Endocrine System

Dr. Annette M. ParrottGPC

BIOL1612

Endo crine System“inside” “secrete”

• Odd organ system– Compared to nervous sys. &

digestive sys.

• Endocrine glands usually not connected– Considered a “system”

because of functional similarity

• Secrete chemical messages called hormones to target cells “to excite”

Also skin, heart, GI tract, placenta, kidneys, adipose tissue

Principal functions of the endocrine system

• Maintenance of the internal environment in the body (maintaining the optimum biochemical environment).

• Integration and regulation of growth and development.

• Control, maintenance and instigation of sexual reproduction and development.

Glands with a sensing and signaling system which regulates the duration and magnitude of hormone release via feedback from the target cell.

Types of hormones

• Hormones are categorized into four structural groups, with members of each group having many properties in common: – Peptides and proteins (polypeptides)– Amino acid derivatives – Steroids (cholesterol based)

– Fatty acid derivatives - Eicosanoids (mostly paracrines, i.e. leukotrines, prostaglandins)

• Peptides– “Chains” of amino acids

• 4 – 200+ amino acids

– Water soluble– Largest # of hormones

• Hypothalamus• Pituitary (Ant. & Post.)• Islets of Langerhans• Parathyroid hormone• Digestive system

hormones

Types of hormones

Peptide/protein hormone synthesis

• Amino Acid Based– Tyrosine derivatives

• Thyroid hormones– Thyroxine (T4)

– Triiodothyronine (T3)

• Catecholamines/Adrenal medulla– Epinephrine– Norepinephrine

» Both neurohormones & neurotransmitter

– Tryptophan derivatives (precursor to serotonin and the pineal hormone melatonin)

– Glutamic acid (converted to histamine)

Types of hormones

• Steroids– Derivatives of cholesterol

differing in side chains– Four covalently-bonded

rings– Lipid soluble (freely

diffuse, not stored, not packaged)• Adrenal cortex• Gonads

• Examples– Glucocorticoids (cortisol major

representative in mammals) – Mineralocorticoids (aldosterone

most prominent) – Androgens (i.e. testosterone) – Estrogens (i.e. estradiol and

estrone) – Progestogens (i.e. progestins)

Types of hormones

• Fatty Acid Derivatives - Eicosanoids – Eicosanoids are a

large group of molecules derived from polyunsaturated fatty acids.

– The principal groups of hormones of this class are prostaglandins, prostacyclins, leukotrienes and thromboxanes.

Types of hormones

How does hormone type affect it’s activation of target cells?

Mechanisms of Hormone Action

• Lipid-soluble steroids & thyroid hormones– Diffuse through

plasma membrane– Enter nucleus– Forms “hormone-

receptor complex”– H-R complex binds as

transcription factors to chromosome to activate/inactivate gene(s)

Mechanisms of Hormone Action

• Peptides & water-soluble amines– Hormone (A) binds to

receptor on cell surface

– Activates G- protein– Activates adenylate

cyclase• Converts ATP to cAMP

– cAMP activates protein kinases, which produce final effect.

Signal Transduction Pathway Animation Transduction Pathway Epinephrine

Mechanisms of Hormone Action

• Peptides & water-soluble amines– Other Hormone (B) binds to

receptor on cell surface– Activates G- protein– Inhibits adenylate cyclase

• Stops ATP to cAMP

– inhibits final effect of first hormone

Which cells are activated by hormones?

Hormone Targets•A cell is a target because is has a specific receptor for the hormone

•Most hormones circulate in blood, coming into contact with essentially all cells. However, a given hormone usually affects only a limited number of cells, which are called target cells.

•A target cell responds to a hormone because it bears receptors for the hormone.

Which diagram represents… Steroid hormones?Lipid hormones?

Peptide hormones?

Target cellconcept

Receptor

Hormone

Target cell

Target cellconcept

Not all hormonesfind their target

How are chemical signals sent to cells?

Types of cell-to-cell signaling•Classic endocrine hormones travel via bloodstream to target cells

•Neurohormones are released via synapses and travel via the bloostream

•Paracrine hormones act on adjacent cells

•Autocrine hormones are released and act on the cell that secreted them

•Intracrine hormones act within the cell that produces them

Response vs. distance traveled

Endocrine action: the hormone is distributed in blood and binds to distant target cells.

Paracrine action: the hormone acts locally by diffusing from its source to target cells in the neighborhood.

Autocrine action: the hormone acts on the same cell that produced it.

Ways of influencing target cells

Within beside/near self close to

Create a Venn diagram comparing the nervous & endocrine systems

Endocrine vs. Nervous System• Major communication

systems in the body• Integrate stimuli and

responses to changes in external and internal environment

• Both are crucial to coordinated functions of highly differentiated cells, tissues and organs

• Unlike the nervous system, the endocrine system is anatomically discontinuous.

Nervous Sys. vs Endocrine Sys.• The nervous system exerts point-to-

point control through nerves, similar to sending messages by conventional telephone. Nervous control is electrical in nature and fast.

• The endocrine system broadcasts its hormonal messages to essentially all cells by secretion into blood and extracellular fluid. Like a radio broadcast, it requires a receiver to get the message - in the case of endocrine messages, cells must bear a receptor for the hormone being broadcast in order to respond.

Regulation of hormone secretion

• Sensing and signaling: a biological need is sensed, the endocrine system sends out a signal to a target cell whose action addresses the biological need. Key features of this stimulus response system are:·        receipt of stimulus·        synthesis and secretion of hormone·        delivery of hormone to target cell·        evoking target cell response·        degradation of hormone

Receipt of Stimulus• Humoral in response

to changing blood levels• i.e. PTH regulation of Ca2+ via

parathyroid

• Neural in response to nerve fibers • i.e. catecholamines

(norepinephrine & epinephrine) from adrenal medulla

• Hormonal in response to other hormones• i.e. GHRH secreted by

hypothalamus which regulates GH secretion by anterior pituitary

Inputs to endocrine cells

Control of Endocrine ActivityThe concentration of hormone as seen by target cells is determined by three factors:

•Rate of production•Rate of delivery

•Permissiveness/Synergism/Antagonism•Upregulation (insipidus)/downregulation

(Type II, melitus)

•Rate of degradation and elimination

What is a feedback loop?

Feedback Control of Hormone Production

•Feedback loops are used extensively to regulate secretion of hormones

•Negative feedback occurs when a change in a physiological variable triggers a response that counteracts the initial fluctuation

Negative Feedback• Neurons in the hypothalamus

secrete thyroid releasing hormone (TRH), which stimulates cells in the anterior pituitary to secrete thyroid-stimulating hormone (TSH)

• TSH binds to receptors on

epithelial cells in the thyroid gland, stimulating synthesis and secretion of thyroid hormones, which affect probably all cells in the body

• When blood concentrations of thyroid hormones increase above a certain threshold, TRH-secreting neurons in the hypothalamus are inhibited and stop secreting TRH.

Feedback control

• Negative feedback is most common: for example, LH from pituitary stimulates the testis to produce testosterone which in turn feeds back and inhibits LH secretion

• Positive feedback is less common: examples include LH stimulation of estrogen which stimulates LH surge at ovulation

Positive & Negative Feedback

Diseases of the Endocrine System

• Cushing's Syndrome • Acromegaly• Pheochromocytoma• Glucagonoma• Somatostatinoma• Diabetes mellitus• Diabetes insipidus• Hyperthyroidism (Graves disease)• Hypothyroidism (Goiter)• Hypothyroidism (Cretinism in babies)• Hypothyroidism (Myxedema) • Achondroplasia (Dwarfism) • Gigantism• SADS (Seasonal Affective Disorder):

Disorders of the Endocrine System Acromegaly Simple Goiter

Disorders of the Endocrine System

Cushing’s Disease Achondroplasia