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The Endocrine System

• Similar in fxn to the Nervous System

• Both send a message-Δ fxn of cell

• Nervous System-quick on, quick off

• Endocrine System-slow on, slow off

© 2012 Pearson Education, Inc.


Figure 18-1 Organs and Tissues of the Endocrine System

Hypothalamus

Production of ADH, oxytocin, andregulatory hormones

Pituitary Gland

Anterior lobe:ACTH, TSH, GH, PRL, FSH, LH,and MSH

Posterior lobe:Release of oxytocin and ADH

Parathyroid Glands(located on the posterior surface ofthe thyroid gland)

Parathyroid hormone (PTH)

Pineal Gland

Melatonin

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Figure 18-1 Organs and Tissues of the Endocrine System

Thyroid Gland

Thyroxine (T4)Triiodothyronine (T3)Calcitonin (CT)

Adrenal Glands

Adrenal medulla:Epinephrine (E)Norepinephrine (NE)

Adrenal cortex:Cortisol, corticosterone,aldosterone, androgens

InsulinGlucagon

Pancreas (Pancreatic Islets)Testis

Ovary

Thymus: (Undergoes atrophyduring adulthood)Secretes thymosins

Adipose Tissue: Secretes• Leptin

Digestive Tract: Secretesnumerous hormones involved in thecoordination of system functions,glucose metabolism, and appetite

Kidneys: Secrete• Erythropoietin (EPO)• Calcitriol

Gonads:Testes (male):

Androgens (especially testosterone),inhibin

Ovaries (female):Estrogens, progestins, inhibin

Organs with SecondaryEndocrine Functions

Heart: Secretes natriuretic peptides.• Atrial natriuretic peptide (ANP)• Brain natriuretic peptide (BNP)

SeeChapter21

SeeChapter22

SeeChapter25

SeeChapters19 and 26

SeeChapters28 and 29

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• Hormone: organic chemical that changes the function of its target cell– Autocrine-– Paracrine-– Endocrine-– exocrine

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• Maintenance of Homeostasis– Water/electrolytes– Enzyme function– Transport

• Regulate long term processes• Development• Growth• Reproduction


• Circulate freely-don’t last long– Bind to receptor– b.d. by liver or kidneys– b.d. by enzymes in plasma or interstitial fluid

• Bound to a carrier-last a long time– Reserves in the blood stream– Once released from carrier-don’t last long– Same reasons as above


Hormones

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Mechanisms of Hormone Action• The Process of Amplification

– Is the binding of a small number of hormone molecules to membrane receptors

– Leads to thousands of second messengers in cell– Magnifies effect of hormone on target cell



Mechanisms of Hormone Action• Down-regulation

– Presence of a hormone triggers decrease in number of hormone receptors

– When levels of particular hormone are high, cells become less sensitive

• Up-regulation– Absence of a hormone triggers increase in number of

hormone receptors

– When levels of particular hormone are low, cells become more sensitive



Figure 18-3 G Proteins and Hormone Activity

Hormone

Proteinreceptor

G proteinactivated

Hormone

Proteinreceptor

G proteinactivated

Effects on cAMP Levels

Many G proteins, once activated, exert their effects by changing the concentrationof cyclic-AMP, which acts as the second messenger within the cell.

Increasedproduction

of cAMPadenylatecyclase

Acts assecond

messenger

kinase

Activatesenzymes

Opens ionchannels

If levels of cAMP increase,enzymes may be activatedor ion channels may beopened, accelerating themetabolic activity of the cell.

Examples:• Epinephrine and norepinephrine (β receptors)• Calcitonin• Parathyroid hormone• ADh, ACTH, FSH, LH, TSH• Glucagon

Examples:• Epinephrine and norepinephrine (α2 receptors)

In some instances, G proteinactivation results in decreasedlevels of cAMP in thecytoplasm. This decrease hasan inhibitory effect on the cell.

Enhancedbreakdown

of cAMPPDE

Reducedenzymeactivity

Hormone

Proteinreceptor

G protein(inactive)

G proteinactivated

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Figure 18-3 G Proteins and Hormone Activity

Hormone

Proteinreceptor

G protein(inactive)

G proteinactivated

Hormone

Proteinreceptor

G proteinactivated

Effects on Ca2+ Levels

Some G proteins use Ca2+ as asecond messenger.

Examples:

• Epinephrine and norepinephrine (α1 receptors)

• Oxytocin• Regulatory hormones of hypothalamus• Several eicosanoids

Activatesenzymes

Calmodulin

PLC,DAG,and IP3

Opening of Ca2+ channels

Release ofstored Ca2+

from ER or SER

Ca2+ acts assecond messenger

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Endocrine Reflexes • Endocrine Reflexes

– Functional counterparts of neural reflexes– In most cases, controlled by negative feedback

mechanisms• Stimulus triggers production of hormone whose effects

reduce intensity of the stimulus



Endocrine Reflexes • Endocrine reflexes can be triggered by

– Humoral stimuli

• Changes in composition of extracellular fluid

– Hormonal stimuli

• Arrival or removal of specific hormone

– Neural stimuli

• Arrival of neurotransmitters at neuroglandular junctions



Endocrine Reflexes• Simple Endocrine Reflex

– Involves only one hormone

– Controls hormone secretion by the heart, pancreas, parathyroid gland, and digestive tract

• Complex Endocrine Reflex– Involves

• One or more intermediary steps

• Two or more hormones

• The hypothalamus



Figure 18-5 Three Mechanisms of Hypothalamic Control over Endocrine Function

Production of ADHand oxytocin

HYPOTHALAMUS

Control of sympatheticoutput to adrenalmedullae

Secretion of regulatoryhormones to control activityof the anterior lobe of thepituitary gland

Preganglionicmotor fibers

Adrenal gland

Secretion of epinephrineand norepinephrine

Adrenal medulla

Adrenal cortex

Posterior lobeof pituitary gland

Release of ADHand oxytocin

Hormones secreted by the anteriorlobe control other endocrine organs

Anterior lobeof pituitary gland

Infundibulum

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Endocrine Reflexes• Neuroendocrine Reflexes

– Pathways include both neural and endocrine components

• Complex Commands– Issued by changing

• Amount of hormone secreted

• Pattern of hormone release:– hypothalamic and pituitary hormones released in sudden

bursts

– frequency changes response of target cells
