Chapter 7a

Introduction to the Endocrine System

Endocrinology

• Study of hormones• Specialized chemical messengers• Secreted by select cells• Action at distant sites via blood• Effects• Enzymatic reactions• Membrane transport• Gene expression

About this Chapter

• Function and purpose of hormones

• Classification, structure, and synthesis of hormones

• Pathways of nervous to endocrine regulation

• Effects of hormone interactions

• Pathologies of the endocrine system

Figure 7-2 (1 of 2)

Anatomy Summary: Hormones

Figure 7-2 (2 of 2)

Anatomy Summary: Hormones

Chemical Regulating Systems• Hormones: cell–to–cell communication molecules• Made in gland(s) or cells• Transported by blood• Distant target tissue receptors• Activates physiological response

• Pheromones: organism to organism communication

Hormones: Function

• Control of • Rates of enzymatic reactions• Transport of ions or molecules across cell

membranes• Gene expression and protein synthesis

• Exert effects at very low concentrations• Bind to target cell receptors• Half-life indicates length of activity

Hormones: Classification

Be able to give types and example.Compare synthesis, half-life and location of

receptor1. Peptide or protein hormones

• Insulin from amino acids2. Steroid hormones

Sex steroids; estrogen, lipophilic3. Amine hormones• epinephrine

Table 7-1

Hormones: Classification

Figure 7-3

Peptide Hormone Synthesis, Packaging, and Release

1

mRNA

RibosomeTo target

Prohormone

Signalsequence

Transportvesicle

Active hormone

Endoplasmic reticulum (ER) Golgi complex

Secretoryvesicle

ECFCytoplasm Plasma

Peptidefragment

Releasesignal

Capillaryendothelium

Preprohormone

Messenger RNA on the ribosomes binds amino acids into a peptide chain called a preprohormoneThe chain is directed into the ER lumen by a signal sequence of amino acids.

The secretory vesicle releases its contents by exocytosis into the extracellular space.

The hormone moves into the circulation for transport to its target.

Enzymes in the ER chop off the signal sequence, creating an inactiveprohormone.

The prohormone passes from theER through the Golgi complex.

Secretory vesicles containing enzymes and prohormone bud off the Golgi. The enzymes chop the prohormone into one or more active peptides plus additional peptide fragments.

2 3 4 5 6

1

2

3

4

5

6

Peptide Hormone Synthesis, Packaging, and Release

Figure 7-4

Preprohormone Hormone Other peptides Signal sequence

processes to

PreproTRH (242 amino acids) 6 TRH(3 amino acids each)

(a)

(c)

TRH

(b)

+

+ +

+

Pro-opiomelanocortin

Prohormone Active hormone(s) Other peptide fragments

processes to

ACTH lipotropin endorphin Fragment

+

+

Proinsulin Insulin C-peptide

Peptide Hormone-Receptor Complex

• Surface receptor• Hormone binds• Enzyme activation• Open channels

• Second messenger systems• Cellular response

Peptide Hormone-Receptor Complex

• Membrane receptors and signal transduction for peptide hormones

Figure 7-5

TK = Tyrosine kinase

AE = Amplifier enzyme

G = G protein

KEYProteins

Secondmessenger

system

Opens ionchannel

phosphorylate

Cellular response

Steroid Hormones: Features

• Cholesterol-derived• Lipophilic and can enter target cell

• Cytoplasmic or nuclear receptors (mostly)• Activate DNA for protein synthesis• Slower acting, longer half-life• Examples • Cortisol, estrogen, and testosterone

Steroid Hormones: Structure

Figure 7-6

Cholesterol is the parent compound for all steroid hormones.

modified by enzymes to make steroid hormones such as

Estradiol (an estrogen)Aldosterone

Adrenalcortex

Ovary

Cortisol

In ovaryIn adrenal cortex

Steroid Hormones: Action

Figure 7-7

1

Cellmembrane

Interstitialfluid

Cytoplasmicreceptor

Endoplasmicreticulum

Nucleus

Nuclear receptor

DNA

Translation

Cell surface receptor

Rapid responses

Transcriptionproduces mRNA

Steroid hormone

Bloodvessel

Proteincarrier

Newproteins

Steroid hormone receptors are in thecytoplasm or nucleus.

Most hydrophobic steroids are bound toplasma protein carriers. Only unboundhormones can diffuse into the target cell.

Translation produces new proteinsfor cell processes.

Some steroid hormones also bind to membrane receptors that use secondmessenger systems to create rapidcellular responses.

The receptor-hormone complex binds toDNA and activates or represses one ormore genes.

Activated genes create new mRNA thatmoves back to the cytoplasm.

2a

2

54

3

1

2a

2

3

4

5

Amine Hormones: Features

• Derived from one of two amino acids• Tryptophan • Tyrosine

• Ring structure

Amine Hormones: Examples

• Thyroid hormones• Catecholamines • Epinephrine• Norepinephrine • Dopamine

Amine Hormones: Structure

Figure 7-8

Tyrosine

Catecholamines

Dopamine

Norepinephrine

Epinephrine

Thyroid hormones

Thyroxine (Tetraiodothyronine, T4)

Triiodothyronine (T3)

Endocrine Reflex Pathways

• Stimulus• Afferent signal• Integration• Efferent signal (the hormone)• Physiological action• Negative feedback

Endocrine Reflex Pathways: Insulin release

Figure 7-9

Receptor

Efferent path

Effector

Tissue response

Stimulus

Efferent neuron

Sensory neuron

Integrating center

Systemic response

KEYEat a meal

Pancreas

Insulin

Stretch receptorin digestive tract

Glucose uptakeand utilization

CNS

Targettissues

Bloodglucose

Afferent neuron

Efferent neuron

Blood glucose

Neg

ativ

e fe

edba

ck