Endocrine System

What is the endocrine system

secretion of hormones in the bloodstream around the body

What are hormones?

Chemical messengers produced by endocrine glands that travel through the bloodstream

Endocrine system → slower, indirect, and…

longer lasting compared to the nervous system, the effect is longer and range is widespread

Endocrine signaling

hormones → directly into blood

Exocrin signaling

secrete substances through ducts

paracrine signaling

cell signaling → target is nearby

autocrine signaling

cell signaling → bind to receptors on the same cell

Hormone types

• peptide hormones

• steroid hormones

• tyrosine-derivative hormones

peptide hormones

any chain of linked amino acids

steroid hormones

derived from lipid cholesterol

tyrosine-derivative hormones

derived from amino acid Tyrosine

peptide hormones are synthesized by

the endoplasmic reticiulum

how to peptide hormones communicate?

don’t cross cell membrane, only bind to receptors on cell surface

Peptide hormones are water-soluble which means…

they can move freely through blood, but cannot diffuse through lipid bilayer cell membrane

Once a peptide hormone binds,

a cascade effect happens throughout the cell

Secondary messengers

allow signals from outside of cell to be transferred throughout the inside of the cell

4 types of secondary messengers

• cAMP

• IP₃

• DAG

• Calcium

Secondary Messengers - How they work: Step 1

A ligand (signaling molecule) binds to a receptor

Secondary Messengers - How they work: Step 2

The receptor is activated, turning on its associated G-protein

Secondary Messengers - How they work: Step 3

The G-protein activates secondary messenger molecules

Secondary Messengers - How they work: Step 4

Secondary messengers activate signaling cascade

Secondary Messengers - How they work: Step 5

Signaling cascade triggers cellular response → transcriptional factors, gene activation, etc.

cAMP Pathway: Step 1

Ligand binds to the G Protein-coupled receptor → activates G protein

cAMP Pathway: Step 2

G Protein activates Adenylyl Cyclase

cAMP Pathway: Step 3

Adenylyl cyclase converts ATP into cAMP

cAMP Pathway: Step 4

cAMP activates Protein Kinase A

cAMP Pathway: Step 5

Protein Kinase A triggers a cellular response

IP₃ Pathway: Step 1

Ligand binds to the G Protein-coupled receptor → activates G Protein

IP₃ Pathway: Step 2

G Protein activates Phospholipase C

IP₃ Pathway: Step 3

Phospholipase C cuts PIP₃ into two separate molecules: DAG and IP₃

IP₃ Pathway: Step 4

IP₃ diffuses through the cytoplasm and binds to receptors on the endoplasmic reticulum

IP₃ Pathway: Step 5

Calcium is released out of the endoplasmic reticulum and into the cytoplasm

IP₃ Pathway: Step 6

Calcium binds to various proteins, activating them

IP₃ Pathway: Step 7

Activated proteins trigger a cellular response

What happens to DAG after it is cut? (1)

It will activate Protein Kinase C

What happens to DAG after it is cut? (2)

Protein Kinase C triggers a cellular response

Growth Factors

Protein molecules synthesized in rough endoplasmic reticulum

• acts on cells by binding to surface receptors

Steroid Hormones

synthesized from cholesterol in smooth endoplasmic reticulum

steroid hormones pass through…

cell membranes and bind to intracellular receptors

Direct Stimulation

• enter the cell and bind to receptors in cytoplasm or nucleus

• Hormone-receptor complex is formed

• binds to activate DNA → alter gene expression

Adrenal Cortex

produces cortisol and aldosterone

Female Gonads

produce estrogen and progesterone

Male Gonads

produce testosterone

Amino acid (Tyrosine) derivative hormones

derived from single amino acids (usually tyrosine)

• 2 broad categories

• Thyroid hormones

• Catecholamines

Thyroid hormones

hydrophobic and bind intracellularly

Catecholamines

hydrophilic and bind extracellularly, use secondary messenger

Thyroid Hormones

T₃ and T₄

• hydrophobic

• increase transcription

• bind to receptos in nucleus

Catecholamines

Epinephrine and Norepinephrine (can also act as neurotransmitters)

• hydrophilic

• bind to cell surface receptors to activate secondary messenger pathways

Positive Feedback loop

the product of a process causes an increase in that process

• rare in biology

Negative feedback loop

the product of a process causes a decrease in that process

• prime way body maintains homeostasis

Positive Feedback Loop example

breastfeeding

• milk being released, oxytocin gets released → as oxytocin gets released more milk gets released

Negative feedback loop example

Glucose levels

• rising blood glucose level → pancreas detects glucose level → pancreas secretes insulin causing liver to take up the glucose for storage (glycogen) → as body cells intake glucose, blood levels decline and insulin release stops

Hypothalamus

• maintains homeostasis

• monitors internal and external environment

• produces peptide hormones

Hypothalamus is the link between the….

endocrine system and the nervous system

Pituitary stalk (infundibulum) connects…

hypothalamus to pituitary gland

• neuronal connection → posterior pituitary gland

• vascular connection → anterior pituitary gland

Hypothalamus uses

negative feedback mechanisms

• secretion of released and inhibiting hormones

Hypothalamus - Hormone synthesis

• ADH (Vasopressin)

• Oxytocin

Hypothalamus - Releasing Hormones

• GnRH (Gonadotropin-Releasing Hormone)

• CRH (Corticotropin-Releasing Hormone)

• TRH (Thyrotropin-Releasing Hormone)

• GHRH (Growth Hormone-Releasing Hormone)

Hypothalamus - Inhibiting Hormones

• Somatostatin (Growth Hormone - Inhibiting Hormone)

• Dopamine (Prolactin-Inhibiting Hormone)

Hypothalamus - Anterior Pituitary Connection: 1

Releasing/inhibiting hormones synthesized in neuronal cell bodies of hypothalamus

Hypothalamus - Anterior Pituitary Connection: 2

Hormones released into capillary network in hypothalamus

Hypothalamus - Anterior Pituitary Connection: 3

Hyphophyseal portal veins transports hormone to the anterior pituitary

Hypothalamus - Anterior Pituitary Connection: 4

Anterior pituitary is activated or inhibited to. release its own hormones

Hypothalamus - Posterior Pituitary Connection: 1

Hormones synthesized in neuronal cell bodies of hypothalaumus (Oxytocin and ADH)

Hypothalamus - Posterior Pituitary Connection: 2

transported down axons

Hypothalamus - Posterior Pituitary Connection: 3

posterior pituitary for storage

Hypothalamus - Posterior Pituitary Connection: 4

released into bloodstream whenever a signal reaches the posterior pituitary

(neuronal connection)

Anterior Pituitary

• Regulates hormone production by other glands

• direct and indirect acting hormones

Tropic Hormones

anterior pituitary

• act indirectly - regulate other endocrine organs (usually release their own hormones)

Nontropic Hormones

Anterior pituitary

• act directly - directly affect the non-endocrin tissues

Nontropic Hormone types

• Melanocyte-Stimulating Hormone

• Prolactin

• Growth Hormone (GH)

MSH - Melanocyte-Stimulating Hormone

stimulates melanocytes to produce + release melanin

Prolactin

• stimulates milk production in females from mammary gland cells

• can be inhibited by hypothalamus

Growth Hormone (GH)

• stimulates bone + muscle growth

• increases cell size, mitosis, rate of protein synthesis, and various metabolic functions

types of tropic hormones

• Thyroid-Stimulating Hormone (TSH)

• Adrenocorticotropic Hormone (ACTH)

• Follicle-Stimulating Hormone (FSH)

• Luteinizing Hormone (LH)

Thyroid-Stimulating Hormone (TSH)

stimulates thyroid gland → increases size, cell number and rate of secreation of T₃ and T₄

Adrenocorticotropic Hormone (ACTH)

stimulates adrenal cortex → releases glucocorticoids (cortisol)

Follicle-Stimulating Hormone (FSH)

• Males: stimulates teses → spermatogenesis

• Females: stimulates ovarian follicles → release estrogen

Luteinizing Hormone (LH)

• Males: stimulates testes → release testosterone

• Females: triggers ovulation, corpus luteum formation, progesterone release

Posterior Pituitary

• does not synthesize its own hormones

• stores ADH + oxytocin from the hypothalamus for release

ADH (vasopressin)

• regulates water levels in the body

• increases permeability of the collecting duct and distal convoluted tubule of the nephron

• increasing reabsorption of water → increase blood volume and pressure

Oxytocin

• Secreted ruing childbirth → stimulates uterine contractions

• stimulates milk ejection from mammary glands

• induces maternal behavior (“love hormone”)

Pineal Gland

• Secretes melatonin

• hypothalamus, pituitary glad, and pineal gland → in the brain

Melatonin

helps regulate circadian rhythm

• amino-acid derived hormone

Thyroid

• located in front of the trachea

• secretes T₃, T₄, and calcitonin

Triiodothyronine (T₃) and Thyroxine (T_4,)

• Tyrosine-derivative hormones - lipid-soluble (hydrophobic)

• Increase body metabolism

• Increase heart rate

Calcitonin

Peptide hormone → tones down calcium in blood

• inhibits osteoclast activity

• increase of osteoblast activity → builds up bond using calcium

What element do you need to synthesize T₃ or T₄?

You need iodine within the process

Thyroid - Disorders

Goiters: enlargement of thyroid gland

• hyperthyroidism

• hypothyroidism

Hyperthyroidism

over-secretion of T₃ and T₄ → excessively high heart rate and metabolic rate

Hypothyroidism

under-secretion of T₃ and T₄ → very low metabolic rate

Parathyroid

• four pea-shaped structures

• attached to the back of the thyroid

Parathyroid Hormone (PTH)

• Peptide hormone with opposite action to calcitonin

• raises calcium levels in blood by stimulating release from bone

• increases osteoclast activity for bon resorption

• increases kidney reabsorption of calcium

• converts vitamin D to its active form → increases calcium absorption in the gut

• lowers blood phosphate levels by increasing phosphate excretion

thymus

• lymphoid organ

• located in chest → behind the sternum

Thymus Hormones

thymosin

Thymosin

peptide hormone that stimulates naive T-lymphocytes to mature and differentiate into functional T-cells

Adrenal Glands

• locate on top of kidneys

• divided into adrenal cortex and adrenal medula

Adrenal cortex

• outer portion

• produces steroid hormones

Adrenal Medulla

produces tyrosine-derivative hormones

Adrenal Cortex hormone categories

• Glucocorticoids

• Mineralocorticoids

• Cortical Androgens