Endocrine Control

Chapter 26

Endocrine SystemMajor Components

• Hypothalamus• Pituitary gland• Pineal gland• Thyroid gland• Parathyroid glands• Thymus gland• Adrenal glands• Pancreatic islets• Ovaries• Testes

hypothalamus (part of the brain)

pituitary gland, anterior lobe

pituitary gland, posterior lobe

adrenal gland (one pair) cortex medulla

ovaries (one pair of female gonads)

testes (one pair of male gonads)

pineal gland

thyroid gland

parathyroid glands (four)

thymus gland

pancreatic islets

Fig. 26-1, p.449

Hormones

• Secreted by endocrine glands, endocrine cells, and hypothalmic neurons

• Travel through bloodstream to target cells

• Bind to receptors on target cells

Other Signaling Molecules

• Neurotransmitters– From axon endings of neurons

• Local signaling molecules– Prostaglandins– Nitric oxide (NO)

Three-Step Hormonal Action

• Activation of a receptor as it binds the hormone

• Transduction of signal into a molecular form that can work inside the cell

• Functional response of target cell

signal reception

signal transduction

cellular response

Responses to Hormones Vary

• Different hormones activate different responses in the same target cell

• Not all types of cells respond to a particular hormone

Main Hormone Types

Steroid hormones– Lipids derived from cholesterol

Peptide hormones– A few amino acids

Amine hormones– Modified amino acids

Protein hormones– Longer amino acid chains

Receptors

• Intracellular– Steroid hormones– Diffuse across plasma membrane

• Plasma membrane– Peptides and proteins– Too big or polar to diffuse– Second messengers (cAMP)

Steroid Hormones

receptor

hormone-receptor complex

gene product

hormone

• Most diffuse across the plasma membrane and bind to a receptor

• Hormone-receptor complex acts in nucleus to inhibit or enhance transcription

A steroid hormone molecule moves from the blood into interstitial fluid that bathes a target cell. Being a lipid-

soluable molecule, the steroid hormone diffuses across the target cell’s plasma membrane.

The mRNA transcript moves from the nucleus into the cytoplasm. There it becomes translated into a gene product that is required for the response to the hormone signal.

gene product

The hormone diffuses through the cytoplasm, then on through the nuclear envelope. Inside the nucleus, it will bind with a receptor molecule.

receptor

hormone-receptor complex

Now the hormone-receptor complex triggers transcription of gene regions in the DNA.

Fig. 26-2a, p.451

1

3

2

4

5

Peptide Hormone

• Hormone binds to a receptor at cell surface

• Binding triggers a change in activity of enzymes inside the cell

glucagon receptor

cyclic AMP + Pi

ATP

The cAMP activatesprotein kinase A.

glucagon

Protein kinase A converts phosphorylasekinase to active form and inhibits an enzyme required for glucagon synthesis.

ATP+Pi

The cAMP now activates protein kinase A.

A glucagon molecule diffuses from blood into the interstitial fluid that bathes the plasma membrane of a liver cell.

unoccupied glucagon receptor at target cell’s plasma membrane

Protein kinase A also inhibits an enzyme required for synthesis of glycogen.

Glucagon binds with the receptor, and the binding activates adenylate cyclase. This enzyme catalyzes the formation of cAMP inside the target cell.

Protein kinase A converts phosphorylase kinase to active form. This enzyme activates a different enzyme, which breaks down glycogen to its glucose monomers.

Fig. 26-2b, p.451

1

3

5

4

2

cyclic AMP

Hypothalamus and Pituitary

• Glands in brain– Structurally and functionally linked

• Master integrating center for endocrine and nervous systems

• Hypothalmic neurons produce– Neurotransmitters– Hormones

Pituitary Gland

• Pea-sized gland at base of hypothalamus• Two lobes– Posterior lobe stores and releases hormones

made in hypothalamus– Anterior lobe produces and secretes its own

hormones

Posterior PituitarySecretions

• Antidiuretic hormone (ADH)

• Oxytocin (OCT)

cell body in hypothalamus

axons

to the general circulation

Cell bodies in hypothalamus synthesize ADH or oxytocin

ADH, oxytocin move down axons, accumulate in axon endings

Small vessels carry hormones to general circulation

Action potentials cause release of hormones, which capillaries pick up

cell body

axon

capillaries

Stepped Art

Fig. 26-3, p.452

muscles in uterus wall

mammary glands

nephrons in kidneys

ADH

oxytocin

Cell bodies of secretory neurons in hypothalamus synthesize ADH or oxytocin.

The ADH or oxytocin moves downward inside the axons of the secretory neurons and accumulates in the axon endings.

Action potentials trigger the release of these hormones, which enter blood capillaries in the posterior lobe of the pituitary. Small blood vessels

deliver the hormone molecules to the general circulation.

Fig. 26-3, p.452

a

b

c

d

Anterior Pituitary

• Responds to hypothalmic signals• Releasers– Stimulate secretion of pituitary hormones

• Inhibitors– Inhibit release of pituitary hormones

Anterior PituitarySecretions

• Adrenocorticotropin (ACTH)• Thyroid stimulating hormone

(TSH)• Follicle-stimulating hormone

(FSH)• Luteinizing hormone (LH)• Prolactin (PRL)• Growth hormone (GH)

thyroidgland

ACTH TSH FSH LH PRL GH (STH)

testes in malesovaries in females

mammary glands

Hormones secreted from anterior lobe cells enter small blood vessels that lead to the general circulation.

Cell bodies of different secretory neurons in the hypothalamus secrete releasing and inhibiting hormones.

The hormones are picked up by a capillary bed at the base of the hypothalamus.

Bloodstream delivers hormones to a second capillary bed in anterior lobe of pituitary.

Molecules of the releasing or inhibiting hormone diffuse out of capillaries and act on endocrine cells in the anterior lobe.

most cells (growth-promoting effects)

Fig. 26-4, p.453

a

e

b

c

d

adrenalglands

Abnormal Pituitary Output

• Pituitary gigantism • Pituitary dwarfism • Acromegaly

Abnormal Pituitary Outputs

Thymus, Thyroid and Parathyroid

• Thymus– Immune function

• Thyroid– Development and metabolism– Regulated by feedback loops

• Parathyroid– Calcium levels

Hypothalamus

Anterior Pituitary

Thyroid Gland

ResponseStimulus

Blood level of thyroid hormone falls below a set point.

+TRH

TSH

Thyroid hormoneis secreted

Rise in the blood level of thyroid hormone inhibits secretion of TRH and TSH.

–

–

Negative Feedback and Thyroid Function

Thyroid Function

• Requires mineral iodine– Deficiency causes

goiter

Parathyroid and Calcium

• Parathyroid hormone (PTH)– regulates blood calcium– secreted when calcium levels drop– causes bone cells to release calcium from bone

tissue– stimulates calcium reabsorption by kidneys

Calcium and Vitamin D

• Without vitamin D, not enough calcium is absorbed

• Low blood calcium causes oversecretion of PTH– Breaks down existing bone– Causes rickets

Adrenal Glands and Stress

• Adrenal cortex secretes cortisol and aldosterone

• Negative feedback maintains blood cortisol levels

HypothalamusStimulusResponse

Anterior Pituitary

Adrenal Cortex

Cortisol is secreted, with these effects:

adrenal cortex

adrenal medulla

Blood level of cortisol falls below a set point

kidney

Both the hypothalamus and pituitary detect rise in blood level of cortisol and slow its further secretion.

CRH

ACTH

+

–

Cellular uptake of glucose from blood slows in many tissues, especially muscles (not the brain).

Proteins degraded in many tissues, especially in muscles. The free amino acids are converted to glucose and used in the assembly or repair of cell structures.

Fats in adipose tissue degraded to fatty acids that enter blood as an alternative energy source, indirectly conserving glucose for the brain.

–

ab

f

c

d

e

Negative Feedback Control of Adrenal Glands

Stress Response

• Stress can cause nervous system to override feedback loop

• Cortisol levels rise above normal, suppress inflammation

• Persistent high cortisol levels may harm health

The Pancreas and Glucose Homeostasis

stomach

pancreas

smallintestine

Pancreatic Hormones and Glucose Balance

• Glucagon– Secreted by alpha cells in islets– Raises blood glucose level

• Insulin– Secreted by beta cells in islets– Lowers blood glucose level

• Somatostatin– Secreted by delta cells– Blocks insulin and glucagon secretion

Fig. 26-9, p.456

Stimulus Stimulus

alpha cells

glucagon

LIVER

insulin

beta cells

Increase in blood glucose

+

PANCREAS

–

Decrease in blood glucose

alpha cells

glucagoninsulin

beta cells+ –

MUSCLE FAT CELLS Body cells, especially in muscle and adipose tissue, take up and use more glucose.Cells in skeletal muscle and liver store glucosein the form of glycogen.

ResponseDecrease in blood glucose

Cells in liver break down glycogen faster. The released glucose monomers enter blood.

Response Increase in blood glucose

a f

g h

i

je

b c

d

Diabetes Mellitus

Excess glucose accumulates Type 1

• Autoimmune disease

• Usually appears in childhood

• Insulin injections

Type 2

• Target cells don’t respond

• Usually appears in adults

• Diet, drugs

Table 26-2, p.457

Sex Hormones

• Testes and ovaries synthesize the same sex hormones in different amounts– Estrogens– Progesterone– Testosterone

• Influence sexual traits

The Pineal Gland• Photosensitive gland embedded in brain• In absence of light, secretes melatonin• Influences seasonal behaviors• Affects human biological clock– sleep-wake cycles– seasonal affective disorder

Deformed Frogs

• Something in water triggers deformities• Problem thyroid function?• Tadpoles from “hotspots” developed normally

when given extra thyroid hormones• UV, parasites also play a role

Effects of Pollution on Frogs

Table 26-3, p.459