Endocrine [A&P II]

The endocrine system

Regulates long term processes like growth, development & reproduction using hormones as messengers

Target cells

Specific cells with receptors needed to bind and “read" hormonal messages

Hormones

Chemical messengers in blood that attach to receptors in target cells and produce a response

Three classes of hormones

Amino acid derivates, peptide hormones, lipid derivatives

Properties of amino acid hormones

Building blocks of proteins

Water soluble, polar

Cannot pass through cell membrane in most cases (thyroid hormone is the exception)

Two types of amino acid hormones & examples of each

Tyrosine: thyroid hormone, epinephrine, norepinephrine, dopamine

Tryptophan: serotonin, melatonin

Peptide hormone properties

Chains of amino acids

Synthesized as prohormones (precursors), stored inside secretory vesicles until trigger for release

Water soluble, cannot pass through cell membrane

3 types of peptide hormones & examples

Short chain polypeptides: antidiuretic/vasopressin (ADH), oxytocin (OXT)

Small proteins: growth hormone (GH), prolactin (PRL)

Glycoproteins: thyroid stim (TSH), luteinizing (LH), follicle stim (FSH)

**classified based on amount of amino acid chains: short chain < small proteins < glycoproteins

Lipid hormone properties

Hydrophobic, can cross phospholipid cell membrane & bind w/ receptor in cytoplasm or nucleus

2 types of lipid hormones

Steroid hormones (cholesterol derived)

Eicosanoids (arachidonic acid derived)

Steroid hormones properties & examples

Cholesterol derived (lipid)

Circulating steroid hormones in blood plasma are bound to transport proteins, meaning they remain in circulation longer

Crosses cell membrane → direct gene activation

Ex: testosterone, estrogen, progestins, corticosteroids, calcitriol

Eicosanoids properties & examples

Arachidonic acid derived (lipid)

Paracrine factors that affect extracellular fluids

Remains in circulation very briefly

ex: leukotrienes, prostaglandins

In a second messenger system, what is the first messenger?

A hormone that cannot cross the cell membrane & binds to a receptor on the surface

Amplification

Magnification of hormone effect in target cell

Small number of hormone molecules → thousands of second messengers

Down-regulation

Hormone presence → hormone receptors decrease → cell becomes less sensitive to it

Up-regulation

Absence of a hormone → increase in hormone receptors → cell becomes more sensitive to it

3 types of hormone-hormone interaction

Synergism: combined effect of multiple hormones with the same effect magnifies this effect (ex. epinephrine & norepinephrine)

Antagonism: two hormones have opposite effects (ex. insulin & glucagon)

Permissiveness: 1 hormone is necessary for another hormone to have an effect (ex. cortisol & glucagon)

3 types of endocrine stimuli

Humoral stimuli: changes in composition of extracellular fluid (ex. blood glucose)

Hormonal stimuli: arrival or removal of specific hormone (ex. TRH → TSH → T3/T4)

Neural stimuli: arrival of neurotransmitters at neuroglandular junctions (ex. hypothalamus signals → posterior pituitary gland)

In the cAMP & Ca+ second messenger process, what happens directly after the hormone binds to the receptor?

GDP is released → opens up receptor so GTP (a charged molecule) can attach

In the cAMP & Ca+ second messenger process, what happens directly after GTP attaches to the receptor?

The G protein is activated and separates from the receptor

In the cAMP second messenger process, what happens to the G protein directly after detaching from the receptor?

It attaches to and activates the enzyme adenylate cyclase

In the cAMP second messenger process, what occurs directly after adenylate cyclase is activated?

It breaks down ATP → cAMP

In the cAMP second messenger process, what occurs once cAMP levels rise?

Protein kinases are activated, triggering the response of the target cell

In the Ca+ (calcium) second messenger system, what occurs directly after the G protein is activated?

The G protein activates the enzyme phospholipase C (PLC)

In the Ca+ (calcium) second messenger system, what two things does phospholipase C (PLC) trigger?

DAG & IP3 (from breaking down cell membrane phospholipids)

In the Ca+ (calcium) second messenger system, what does DAG do?

Stays in cell membrane (not a real 2nd messenger) → activates enzymes like protein kinase → various metabolic effects

In the Ca+ (calcium) second messenger system, what two things does IP3 do?

1. opens calcium channels → Ca+ flows in

2. → ER/SR (endoplasmic/sarcoplasmic reticulum) → release of Ca+ from ER/SR

Both lead to various metabolic effects

Which hormones result in direct gene activation?

Steroids (lipid soluble) and thyroid hormones (not lipid soluble but small enough to pass through cell membrane)

Pituitary gland (hypophysis) location

Sella turcica, a depression in the sphenoid bone of the skull; inferior to hypothalamus

Two lobes of the pituitary gland and how they communicate with the hypothalamus

Anterior: hormonal stimuli
Posterior: neural stimuli

Which lobe of the pituitary gland does NOT produce its own hormones?

Posterior pituitary

How does the anterior pituitary lobe communicate with the hypothalamus?

Hypothalamal-hypopheseal portal system (blood vessels)

Which hormones does the anterior pituitary gland produce & their targets? (Mnemonic: FLAT PeG)

FSH: follicle stim → testes/ovaries
LH: luteinizing → testes/ovaries

ACTH: adrenocorticotropic → adrenal medulla

TSH: thyroid stim → thyroid

PRL: prolactin → mammary glands

Endorphins

GH: growth → liver

Which 2 hormones are released by the posterior pituitary gland (not produced there)

ADH: antidiuretic

OXT: oxytocin

Thyroid location and gross structure

Anterior to thyroid cartilage of larynx, consists of 2 lobes connected by isthmus

Thyroid follicles structure

Hollow spheres, follicle cavity containing viscous colloid, walls consist of simple cuboidal epithelium

Follicular cells (thyroid) produce the hormones:

T3/T4, differentiated by 3 vs 4 iodine

C cells/parafollicular cells (thyroid) produce the hormone…

Calcitonin, decreases blood calcium

Parathyroid glands produce the hormone:

PTH (parathyroid hormone), increases blood calcium

Layers of the adrenal cortex (superficial → deep)

Zona glomerulosa

Zona fasciculata

Zona reticularis

The zona glomerulosa of the adrenal cortex produces:

Mineralocorticoids ex. aldosterone

→ kidneys, regulating minerals/salts

The zona fasciculata of the adrenal cortex produces:

Glucocorticoids ex. cortisol

→ breaks down non-carbs (proteins/lipids) via gluconeogenesis (as opposed to glucogenesis, which breaks down carbs)

The zona reticularis of the adrenal cortex produces:

Gonadocorticoids ex. testosterone, estrogen

(not as much as the actual gonads)

The adrenal medulla is stimulated by ____ and releases ____ (ex: ___, ___)

Sympathetic nervous system

Catecholamines

Epinephrine & norepinephrine

Pineal gland location

Epithalamus (above thalamus)

The pineal gland contains _____ (cells) which produce _____ (hormone)

Pinealocytes

Melatonin

Areas of pancreas responsible for endocrine functions

Pancreatic islets/islets of langherhans

2 main types of pancreatic cells and the hormones they produce

Alpha cells: glucagon (raises blood sugar)
Beta cells: insulin (lowers blood sugar)