Endocrine SystemChemical Control

Messenger Molecules

• Cells must communicate with one another to coordinate cell processes within tissues and to maintain homeostasis.

• Cell-to-cell communication is carried out via messenger molecules.

Three types of chemical

signals are used for cell-

to-cell communicatio

n.

Four methods of cell-to-cell communication are found in the human body, ranging from

direct to remote communication.

Endocrine hormones

• Produced by endocrine (“ductless”) glands and secreted into the bloodstream.

• Endocrine hormones may affect a wide array of target cells to produce multiple effects.

• Two types: peptides (small proteins) and steroids (lipids).

Hormones and Receptors

Peptide Hormones• Peptide hormones do not enter the cell

directly. These hormones bind to receptor proteins in the cell membrane.

• When the hormone binds with the receptor protein, a secondary messenger molecule initiates the cell response.

• Because peptide hormones are water soluble, they often produce fast responses.

(cytoplasm)

(nucleus)

peptide or aminoacid-derived

hormone(first messenger)

(extracellularfluid)

cyclic AMP-synthesizing

enzyme

cyclic AMP

ATP

inactiveenzyme

(second messenger)

activeenzyme

reactant

product

plasma membrane

nuclearenvelope

receptor

The hormone binds toa receptor on the plasmamembrane of a target cell

1

The activated enzymescatalyze specific reactions

4

The secondmessenger activates

other enzymes

3

Hormone–receptor bindingactivates an enzyme that catalyzes

the synthesis of a second messenger,such as cyclic AMP

2

Steroid Hormones

• Steroid hormones enter through the cell membrane and bind to receptors inside of the target cell.

• These hormones may directly stimulate transcription of genes to make certain proteins.

• Because steroids work by triggering gene activity, the response is slower than peptide hormones.

gene

plasmamembrane

ribosome

hormone receptor

steroid hormone

mRNA

(nucleus)

RNA polymerase

DNA

(cytoplasm)

new protein

(extracellularfluid)

A steroid hormonediffuses through theplasma membrane

The hormone binds to areceptor in the nucleus or toa receptor in the cytoplasm

that carries it into the nucleus The hormone–receptorcomplex binds to DNA andcauses RNA polymerase tobind to a nearby promoter

site for a specific gene

RNA polymerase catalyzesthe transcription of DNA into

messenger RNA (mRNA)

The mRNA leaves thenucleus, then attaches to aribosome and directs the

synthesis of a specific proteinproduct

1

2

3

4

5

nuclearenvelope

Role of the Hypothalamus

• The thalamus receives sensory information, relays some to the hypothalamus.

• Hypothalamus monitors the body for temperature, pH, other conditions.

• Hypothalamus signals pituitary gland if conditions need to be corrected.

Role of the Pituitary• The pituitary is the “master gland” that signals

other glands to produce their hormones when needed.

• The anterior lobe of the pituitary receives signals from the hypothalamus, and responds by sending out the appropriate hormone to other endocrine glands.

• The posterior pituitary receives oxytocin or antidiuretic hormone (ADH) from the hypothalamus, relays them to the body as necessary.

hypothalamus

pituitary(anterior lobe)

capillarybed

endocrinecell

blood flow

bloodflow

capillarybed

pituitary(posterior lobe)

Oxytocin and ADH(blue triangles) are

secreted into the bloodvia capillaries in theposterior pituitary

Endocrine cells of theanterior pituitary secretehormones (red squares)in response to releasinghormones; the pituitary

hormones enter thebloodstream

Neurosecretory cells ofthe hypothalamus produce

oxytocin and ADH

Releasing or inhibiting hormones(green circles) are secreted into

capillaries feeding the anterior lobeof the pituitary

Neurosecretory cellsof the hypothalamus produce releasing andinhibiting hormones

1

2

1

2

3

Pituitary HormonesPituitary Hormone Functions

Follicle-stimulating hormone

Stimulates egg maturation in the ovary and release of sex hormones.

Lutenizing hormone Stimulates maturation of egg and of the corpus luteum surrounding the egg, which affects female sex hormones and the menstrual cycle.

Thyroid-stimulating hormone

Stimulates the thyroid to release thyroxine.

Adrenocorticotropic hormone

Causes the adrenal gland to release cortisol.

Melanocyte-stimulating hormone

Stimulates synthesis of skin pigments.

Growth hormone Stimulates growth during infancy and puberty.

Antidiuretic hormone Signals the kidney to conserve more water.

Oxytocin Affects childbirth, lactation, and some behaviors.

Endocrine HormonesGland Hormones Functions

Thyroid Thyroxine Regulates metabolism

Calcitonin Inhibits release of calcium from the bones

Parathyroids Parathyroid hormone Stimulates the release of calcium from the bones.

Islet cells (in the pancreas)

Insulin Decreases blood sugar by promoting uptake of glucose by cells.

Glucagon Increases blood sugar by stimulating breakdown of glycogen in the liver.

Testes Testosterone Regulates sperm cell production and secondary sex characteristics.

Ovaries Estrogen Stimulates egg maturation, controls secondary sex characteristics.

Progesterone Prepares the uterus to receive a fertilized egg.

Adrenal cortex Epinephrine Stimulates “fight or flight” response.

Adrenal medulla

Glucocorticoids Part of stress response, increase blood glucose levels and decrease immune response.

Aldosterone Regulates sodium content in the blood.

Testosterone (in both sexes)

Adult body form (greater muscle mass), libido.

Pineal gland Melatonin Sleep cycles, reproductive cycles in many mammals.

Homeostasis and Hormones

• Examples:

• Thyroid and temperature control

• Thyroid, Parathyroid, and calcium

• Pancreas and glucose control

Temperature Control

Blood Calcium

Blood Sugar Control

Other hormone roles

• Controlling sleep cycles (melatonin)

• Controlling reproductive cycles (melatonin, sex hormones)

• Growth (growth hormone)

• Responding to stress or emergencies (epinephrine and other hormones)

Hormones Everywhere!• Many other organs besides the

endocrine glands produce hormones.

• Kidneys produce several hormones that regulate blood pressure, which is essential for kidney function.

• The digestive system produces several hormones that regulate appetite.

Low body fat stimulates leptin production, which stimulates appetite. The mouse is obese

because its low leptin levels give it an enormous appetite. Leptin injections return the mouse’s weight to normal. Humans sometimes have a leptin issue, too, but the problem is a bad leptin receptor on body cells. Alas, leptin

injections won’t cure that.

The obese mouse on the left does not

produce enough leptin, a hormone

produced by fat cells.

Finally…

• Summarize the roles of the pituitary and other endocrine glands in maintaining homeostasis in the human body.