The Endocrine System

Chapter 15

Hormones

• Secreted by endocrine glands,

endocrine cells, and certain neurons

• Travel through the bloodstream to

nonadjacent target cells

Other Signaling Molecules

• Neurotransmitters

• Local signaling molecules

• Pheromones

Discovery of Hormones

• Bayliss and Starling severed nerves to dog’s

intestine; left blood vessels intact

• Pancreas still responded

• Extracts of glandular epithelium also

provoked pancreatic response

• Extracts contained secretin

Hormone Interactions

• Opposing interaction

• Synergistic interaction

• Permissive interaction

Endocrine System

Main Sources • Pituitary gland• Adrenal glands• Thyroid gland• Parathyroid

glands• Pineal gland• Thymus gland

Responses to Hormones Vary

• Different hormones activate different

responses in the same target cell

• Not all types of cells respond to a

particular hormone

Two Main Hormone Types

• Steroid hormones– Derived from cholesterol– Estrogens, progestins, androgens (such as

testosterone), cortisol, aldosterone

• Peptide hormones– Peptides, proteins, or glycoproteins– Glucagon, ADH, oxytocin, TRH, insulin,

somatotropin, prolactin, FSH, LH, TSH

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

Protein 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

cAMP activatesprotein kinase A

glucagon

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

The Hypothalamus

• Region in the

forebrain

• Contains hormone-

secreting cells

• Interacts with

pituitary

hypothalamus

pituitary gland

Pituitary Gland

• Pea-sized gland at base of hypothalamus

• Two lobes

– Posterior lobe stores and secretes hormones

that were synthesized in the hypothalamus

– Anterior lobe produces and secretes its own

hormones

Posterior Lobe

• Antidiuretic hormone (ADH)

• Oxytocin (OCT)

cell body in hypothalamus

axons

to the general circulation

Anterior Pituitary

• ACTH• TSH• FSH• LH• PRL• STH

Normal Hormone Production

• Generally, the body produces only very

small amounts of hormones

• To isolate 1 milligram of TRH,

researchers dissected 7 metric tons of

hypothalamic tissue

Abnormal Somatotropin Output

• Gigantism

• Pituitary dwarfism

• Acromegaly

Feedback Mechanisms

• Negative feedback

– An increase in concentration of a hormone

triggers activities that inhibit further secretion

• Positive feedback

– An increase in concentration of a hormone

triggers activities that stimulate further

secretion

Cortisol

• Cortisol secretion

– Inhibits blood glucose uptake by muscle and

other tissues

– Causes breakdown of proteins to amino

acids and conversion to glucose

– Causes degradation of adipose tissue to fatty

acids for use as energy source

Feedback Control of Cortisol Secretion

• Hypothalamus senses rise in glucose and

secretes less releasing hormone (CRH)

• Anterior pituitary responds by secreting

less ACTH

• Adrenal cortex slows its secretion of

cortisol

Localized Feedback in Adrenal Medulla

• Norepinephrine secreted by neurons in the medulla accumulates in the synaptic gap

• Some molecules bind to receptors on the axon endings that secreted them

• This prevents further secretion of norepinephrine by that axon

Thyroid Gland Disorders

• Goiter

• Hyperthyroidism

• Hypothyroidism

Calcium Regulation

• Parathyroid hormone (PTH) is the main regulator of calcium in the blood

• It is secreted when calcium levels drop

• PTH causes bone cells to digest bone tissue and release calcium

• PTH also stimulates calcium reabsorption by the kidneys and absorption by the gut

Control of Glucose Metabolism

insulin

Glucose rises

Glucose fallsGlucose is absorbed

Cells use glucose

glucagonGlycogen to glucose

Glucose uptake

Glucose to glycogen

Diabetes Mellitus

• Disease in which excess glucose accumulates in blood, then urine

• Effects include– Excessive urination– Constant thirst– Weight loss– Ketone formation and acid-base imbalances

Two Types of Diabetes

• Type 1

• Autoimmune disease

• Usually appears in

childhood

• Treated with insulin

injections

• Type 2

• Target cells don’t

respond

• Usually appears in

adults

• Treated with diet,

drugs

The Pineal Gland

• Photosensitive gland embedded in brain

• In the absence of light, secretes melatonin

• Affects the human biological clock

• May also play a role in human puberty and

in seasonal affective disorder

Local Signaling Molecules

• Prostaglandins

– Produced and secreted in response to local

changes

– Sixteen types with a variety of effects

• Growth factors

– Affect cell division rates in tissues