ENDOCRINE SYSTEM - PART 1

Endocrine System

composed of endocrine glands and specialized endocrine cells

Hormones

chemical messengers secreted by the endocrine glands
o Stimulate a specific response

Target tissues or effectors

specific sites where hormones go through

Exocrine glands

glands with ducts; secretions outside of the body; ex: saliva, sweat, breast milk and digestive enzymes

Chemical messengers

allow cells to communicate with each other to regulate body activities

Autocrine Chemical Messenger

Secreted by cells in a local area; influences the activity of the same cell or cell type from which it was secreted
Eicosanoids (prostaglandins, thromboxanes, prostacyclins, leukotrienes)

Paracrine Chemical Messenger

Produced by a wide variety of tissues and secreted into extracellular fluid; has a localized effect on other tissues
Somatostatin, histamine, eicosanoids

Neurotransmitter Chemical Messenger

Produced by neurons; secreted into a synaptic cleft by presynaptic nerve terminals; travels short distances; influences postsynaptic cells
Acetylcholine, epinephrine

Endocrine Chemical Messenger

Secreted into the blood by specialized cells; travels some distance to target tissues; results in coordinated regulation of cell function
Thyroid hormones, growth hormone, insulin, epinephrine, estrogen, progesterone, testosterone, prostaglandins

Functions of the Endocrine System

1. Regulation of metabolism
2. Control of food intake and digestion
3. Modulation of tissue development
4. Regulation of ion levels
5. Control of water balance
6. Regulation of cardiovascular functions
7. Control of blood glucose and other nutrients
8. Control of reproductive function
9. Stimulation of uterine contractions and milk release
10. Modulation of immune system function

Hormones

Two chemical categories: lipid-soluble and water-soluble

Lipid-Soluble Hormones

➢ Nonpolar
➢ Steroid hormones, thyroid hormones, and fatty acid derivative hormones, such as certain eicosanoids

Transport of Lipid-Soluble Hormones

➢ Easily removed from the blood because of their small size and low solubility (if unprotected)

Three mechanisms that could potentially result in the removal of unprotected lipid-soluble hormones from the body:

o Breakdown by enzymes in the liver or enzymes in the lungs
o Excretion into urine by the kidneys
o Breakdown by enzymes in the bloodstream

Binding proteins

“chaperone” the hormone
o Reduces the rate at which lipid-soluble hormones are removed from the blood

Water-Soluble Hormones

➢ Polar
➢ Protein hormones, peptide hormones, most amino acid derivative hormones

Transport of Water-Soluble Hormones

➢ Circulate freely in the blood
➢ Dissolve directly into the blood and delivered to their target tissue without attaching to a binding protein

Proteases

degrade protein and peptide hormones in the blood; the breakdown products are excreted in the urine

Three modifications made to hormone molecules that help protect them from being destroyed:

o Having a carbohydrate attached to them
o Having a terminal end protected from protease activity
o Having binding proteins. Bound hormones circulate in the plasma longer that free water-soluble hormones do

Stimulation of Hormone Release

1. Control by Humoral Stimuli
2. Control by Neural Stimuli
3. Control by Hormonal Stimuli

Control by Humoral Stimuli

exhibited by hormones that are sensitive to circulating blood levels of certain molecules such as glucose or calcium

Calcium

stimulates the release of PTH

Elevated blood glucose levels

stimulate pancreas to secrete insulin

Elevated blood potassium levels

stimulate the adrenal cortex to secrete aldosterone

Control by Neural Stimuli

cause hormone secretion in direct response to action potentials in neurons, as occurs during exercise

Releasing hormones

hormones from the hypothalamus that cause the release of other hormones

Neuropeptides

neurons secrete chemical messengers directly into the blood when they are stimulated

Control by Hormonal Stimuli

When a hormone is secreted, and it stimulates the secretion of other hormones

Tropic hormones

hormones from the anterior pituitary gland that stimulate hormones from other endocrine glands

Inhibition of Hormone Release

1. Inhibition of Hormone Release by Humoral Stimuli
2. Inhibition of Hormone Release by Neural Stimuli
3. Inhibition of Hormone Release by Hormonal Stimuli

Inhibition of Hormone Release by Humoral Stimuli

➢ Often when a hormone’s release is sensitive to the presence of a humoral stimulus, there exists a companion hormone whose release is inhibited by the same humoral stimulus

Blood pressure

adrenal cortex secreted aldosterone to raise BP; if blood pressure goes up, the atria of the heart secrete atrial natriuretic peptide (ANP) which lowers BP

Inhibition of Hormone Release by Neural Stimuli

➢ Neural stimuli can prevent hormone secretion
➢ If the neurotransmitter is inhibitory, the target endocrine gland does not secrete its hormone

Inhibition of Hormone Release by Hormonal Stimuli

➢ Some hormones prevent the secretion of other hormones

Inhibiting hormones

hormones from the hypothalamus that prevent the secretion of tropic hormones from the anterior pituitary gland

Negative Feedback

➢ Prevents further hormone secretion once a set point is achieved
o May inhibit the action of other stimulatory hormones to prevent the secretion of the hormone in question

Positive Feedback

➢ Self-promoting system whereby the stimulation of hormone secretion increases over time

Hormone Receptors

target cell proteins that hormones bind to when exerting their actions

Receptor site

portion of each receptor molecule where a hormone binds

Specificity

tendency for each type of hormone to bind to one type of receptor and not to others

Classes of Receptors

1. Nuclear Receptors
2. Membrane-Bound Receptors

Lipid-soluble hormones

bind to nuclear receptors located inside the nucleus of the target cell

Water-soluble hormones

bind to membrane-bound receptors which are integral membrane proteins since they cannot pass through the cell membrane

Action of Nuclear Receptors

1. Nuclear receptors have portions that allow them to bind to the DNA in the nucleus once the hormone is bound
➢ Hormone-receptor complex – activates genes which in turn activate the DNA to produce mRNA
➢ mRNA – increases the synthesis of certain proteins that produce the target cell’s response
2. Nuclear receptors cannot respond immediately because it takes time to produce the mRNA and the protein

Membrane-Bound Receptors and Signal Amplification

1. Membrane-bound receptors activate a cascade of events once the hormone binds
2. Some membrane-bound receptors are associated with membrane proteins called G proteins.
➢ When a hormone binds to a membrane-bound receptor, G proteins are activated
➢ Alpha subunit of the G protein – can bind to ion channels and cause them to open or change the rate of synthesis of intracellular mediators such as cyclic adenosine monophosphate (cAMP)
3. Second-messenger systems – act rapidly because they act on already existing enzymes and produce an amplification effect

Membrane-Bound Receptors that Activate Adenylate Cyclase-Coupled G Proteins

➢ Three subunits of G proteins

Three subunits of G proteins

o Alpha
o Beta
o Gamma

G proteins

one of the subunits binds to guanine nucleotides

Inactive state

guanine diphosphate molecule bound to the alpha subunit

Active state

guanine triphosphate bound to the alpha subunit

Signal Amplification

a single hormone activates many second messengers which activates enzymes that produce an enormous amount of final product

Pituitary Gland

➢ Also called hypophysis
➢ Body’s master gland
➢ Housed in a depression of the sphenoid bone inferior to the hypothalamus of the brain
➢ Lies posterior to the optic chiasm

Infundibulum

connects the pituitary gland and hypothalamus

two parts of the pituitary gland

o Anterior pituitary
o Posterior pituitary

Anterior pituitary

▪ Made up of epithelial cells derived from the embryonic oral cavity
▪ Growth hormone, thyroid-stimulating hormone, adrenocorticotropic hormone, melanocyte-stimulating hormone, luteinizing hormone or interstitial cell-stimulating hormone, follicle-stimulating hormone, prolactin

Posterior pituitary

▪ Extension of the brain
▪ Composed of neurons
▪ Antidiuretic hormone, oxytocin

Hypothalamus

➢ Serves as a control center for the autonomic nervous system and endocrine system
➢ Region of the diencephalon located inferior to the thalamus
➢ Controls the pituitary gland in two ways:
o Hormonal control
o Direct innervation

Anterior pituitary

synthesizes hormones whose secretion is under the control of the hypothalamus
➢ Secretions from the anterior pituitary are controlled by hormones that pass through the hypothalamic-pituitary portal system from the hypothalamus

Posterior pituitary

storage location for two hormones synthesized by special neurons in the hypothalamus
➢ Hormones secreted from the posterior pituitary are controlled by action potentials carried by axons that pass from the hypothalamus to the posterior pituitary

Hormones of the anterior pituitary

growth hormone, thyroid-stimulating hormone, adrenocorticotropic hormone, gonadotropins (LH and FSH), prolactin, melanocyte-stimulating hormone

Growth Hormone

➢ Stimulates the growth of bones, muscles, and other organs by increasing gene expression
➢ Slows protein breakdown during period of food deprivation and favors lipid breakdown

Pituitary dwarf

deficiency of growth hormone

Giantism

excess growth hormone resulting in an abnormally tall person

Insulin-like growth factors (somatomedins)

influences the effect of growth hormone

Thyroid-Stimulating Hormone

➢ Binds to membrane-bound receptors on cells of the thyroid gland
➢ Stimulates the secretion of thyroid hormone and growth of the thyroid gland

Adrenocorticotropic Hormone

➢ Binds to membrane-bound receptors on cells of the adrenal cortex
➢ Increases the secretion of cortisol or hydrocortisone

Gonadotropins

➢ Bind to membrane-bound receptors on the cells of the gonads
➢ Regulate growth, development, and functions of the gonads

Two major gonadotropins:

o Luteinizing hormone
o Follicle-stimulating hormone

Luteinizing hormone

▪ Females: stimulates ovulation; promotes secretion of reproductive hormones, estrogen and progesterone
▪ Males: stimulates interstitial cells of the testes to secrete testosterone

Follicle-stimulating hormone

▪ Stimulates the development of follicles in the ovaries and sperm cells

Prolactin

➢ Binds to membrane-bound receptors in the cells of the breast
➢ Helps promote development of the breast during pregnancy
➢ Stimulates the production of breastmilk

Melanocyte-Stimulating Hormone

➢ Binds to membrane-bound receptors on melanocytes
➢ Synthesizes melanin

hormones of the posterior pituitary

antidiuretic hormone, oxytocin

Antidiuretic Hormone

➢ Binds membrane-bound receptors
➢ Increases water reabsorption by kidney tubules
➢ Causes blood vessels to constrict when released in large amounts
➢ Also called vasopressin

Diabetes insipidus

lack of ADH; production of a large amount of dilute urine

Oxytocin

➢ Binds to membrane-bound receptors
➢ Causes contraction of the smooth muscle cells of the uterus as milk letdown from the breasts in lactating women

Thyroid Gland

➢ Synthesizes and secretes three hormones:
o Triiodothyronine (T3)
o Thyroxine (tetraiodothyronine; T4)
o Calcitonin
➢ Made up of two lobes connected by a narrow band called the isthmus
o Located on each side of the trachea inferior to the larynx

Thyroid follicles

small spheres with walls composed of simple cuboidal epithelium
o Synthesize T3 and T4
o Each follicle is filled with the protein thyroglobulin

thyroglobulin

Thyroid hormones are attached to this

Thyroid hormones

attach to nuclear receptors in their target cells; regulate the metabolic rate of the body

Goiter

abnormally enlarged thyroid gland; can result from underproduction or overproduction of hormone or from a deficiency of iodine in the diet

Hypothyroidism

an underactive thyroid gland; a glandular disorder resulting from insufficient production of thyroid hormones

Cretinism

severe hypothyroidism resulting in physical and mental stunting

Hyperthyroidism

an overactive thyroid gland; pathologically excessive production of thyroid hormones or the condition resulting from excessive production of thyroid hormones

Graves disease

autoimmune disease that causes hyperthyroidism

Exophthalmia

bulging of the eyes

Parafollicular cells or C cells

secrete calcitonin

Calcitonin

thyroid hormone that tends to lower the level of calcium in the blood plasma and inhibit resorption of bone

Parathyroid Glands

➢ Four parathyroid glands embedded in the posterior wall of the thyroid gland
➢ Secretes parathyroid hormone
o Regulation of blood calcium levels
o Increases vitamin D formation

Hyperparathyroidism

excessive secretion of parathyroid hormone resulting in abnormally high levels of calcium in the blood

Hypoparathyroidism

inadequate secretion of parathyroid hormone resulting in abnormally low levels of calcium in the blood

Adrenal Glands

➢ Two small glands located superior to each kidney

Inner part of the adrenal gland

adrenal medulla

outer part of the adrenal gland

adrenal cortex

Adrenal Medulla

➢ Inner part
➢ Secretes two major hormones:
o Epinephrine – 80%
o Norepinephrine – 20%

Epinephrine and norepinephrine

fight-or-flight hormones; prepare the body for intense physical activity

Major effects of epinephrine and norepinephrine

o Release of stored energy sources to support increased physical activity
o Increased blood pressure
▪ Increased heart rate
▪ Constriction of blood vessels in skin and internal organs
o Increased blood flow to skeletal muscles
o Increased metabolic rate of several tissues

Adrenal Cortex

➢ Outer part
➢ Secretes three classes of steroid hormones:
o Mineralocorticoids
o Glucocorticoids
o Adrenal androgens

Mineralocorticoids

o Regulate ion balance in the glomerulosa of the adrenal cortex

Aldosterone

produced in greatest amounts
▪ Secreted under low BP conditions
▪ Help regulate blood sodium ion and potassium ion levels and water volume