chapter 17 endocrine system

function

coordination and integration

endocrine glands

are ductless glands that synthesize and release hormones into the blood stream to communicate for long distant communication

Hormones

are used as chemical messengers that have target cell specificity - they have specific receptors for a hormone that they bind and respond to

hormones are reliant upon

bloodstream (usually) to deliver the message. Randomly leave blood and enter interstitial fluid

hormones bind to

target cell's receptors

hormones transported in blood

influences metabolic activities

responses in the endocrine system are

slow but long lasting Unlike the nervous system

the endocrine system targets

any cells in the body with correct receptors which can be very widespread

Exhibits longer reaction times

Hormones can have different response times: immediate - hours/days (or even inactive until entering target cells)

has longer-lasting effects (minutes to days and weeks)

Response Duration

is usually limited: seconds to hours•

Effects may disappear rapidly as blood levels drop, but some may persist for hours at low blood levels

the endocrine system acts with

nervous system to coordinate and integrate activity of body cells

endocrine glands

are ductless and contain epithelial tissue that makes and releases hormones within a connective tissue framework

endocrine glands include

pituitary, thyroid, parathyroid, adrenal, pineal

exocrine glands

have ducts

Hypothalamus

is a neuroendocrine organ

Regulating development, growth, and metabolism

- hormones help regulate embryonic cell division and differentiation

- hormones regulate metabolism (both anabolism and catabolism)

maintaining blood composition and volume

regulate blood solute concentrations, blood volume, cellular concentration, and platelets

Controlling digestive processes

Hormones influence secretory processes and movement of materials in digestive tract

Controlling reproductive activities

Hormones affect development and function of reproductive systems and the expression of sexual behaviors

some glands have exocrine and endocrine functions including

Pancreas, gonads (ovaries, testes), placenta, kidneys

adipose tissue

adipose cells release leptin - appetite control; stimulates increased energy expenditure

Heart: Atrial Natriuretic Peptide (ANP)

decreases blood Na+ concentration, therefore blood pressure and blood volume

Kidneys: Erythropoietin

signals production of red blood cells

Skin: Cholecalciferol

precursor of vitamin D (calcitriol = active form of vitamin D) that helps absorb calcium from intestine

3 types of stimuli for hormone release from endocrine gland

1. Hormonal

2. Humoral

3. Neural

hormonal stimuli

Stimulus: hormone release (TROPIC)

hormonal stimuli response

A different hormone is released - tropic response to other hormones

hormonal stimuli example

Hormones from the hypothalamus can trigger release of different hormones from the gonads, adrenal cortex, thyroid gland

humoral stimuli

• Stimulus: Change in critical ions or nutrients in blood

humoral stimuli response

Response to changes in blood chemistry stimulates hormone release

humoral stimuli example

Low calcium detected in capillary bed to Parathyroid hormone (from parathyroid gland) is released to blood Calcium increases. PTH causes Ca2+ concentrations to increase, and stimulus is removed

neural stimuli

Neural input

neural stimuli response

Hormone is released in response to nervous system

- Nervous system can adjust hormone levels when needed (modulation)

• Can modify stimulation or inhibition of endocrine glands

What is a hormone?

Chemical messenger secreted by cells - travel through the blood to regulate metabolic function of the other cells in the body.

Hormones can be:

amino acid or a steroid

hormone can act

on the same cells that secrete it or on other cells

hormones can regulate

itself or be triggered by an outside source to turn on or off

autocrine and paracrine secretions

local chemical messengers; NOT part of endocrine system (no bloodstream)

Autocrine

chemicals that exert effects on same cells that secrete them

• Example - prostaglandins released by smooth muscle cells

Paracrine

locally acting chemicals that affect cells other than those that secrete them (neighboring cells)

• Example - somatostatin -inhibits insulin secreting cells

Eicosanoids

a type of local hormone formed from fatty acids within phospholipid bilayer of membrane

• Synthesized through an enzymatic cascade

Prostaglandins

are eicosanoids

• Stimulate pain and inflammatory responses

• Aspirin and other nonsteroidal anti-inflammatory drugs block prostaglandin formation

Steroids

synthesized from cholesterol (ex: sex hormones). Gonadal and adrenocortical hormones are steroids (cortisol)

• Are lipid-soluble and can cross the plasma membrane

• Steroid hormones bind to their corresponding receptors in the cytoplasm of responsive cells

biogenic amines

- modified amino acids

• Includes: catecholamines, thyroid hormone, melatonin

• Water-soluble except for thyroid hormone (TH)

• TH is nonpolar (made from a pair of tyrosines) and lipid soluble

Amino Acid (protein) based

proteins (ex: tropic hormones).

• Most hormones are amino acid based. Bind externally on cell receptors!

-Tropic hormones: hormones that regulate other glands to release their hormones.

Amines

formed by amino acids, include norepinephrine, epinephrine

peptides

formed by amino acids and include ADH, OT, TRH, SS, GnRH

proteins

made from amino acids and include PTH, GH, and PRL

Glycoproteins

are made from protein and carbohydrates which include FSH, LH, and TSH

Steroids

are made from cholesterol and include estrogens, testosterone, aldosterone, and cortisol.

lipid soluble hormones

require a carrier protein

• Lipid-soluble hormones do not dissolve readily in blood

• Carriers are water-soluble proteins made by the liver

• Carriers protect hormones from early destruction

binding between hormone and carrier is temporary

• Attachment, detachment, reattachment are common

• Most of the hormone (90% or more) is bound hormone

• Only unbound (free) hormone can exit blood and bind to target cell receptors

Most water-soluble hormones

- travel freely through blood

- a few use carrier proteins to prolong their life

A hormone's blood concentration depends on

how fast it is synthesized and eliminated

Hormone Release

and its concentration in blood are positively correlated

• An increase in release results in higher the blood concentration and vice versa

Hormone elimination

occurs in multiple ways

- Enzymatic degradation in liver cells

- Removal from blood via kidney excretion or target cell uptake

- The faster the elimination rate, the lower the blood concentration

Half-life

time necessary to reduce a hormone's concentration to half of its original level

• Depends on how efficiently it is eliminated

• Hormones with short half-life must be secreted frequently to maintain normal concentration

Water-soluble hormones generally have

short half-life

• For example, half-life of a few minutes for small peptide hormones

Steroid hormones generally have

a long half-life

• Carrier proteins protect them

• For example, testosterone half-life is 12 days

Lipid Soluble Hormones

Internal hormonal Mechanism

hydrophobic hormones

can diffuse across target cell membrane (small, nonpolar, and lipophilic)

intracellular receptor

in the cytosol or nucleus

alters gene expression

directly targets and activates genes

• Results in transcription of an mRNA, which is translated to a protein

• The protein may have structural or metabolic effects

plasma membrane mechanism

external hormonal mechanism

hydrophilic hormones

(Water-soluble hormones) - basically, all your AA hormones except thyroid hormone.

cell receptor is embedded in theplasma membrane

Hormone binds its receptor, receptor uses SECOND MESSENGER signaling (the hormone itself - first messenger). Signaling mechanisms/cascades - amplify that signal

water soluble hormones

• Multiple results possible with different signal transduction pathways

• Activation or inhibition of enzymatic pathways

• Growth through cellular division

• Release of cellular secretions

• Changes in membrane permeability

• Muscle contraction or relaxation

intracellular enzyme cascade

•Signal is amplified at each enzymatic step

•Just a few hormone molecules can change many molecules within cell

•There are many places to regulate pathway activities

signaling pathway controls

Cells possess mechanisms to quickly inactivate intermediate

For example, to break down second messengers

Target Cell Specificity

The Target cell (with a specific receptor for that hormone) being activated depends on several factors

1. Blood levels of hormone - how much is released

2. Relative number of receptors on/in target cell -how many receptors on that cell receive the hormone

3. Affinity (strength) of binding between receptor and hormone

4. Its simultaneous response to other hormones

Adaptation to Feedback

Amounts of circulating hormones in the bloodstream can influence the NUMBER of receptors for that hormone:

Up-regulation

target cells form more receptors in response to low hormone levels

Down-regulation

target cells lose receptors in response to high hormone levels

• Desensitizes the target cells to prevent them from overreacting to persistently high levels of hormone.

Hormones can also influence other hormones and hormone receptors

Progesterone can down-regulate the estrogen receptors in the uterus

Synergism

hormones work together to produce greater effect

Permissiveness

first hormone allows action of second hormone

antagonistic

one hormone causes opposite effect of another hormone

negative feedback

systems control blood levels of most hormones

• Increased hormone effects on target organs can inhibit further hormone release

• Output shuts off the original effect of the stimulus

• Levels vary only within narrow, desirable range

• any change/deviation from the system is opposed and resisted

Anatomic Relationship of the Hypothalamus and the Pituitary Gland

The hypothalamus controls the pituitary, which controls several other endocrine organs

pituitary gland

• Lies inferior to hypothalamus in sella turcica of sphenoid bone

• Pea-sized

• Connected to hypothalamus by infundibulum

• Partitioned into anterior and posterior pituitary

posterior pituitary

composed of neural tissue that secretes neurohormones

- connected via hypothalamic-hypophyseal tract (neurons)

Posterior lobe + infundibulum

neurohypophysis

anterior pituitary

adenohypophysis: consists of glandular tissue. Hypophyseal portal system (releasing/inhibiting hormones from hypothalamus)

- connected via hypophyseal portal system (bloodstream)

Hypothalamic neurons project through

infundibulum and release hormones in posterior pituitary

•Somas in supraoptic nucleus and paraventricular nucleus

•Axons in hypothalmo-hypophyseal tract of infundibulum

•Synaptic knobs within posterior pituitary

posterior pituitary secretes

ADH and oxytocin

Oxytocin and ADH

are synthesized in the hypothalamus then transported along the hypothalamic-hypophyseal tract to be stored in axon terminals in the Posterior Pituitary and released into blood when neurons fire

Oxytocin stimulus

impulses from hypothalamic neurons in response to cervical/uterine stretching. Primarily positive feedback.

oxytocin response

stimulates uterine contraction, initiates labor, initiates milk ejection in the breast, emotional bonding.

Oxytocin inhibition

lack of appropriate neural stimuli

Acts as neurotransmitter in brain

• In the brain, oxytocin acts as a chemical messenger and has an important role in many human behavior

adh

(anti-diuretic hormone/ Vasopressin

ADH stimulus

impulses from hypothalamic neurons in response to increased blood solute concentration or decreased blood volume

• Hypothalamus osmoreceptors monitor solute concentrations

ADH response

stimulates water reabsorption in kidney tubule cells, inhibits urine formation "rescues water"

ADH inhibition

adequate hydration. Inhibited by alcohol, diuretics

adh disease correlation

too little - diabetes insipidus; too much-SIADH - Syndrome of inappropriate antidiuretic hormone secretion

• High concentrations cause vasoconstriction

Hypothalamo-hypophyseal portal system

of blood vessels connects hypothalamus to anterior pituitary

Hypothalamus hormonally stimulates

anterior pituitary to release its hormones

Hypothalamus secretes

regulatory hormones --- Travel via portal blood vessels to Anterior pituitary---secretes hormones into general circulation

anterior pituitary (adenohypophysis)

Hypothalamus secretes releasing and inhibiting hormones to anterior pituitary to regulate hormone secretion

thyroid-stimulating hormone (TSH); thyrotropin

• Release triggered by TRH from hypothalamus

• Causes release of thyroid hormone (TH) from thyroid gland

Prolactin (PRL)

• Release triggered by PRH, inhibited by PIH from hypothalamus

• Causes milk production, mammary gland growth in females