto stimulate release of hormone(s) with direct metabolic

or developmental effects


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Nontropic Hormones

y Nontropic hormones produced by the anterior pituitary:y Prolactin

y Prolactin stimulates lactation in mammals but has diverse effectsin different vertebrates

y Melanocyte-stimulating hormone (MSH)

y MSH influences skin pigmentation in some vertebrates and fatmetabolism in mammals

y F-endorphin

y Endorphins inhibit pain


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Growth Hormone

y Growth hormone (GH) has tropic and nontropic actions

y It promotes growth directly and has diverse metaboliceffects

y It stimulates production of growth factors


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Nonpituitary hormones help regulate metabolism,homeostasis, development, and behavior

y

Many nonpituitary hormones regulate various functionsin the body


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Thyroid Hormones

y

The thyroid gland consists oftwo lobes on the ventralsurface of the trachea

y It produces two iodine-containing hormones:triiodothyronine (T3) andthyroxine (T4)


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y The hypothalamus and anterior pituitary controlsecretion of thyroid hormones through two negativefeedback loops


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Hypothalamus

TRH

Anteriorpituitary

TSH

Thyroid

T3 T4


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` Thyroid hormones stimulate metabolism and influencedevelopment and maturation

` Hyperthyroidism, excessive secretion of thyroid

hormones, can cause Graves disease in humans


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y The thyroid gland also produces calcitonin, whichfunctions in calcium homeostasis


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Parathyroid Hormone and Calcitonin: Control of

Blood Calcium

y Two antagonistichormones, parathyroid

hormone (PTH) andcalcitonin, play the majorrole in calcium (Ca2+)homeostasis in mammals


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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 kidneysActivevitamin D

IncreasesCa2+ uptakein intestines


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y

Calcitonin stimulates Ca2+

deposition in bones andsecretion by kidneys, lowering blood Ca2+ levels

y PTH, secreted by the parathyroid glands, has theopposite effects on the bones and kidneys, and thereforeraises Ca2+ levels

y PTH also has an indirect effect, stimulating the kidneysto activate vitamin D, which promotes intestinal uptake

of Ca2+ from food


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Insulin and Glucagon: Control of Blood Glucose

y The pancreas secretes insulinand glucagon, antagonistichormones that help maintain

glucose homeostasis

y Glucagon is produced byalpha cells

y Insulin is produced by betacells


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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 intothe blood.

Glucagon


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Target Tissues for Insulin andG

lucagon

y Insulin reduces blood glucose levels by

y Promoting the cellular uptake of glucose

y Slowing glycogen breakdown in the livery Promoting fat storage


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y Glucagon increases blood glucose levels byy Stimulating conversion of glycogen to glucose in the liver

y Stimulating breakdown of fat and protein into glucose


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Diabetes Mellitus

y Diabetes mellitus is perhaps the best-known endocrinedisorder

y It is caused by a deficiency of insulin or a decreased responseto insulin in target tissues

y It is marked by elevated blood glucose levels


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` Type I diabetes mellitus (insulin-dependent) is anautoimmune disorder in which the immune systemdestroys pancreatic beta cells

` Type II diabetes mellitus (non-insulin-dependent)involves insulin deficiency or reduced response of targetcells due to change in insulin receptors


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Adrenal Hormones: Response to

Stress

y The adrenal glands areadjacent to the kidneys

y Each adrenal gland actuallyconsists of two glands: theadrenal medulla and adrenalcortex


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.from the Adrenal Medullay The adrenal medulla secretes epinephrine (adrenaline)

and norepinephrine (noradrenaline)

y They are secreted in response to stress-activatedimpulses from the nervous system

y They mediate various fight-or-flight responses


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` Cortisol triggers the metabolism of proteins and fats to produceglucose. Cortisol also suppresses the immune system, which is

probably one reason that chronic stress is unhealthy.` Cortisol secretion is suppressed by negative feedback to the

hypothalamus and anterior pituitary.


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` The adrenal cortex also secretes aldosterone, which stimulatesthe kidneys to absorb sodium and thus water. This increasesthe blood pressure. When homeostasis is reached, negativefeedback shuts off the secretion of aldosterone.

` Low aldosterone causes low blood pressure, an imbalance ofelectrolytes (sodium and potassium ions) in the blood, andunhealthy weight loss, which are symptoms ofAddisons

disease.


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Stress Hormones from the Adrenal

Cortexy Hormones from the adrenal cortex also function in

response to stress

y They fall into three classes of steroid hormones:

y Glucocorticoids, such as cortisol, influence glucosemetabolism and the immune system

y Mineralocorticoids, such as aldosterone, affect salt andwater balance

y Sex hormones are produced in small amounts


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Stress

Hypothalamus

Anterior pituitary

Blood vessel

ACTH

ACTH

Releasing

hormone

Nervecell

Nerve cell

Long-term stress response

Effects of

mineralocorticoids:

1. Retention of sodiumions and water bykidneys

2. Increased bloodvolume and bloodpressure

Effects of

glucocorticoids:

1. Proteins and fatsbroken down andconverted to glucose,leading to increasedblood glucose

2. Immune system maybe suppressed

Short-term stress response

Effects of epinephrine and norepinephrine:

1. Glycogen broken down to glucose;increased blood glucose

2. Increased blood pressure

3. Increased breathing rate

4. Increased metabolic rate

5. Change in blood flow patterns, leading to

increased alertness and decreased

digestive and kidney activity

NervesignalsSpinal cord

(cross

section)

Adrenalgland

Kidney


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Gonadal Sex Hormones

yThe gonads, testes andovaries, produce most of thesex hormones:androgens(testosterone),

estrogens, and progestins


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` The testes primarily synthesize androgens, mainlytestosterone, which stimulate development andmaintenance of the male reproductive system

` Testosterone causes increase in muscle and bone massand is often taken as a supplement to cause musclegrowth, which carries health risks


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yEstrogens, most importantly estradiol, are responsiblefor maintenance of the female reproductive system andthe development of female secondary sex characteristics

y In mammals, progestins, which include progesterone,are primarily involved in preparing and maintaining theuterus


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Melatonin and Biorhythms

y The pineal gland, located inthe brain, secretesmelatonin


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Light/dark cycles control release of melatonin

y Primary functions of melatonin appear to relate tobiological rhythms associated with reproduction

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Biology 30 Endocrine System