chapter 30 chemical signaling within the animal body

Chapter 30

Chemical signaling within the animal body

Hormones

• A hormone is a chemical signal produced in the body.• It typically acts at a site

distant from where it was produced.

• Most hormones are produced in glands that are completely enclosed in tissue.• These glands are called

endocrine glands.

Hypothalamus

PinealglandPituitarygland

Thyroidgland

Pancreas

Testes(in males)

Ovaries(in females)

Adrenalglands

Thymus

Parathyroid glands(attached to the thyroid)

Hormones

• There are three big advantages to using chemical hormones as messengers rather than speedy electrical signals (like nerve signals).• Chemical molecules can spread to all tissues

via the blood.• Chemical signals can persist much longer than

electrical ones.• Many different kinds of chemicals can act as

hormones.

Hormones

• The glands that produce hormones are generally controlled by the nervous system.• The endocrine system and the motor nervous

system are the two main routes the CNS uses to issue commands to the organs of the body.• The two are so closely linked that they are

often considered a single system—neuroendocrine system.

• The hypothalamus is the main switchboard of the neuroendocrine system.

Hormones

• The CNS regulates the body’s hormones through a chain of command.• For example, the hypothalamus controls the

pituitary gland with thyrotropic-releasing hormone (TRH).• This causes the pituitary to release or

thyroid-stimulating hormone (TSH).• TSH then causes the thyroid gland to

release thyroid hormones.• The hypothalamus also secretes inhibiting

hormones that keep the pituitary from secreting specific hormones.

Hormones

• Hormones are effective messengers within the body because a particular hormone can influence a specific target cell.• Cells that the body has targeted to respond to a

particular hormone have receptor proteins shaped to fit that hormone and no other.

Hormones

• Hormones secreted by endocrine glands belong to four different chemical categories:

• polypeptides• glycoproteins• amines• steroids

Hormones

• The path of communication taken by a hormonal signal is a series of simple steps:

1. Issuing the command - the hypothalamus controls the release of many hormones.

2. Transporting the signal - most are transported throughout the body by the bloodstream.

3. Hitting the target - the hormone binds to a receptor on the target cell.

4. Having an effect - when the hormone binds to the receptor protein, the protein changes shape and triggers a change in cell activity.

Key Biological Process: Hormonal communication

1

1

2

2

3

3

3 4

4

4

Dehydration

Reduced urinevolume causesincreased waterretention.

Blood volume andpressure drops.

Osmotic concentrationin the blood increases.

Osmoreceptors

Hypothalamus

Generally, a part ofthe neuroendocrinesystem receivessensory informationand issues acommand in the formof a chemicalmessenger (hormone).

Posteriorpituitary

Antidiuretichormone (ADH)

The hormone istransported to targetcells via thebloodstream.

The hormonereaches the targetcells and binds to thecell receptors.

The hormone-recept or complextriggers changes in thetarget cells.

Increasedvasoconstrictionleads to higherblood pressure.

Bloodstream

ADH

ADH

How Hormones Target Cells

• The steroid hormones are recognized by protein receptors located in the cytoplasm or nucleus of the target cell.• Steroids are manufactured from cholesterol.• Steroid hormones can pass across the lipid

bilayer of the cell plasma membrane.


• The complex of a steroid hormone and its receptor inside the target cell bind to DNA in the nucleus.• This activates the transcription of a specific

gene and a protein is subsequently synthesized.

How steroid hormones work

5

1

3

4

1

2

3

E

E

E

Transportprotein

Cytoplasm

Target cell

Receptorprotein

Steroid hormone-receptor complex

2

Nucleus

DNAmRNA

Progesteronereceptor

Proteinsynthesis

Tissue fluid Blood plasma

Steroidhormone

Plasma membrane

Estrogen (E) is a lipid soluble steroidhormone and thus readily passes through theplasma membrane of cells lining the uterus.

Inside the cell, estrogen binds to a specific receptorprotein associated with the DNA in the nucleus.

The estrogen-receptor complex activates thetranscription of genes.

Later, when progesterone enters the cell, it binds tothe receptor and stimulates the cell to produce enzymesthat help prepare the uterus to nourish an embryo in theevent of a pregnancy.

5

4 Protein synthesis is induced. In this case,the protein produced is a receptor foranother steroid protein, progesterone.


• The receptors for peptide hormones are embedded in the plasma membrane.• The binding of the hormone to the receptor

triggers changes in the cytoplasmic end of the receptor protein.

• Using second messengers, this change is amplified and causes changes in the cell

• Second messengers activate enzymes.• one of the most common is cyclic AMP (cAMP).

How peptide hormones work

P

P

2

3

1

2

3

1

The peptide hormonebinds with its membranereceptor.

The hormone-receptorcombination triggers aseries of biochemicalreactions that producesthe second messenger.

The second messengertriggers a series ofreactions that leads toaltered cell functions.

Receptor

Productionof secondmessenger

Alteration ofcell activity

Peptidehormone


• A single hormone binding to a receptor in the plasma membrane can result in the formation of many second messengers in the cytoplasm.• Cyclic AMP is made from ATP by an enzyme

that removes two phosphate units.• Each second messenger can activate many

molecules of a certain enzyme.

Key Biological Process: Second messengers

1

2

3

1 Adenylyl cyclase converts ATP into cyclic AMP(cAMP), and cAMP acts as a second messengerthat activates enzymes called protein kinases.

After a peptide hormone binds to its receptor, thehormone-receptor complex activates adenylyl cyclase.

Protein kinases catalyze a wide variety ofactions, depending on the nature of the firstmessenger. Because of the presence of asecond messenger, the effect on the cell isgreatly amplified.

Altered cell function(regulates enzymes,synthesizes proteins, secretes molecules)

Protein kinase(inactive)

ATP

Protein kinase(active)

(Secondmessenger)cAMP

Adenylylcyclase

2 3

Hormone(first messenger)

Receptor

The Hypothalamus and the Pituitary

• The pituitary gland is located beneath the hypothalamus and is the location where nine hormones are produced.• These hormones act principally to influence

other endocrine glands.• The pituitary consists of two lobes:

• Posterior pituitary regulates water conservation and, in women, milk letdown and uterine contraction.

• Anterior pituitary regulates other endocrine glands.


• The hypothalamus and the posterior pituitary are connected by a tract of neurons.• Hormones are produced by cell bodies in the

hypothalamus and transported to the posterior pituitary.• Antidiuretic hormone (ADH) regulates the

kidney’s retention of water.• Oxytocin initiates uterine contractions during

childbirth and milk release in mothers.


• The anterior pituitary is a complete gland that produces the hormones that it secretes.• Thyroid-stimulating hormone (TSH)

stimulates the thyroid gland to produce thyroxine, which in turn stimulates oxidative respiration.

• Adrenocorticotropic hormone (ACTH) stimulates the adrenal gland to produce hormones.

• Growth hormone (GH) simulates the growth of muscle and bone throughout the body.


• Follicle-stimulating hormone (FSH) • In females, it triggers the maturation of egg

cells and stimulates the release of estrogen.• In males, it regulates sperm development.

• Luteinizing hormone (LH)• In females, it triggers ovulation of a mature

egg.• In males, it stimulates the gonads to produce

testosterone.


• Prolactin (PRL) stimulates the breasts to produce milk.

• Melanocyte-stimulating hormone (MSH) stimulates, in reptiles and amphibians, color changes in the epidermis.• Its function in humans is poorly understood.

Figure 30.4 The role of the pituitaryCopyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Antidiuretic

hormone(ADH)

Adrenocorticotropic

hormone (ACTH)

Gro

wth

hor

mon

e (G

H)

Prolactin (PRL)

Oxytocin

Melanocyte-stimulating

hormone (MSH)Gonadotropic hormones:Follicle-stimulatinghormone (FSH) andluteinizing hormone (LH)

Adrenalcortex

Thyroid gland

Thyroid-stimulating hormone(TSH)

Kidneytubules

Hypothalamus

Mammary glandsin mammalsBone

and muscle Testis

Ovary

Anteriorpituitary

Posteriorpituitary

Muscles ofmammary glands and

uterus


• The hypothalamus controls production and secretion of the anterior pituitary hormones by means of a family of special hormones.• Neurons in the

hypothalamus secrete both releasing and inhibiting hormones.

Cell body

Axons toprimarycapillaries

Hormones

Portal venules

Hypophysealportal system

Anteriorpituitary

Posterior pituitary

Pituitary stalk

Primarycapillaries


• Negative feedback (feedback inhibition) often controls the release of hormones from the hypothalamus and anterior pituitary.• When enough of the target

hormone has been produced, the hormone then feeds back to the hypothalamus and anterior pituitary and inhibits the release of stimulating hormones.

–

–

HypothalamusInhibition

Releasing hormones(TRH, CRH, GnRH)

Anterior pituitary

Target glands

Hormones

(Thyroid, adrenal cortex, gonads)

Tropic hormones(TSH, ACTH, FSH, LH)

Inhibition

The Pancreas

• The pancreas has both exocrine and endocrine functions.• It secretes digestive enzymes and

hormones.• The hormones, produced in the

islets of Langerhans, are insulin and glucagon.• Insulin promotes the uptake

of glucose and the accumulation of glycogen in the liver and triglycerides in fat cells.

• Glucagon causes liver cells to release stored glucose and to break down triglycerides.

Between meals

Blood glucose

Pancreas

Blood glucose

After a meal

Pancreatic islets Pancreatic islets

Insulin secretion

Glucagon secretion

Insulin secretion

Glucagon secretion

Cellular uptakeof glucose

Release of stored glucose,break down of fat

The Pancreas

• Diabetes mellitus is a serious disorder in which affected individuals are unable to take up glucose from the blood.• There are two kind of diabetes mellitus:

• Type I is a hereditary autoimmune disease in which the islets of Langerhans are attacked, resulting in abnormally low insulin secretion.

• Type II is when cells don’t respond to insulin, sometimes due to a reduction in the number of insulin receptors in the target tissue.

The Thyroid, Parathyroid, and Adrenal Glands

• The thyroid gland makes several hormones.• Thyroxine increases metabolic rate and

promotes growth.• Calcitonin inhibits the release of calcium from

bones and promotes the uptake of calcium by bones.


• The parathyroid glands are four small glands attached to the thyroid.• These glands produce parathyroid hormone

(PTH), a hormone that is absolutely essential for survival because it regulates calcium levels in the blood.• Calcium ions are necessary for muscle

contractions, such as those of the heart.• PTH stimulates the release of calcium from

bone.• Calcitonin (released from the thyroid gland)

has the opposite effect.

Maintenance of proper calcium levels in the blood

Ca++

Ca++

Ca++

Ca++

Ca++

Ca++

Ca++

Ca++

Ca++

LOW CALCIUM LEVELSTIMULATES PTHSECRETION Inactive

osteoblast

Bonematrix

(a) (b)

Bonematrix

HIGH CALCIUM LEVELSTIMULATES CALCITONINSECRETION

PTH stimulatesosteoclast(breaking downbone matrix)

Osteocyte(in lacuna)

Calcitoninstimulatesactiveosteoblast(increasingbonematrix)


• The adrenal glands are located just above the kidney and each is comprised of two parts.• The medulla is the inner core and produces

epinephrine and norepinephrine.• The cortex is the outer region and produces

the steroid hormones cortisol and aldosterone.


• The adrenal medulla releases epinephrine (adrenaline) and norepinephrine in times of stress.• These hormones act as emergency signals that

stimulate rapid deployment of body fuel.

• The adrenal cortex produces • Cortisol, which acts to maintain nutritional well-

being.• It is also released in times of stress but can

become a chronic problem if stress continues.• Aldosterone, which affects water reabsorption in

the kidney and affects both blood volume and blood pressure.

chapter 30 chemical signaling within the animal body

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