Endocrine System

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• Water soluble• Largest # of

hormones– Hypothalamus– Pituitary (Ant. &

Post.)– Islets of

Langerhans– Parathyroid

hormone– Digestive

system hormones

Types of hormones: Proteins

• 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– Lipid soluble (freely diffuse,

not stored, not packaged)

• 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: Lipids• Eicosanoids

– derived from polyunsaturated fatty acids.

– The principal groups of hormones of this class are prostaglandins, prostacyclins, leukotrienes (inflammation) and thromboxanes (platelet aggregation).

Mechanisms of Hormone Action• Lipid-soluble steroids &

thyroid hormones– Diffuse through plasma

membrane– Enter nucleus– Forms “hormone-receptor

complex”, binds as TFs to chromosome to activate/ inactivate gene(s)

• 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

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?

Components of an automatic control system

Variable characteristic of the internal environment that is controlled by this mechanism (internal temp in this example)Sensor (receptor) detects changes in variable and feeds that information back to the integrator (control center) (thermometer in this example)Integrator (control center) integrates (puts together) data from sensor and stored "setpoint" data (thermostat in this example)Setpoint "ideal" or "normal" value of the variable that is previously "set" or "stored" in memoryEffector mechanism (furnace in this example) that has an "effect" on the variable (internal temperature in this example)

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

• A steroid hormone affects target cells by directly binding with:– A. cAMP– B. nuclear receptors which activate genes– C. protein receptors on the target cell’s surface– D. The RER– E. The second messenger

Endocrine Glands• Hypothalamus• Pituitary

– Anterior lobe– Posterior lobe

• Thyroid gland• Parathyroid

glands• Adrenal Glands

– Cortex– Medulla

• Islets of Langerhans

• Gonads– Ovaries– Testes

• Pineal gland• Thymus• others

Hormonal Communication

Hypothalamus

• Part of brain– Regulates ANS,

emotions, feeding/satiety, thirst, body temperature, etc.

– Hormones related to these functions• “Releasing

hormones”• Axonal transport to

posterior lobe

Anterior Pituitary

• “Releasing” hormones regulate AP aka adeno hypo physis “glands” “under” “growth”

• All proteins– TSH (thryoid stimulating

hormone/thyrotropin)

– ACTH (adrenocorticotropic hormone)

– FSH (gonadotropin)– LH (gonadotropin)

• Tropins/tropic hormones

– GH (growth hormone)– Prolactin-releasing H

Hypothalamic Pituitary Axis Animation : IP Web

Anterior Pituitary

Parathyroid Glands• Four small glands embedded

in posterior of thyroid– Parathyroid hormone (PTH)– Stimulates osteoclasts to free

Ca2+ from bone– Stimulates Ca2+ uptake from

intestine by stimulating conversion of vitamin D to calcitrol

– Stimulates Ca2+ reuptake from kidney

Hormonal Regulation of Calcium

Feedback Loop• Negative feedback in

calcium homeostasis. A rise in blood Ca2+ causes release of calcitonin from the thyroid gland, promoting Ca2+ deposition in bone and reducing reabsorption in kidneys.

• A drop in blood Ca2+ causes the parathyroid gland to produce parathyroid hormone (PTH), stimulating the release of Ca2+ from bone.

• PTH also promotes reabsorption of Ca2+ in kidneys and uptake of Ca2+ in intestines.

Pancreas• Consists of two major

types of secretory tissues which reflects its dual function – Exocrine gland

• secretes digestive juice • localized in the acinar cells

– Endocrine gland • releases hormones• localized in the islet cells

(islets of Langerhans)

Pancreatic Islets• “About a million”

embedded in pancreas• Control centers for

blood glucose– Insulin from beta cells– Glucagon from alpha

cells

Insulin Glucagon

Islets of Langerhans• Insulin stimulates

glucose uptake, glycogenesis

• Glucagon stimulates glycogenolysis, glucose release from liver (vs gluconeogenesis)

Feedback Loop

•A rise in blood glucose causes release of insulin from beta cells the pancreas, promoting glucose uptake in cells and storage as glycogen in the liver.

•A fall in blood glucose stimulates alpha cells in the pancreas to secrete glucagon, which causes the liver to break down glycogen and release glucose.

Pancreas Homeostatic ImbalancesDiabetes “siphon” mellitus mel= “honey”

Symptoms:•Polyuria•Polydipsia•Polyphagia

Blood Level Regulation in Diabetics

Non-Endocrine Gland Hormones

• Stomach (gastrin)• Small intestine (duodenumintesetinal

gastrin, secretin, cholecystokinin)• Heart (atrial natriuretic peptide)• Kidneys (erythropoietin, active vitamin D3)• Adipose tissue (leptid, resistin)• Skin • Placenta (human chorionic gonadotropin,

human placental lactogen, relaxin)

Functions regulated by the Endocrine System

• Growth• Healing• Water balance & Blood Pressure• Calcium Metabolism• Energy Metabolism• Stress• Regulation of other Endocrine

Organs

1. Juvenile diabetes mellitis (type 1) is:a. insulin dependentb. Non-insulin dependentc. Diabetes insipidusd. Goiter associatede. Caused by thyroid deficiency

2. Which of the following processes is not regulated by adrenal cortical hormones:a.Adaptatoin to stressb.Blood pressurec. Glucose utilizationd.Labor and deliverye.Sodium/potassium balance

Essential knowledge 3.D.2: Cells communicate with each other through direct contact with other cells or from a distance via chemical signaling.

a. Cells communicate by cell-to-cell contact.b. Cells communicate over short distances by using local

regulators that target cells in the vicinity of the emitting cell.

c. Signals released by one cell type can travel long distances to target cells of another cell type.– 1. Endocrine signals are produced by endocrine cells that release

signaling molecules, which are specific

Resources • Endocrinology Questions:• Endocrinology Quiz:• Pathophysiology of the Endocrine System: • Endocrine Surgery: • Dr. Ross BIO 218 A&P:• McGraw Hill Tutorial:

http://www.mhhe.com/biosci/esp/2002_general/Esp/default.htm• Lion Den: http://www.lionden.com/ap.htm• Medical Mneumonics: http://www.medicalmnemonics.com/cgi-bin/browse.cfm • Peptide Hormone Signal Transduction Animation: • Pathophysiology of the Endocrine System: • 106 Animated Tutorials:• BIOMedia: http://bio1152.nicerweb.com/Locked/media/ • Signal Transduction Pathway:• Human Physiology U of Colorado: • Textbook in Medical Physiology And Pathophysiology:   • Pituitary Tumors: