Chapter 2: The Endocrine System Flashcards
Endocrine system
Consists of endocrine glands which have cells that release chemical messages (i.e. hormones) into the blood stream
Endocrine
Autocrine
Paracrine
Not mutually exclusive.
Autocrine
Pertaining to a signal secreted by a cell into the environment that affects the transmitting cell.
Endocrine gland
A ductless gland from which hormones are released into the blood system in response to specific physiological signals.
Paracrine
Secretion of locally acting biological substances from cells.
General features of the endocrine system
- Endocrine glands are ductless and secrete hormones directly into the blood.
(Exocrine glands DON’T have ducts/tubes).
Some endocrine glands are not entirely glandular in nature.
- Secreted into the blood stream
- Rich blood supply
- Can travel in the blood to virtually every cell in the body
- Hormone receptors (lock) are specific binding sites that interact with a particular hormones of class of hormones (key)
PINEAL GLAND
Located within the brain, between the telencephalon and diencephalon
Pinealocytes
Melatonin:
sleep, biological rhythms
Over the course of vertebrate evolution, the primary function of pineal cells has shifted from photoreception to neurosecretion.
HYPOTHALAMUS
Part of the brain located below the thalamus
Groups neuronal cell bodies (nuclei)
A variety of vital endocrine, behavioral, and autonomic functions can be localized to particular nuclei of the hypothalamus
In the median eminence, specialized neurons called neurosecretory cells secrete neurohormone (hormones released from a neuron) into the blood vessels of the pituitary
HYPOTHALAMUS Inputs
Sensory inputs:
Smell, taste, visual, somatosensory
Blood borne signals:
Sugar, mineral levels, immune products, hormones
Neural inputs:
Hippocampus, amygdala, cortex, brainstem, spinal cord
Hypothalamus:
Detects changes in internal & external environment and responds by changing its output of hormones
Hypothalamic Releasing Hormones
Gonadotropin Releasing Hormone (GnRH)
Corticotropin Releasing Hormone (CRH)
Thyrotropin Releasing Hormone (TRH)
Somatocrinin or Growth Hormone Releasing Hormone (GHRH)
Hypothalamic Inhibitory Hormones
Somatostatin or GH Inhibiting Hormone (GHIH)
Dopamine (DA) inhibits prolactin secretion from the anterior pituitary
Prolactin
A protein hormone that is highly conserved throughout vertebrate evolution and has many physiological functions, which can be broken down into 5 basic classes:
(1) reproduction
(2) growth & development
(3) water and electrolyte balance
(4) maintenance of integumentary structures
(5) actions on steroid-dependent target tissues or synergisms with steroid hormones to affect target tissues.
Prolactin inhibitory hormone (PIH)
Dopamine inhibits prolactin secretion from the anterior pituitary.
PITUITARY GLAND
Also called the hypophysis
Pituitary connected to base of hypothalamus by infundibulum (contains axons and blood vessels)
“master gland”
but it is highly regulated..
2 distinct components:
Anterior pituitary (pars anterior or adenohypophysis)
Posterior pituitary (pars nervosa or neurohypophysis)
Have different embryological origins
Hypothalamus communicates with anterior and posterior pituitary differently
Posterior pituitary
pars nervosa or neurohypophysis
this section is derived from a downgrowth of neural tissue from the hypothalamus.
The posterior pituitary releases oxytocin (stimulates contraction of uterine muscles; stimulates release of milk by mammary glands) and vasopressin (stimulates increased water reabsorption by kidneys; stimulates constriction of blood vessels).
Hormones are secreted directly from neurons into the blood
Nonapeptides:
The hormones of the posterior pituitary are of peptide class, specifically nonapeptides (they contain 9 amino acids)
Anterior pituitary
Adenohypophysis or pars anterior
this part has an embryological origin as an epithelial upgrowth from the foetal oral cavity called Rathke’s Pouch.
The anterior lobe of the pituitary secretes many tropic hormones—including TSH, FSH, LH, ACTH, growth hormone, and prolactin—that travel through the bloodstream and regulate endocrine glands throughout the body.
Receives little if any neural input
Neurohormones from the hypothalamus reach the anterior pituitary via the portal blood system
Causes release of tropic hormones
Oxytocin
Posterior Pituitary Hormone
Social, reproductive, and parental behaviors
Uterine contractions during childbirth
Brand Name: Pitocin
Milk let-down reflex
Nonapeptides
Tropic hormones
Hormones from the anterior pituitary that stimulate various physiological processes,either by acting directly on target tissues or by causing other endocrine glands to release hormones.
Vasopressin
Also known as Antidiuretic Hormone (ADH)
Main target: Kidney, arterioles
Water balance (retention); Alcohol, urination, and dehydration-related hangover
Blood pressure:
During hemorrhage, ADH acts as a vasoconstrictor to increase blood pressure
Nonapeptide
gonadotropins
Luteinizing Hormone (LH) & Follicle Stimulating Hormone (FSH) are called gonadotropins because in response to GnRH, LH & FSH stimulate hormone production in the gonads, as well as the development and maturation of gametes (sperm or eggs)
Thyroid Stimulating Hormone (TSH)
stimulates thyroid
Prolactin (Prl)
In response to TRH, PRL promotes lactation
Alcohol is an inhibitor of Vasopressin
….
Thyroid gland
H-shaped structure that partially surrounds upper trachea
Consists of many sphere-shaped follicles that produce, store, and secrete thyroid hormones
Unique because it can store a large amount of hormone
Thyroid Hormones
2 thyroid hormones released in response to TSH from the anterior pituitary:
Triiodothyronine (T3)
Thyroxine (T4)
Derived from the amino acid tyrosine and require iodine for their production
3 general effects:
1) Alter growth
2) Influence reproduction
3) Affect metabolism
Disorders of the thyroid:
Goiter (Iodine)
Hyperthyroidism
Graves Disease (autoimmune disorder)
Hypothyroidism (common in women who have just given birth)
Hashimoto’s Thyroidosis (autoimmune disorder)
Cretinism
Permissive effects: Always the bridesmaid never the bride
Needs to be there or serious issues occur, but as long as its barely there, its ok
Almost every cell in the body is effected by the thyroid’s hormones
Pancreas
Both an endocrine and exocrine gland
Exocrine cells produce and secrete digestive juices into the intestines which breakdown food
Endocrine tissues are embedded in exocrine tissues called Islets of Langerhans – secrete hormones that control blood glucose levels
Pancreatic Hormones
Insulin (β cells): lowers blood sugar by promoting the liver to store glycogen
Glucagon (α cells): increases blood sugar by stimulating the breakdown of glycogen in the liver
Somatostatin (δ cells):regulate and fine-tune the insulin and glucagon-producing cells
Diabetes
Type 1: body attacks insulin production cells and kills them
Type 2: the body makes insulin, but the cells are rendered insensitive to insulin, and don’t respond
Gastrointestinal Tract
Different than other endocrine glands because:
The endocrine tissues of the gastrointestinal tract are scattered through the gut and are NOT located in a single glandular organ
The gastrointestinal hormones regulate the cells and organs in which they are produced
3 major hormones that control various aspects of digestion, stimulating exocrine glands of the gut and pancreas
Gastrin
Secretin
Cholecystokinin (CCK)
Many of these hormones have also been discovered in the brain and affect behaviors, including feeding
Ghrelin
Stomach hormone
Stimulates: Food intake “Hunger Hormone” GH secretion from the pituitary Adiposity
Causes fat cells to grow.
Also regulates emotion and anxiety, so you can’t just block it.
And if you did, other hunger hormones would take over.
Leptin
Secreted from adipose (fat) cells
Acts on receptors in the CNS and at other sites to:
Induce energy expenditure
Inhibit food intake (“satiety hormone”)
Animal without leptin became morbidly obese.
Some people have bodies and brains have receptors that don’t respond to leptin, and become obese.
Adrenal Glands
Bilateral glands above kidneys
Like the pituitary, adrenals are 2 glands with distinct embryological origins: Adrenal medulla (inner) Adrenal cortex (outer)
The adrenal glands develop from 2 separate embryological tissues: the neural crest ectoderm and the intermediate mesoderm.
The medulla originates from neural crest cells migrating from sympathetic ganglion. Mesodermal cells then surround the medulla.
The adrenal cortex develops from the intermediate mesoderm. The fetal cortex develops in the centre, with the permanent cortex surrounding it.
Both parts of the adrenals work together to regulate metabolism and cope with stress
Hormones of the Adrenals
Epinephrine & Norepinephrine:
Released in response to stress and activation of the SNS
Affect the circulatory and metabolic systems to prepare the body for action and cope with emergencies (“flight or flight” response)
Glucocorticoids
e.g. Cortisol: humans; corticosterone; rodents and other animals.
Released in response to stress.
Affects carbohydrate metabolism resulting in increased blood levels of glucose.
Mineralocorticoids
e.g. Aldosterone
Primarily responsible for causing the kidney to retain sodium and reduce the urine production.
Sex hormones
e.g. Testosterone
Congenital Adrenal Hyperplasia
Gonads
Male: testes
Female: ovaries
Forms from the same tissue
Two functions compartmentalized to different cell types:
- -Production of steroid hormones
- -Production and maturation of gametes (sperm & eggs)
Males: constant in function
Females: cyclic in function
Function of gonads regulated by gonadotropins (LH & FSH) from the anterior pituitary
The hormones of the gonads are important for:
- -Gamete maturation
- -Development for secondary sex characteristics
- -Reproductive behaviors
Testes
Male gonads
Sertoli cells: produce sperm; a majority of the testes cells are stroll cells
Seminiferous tubules: sperm maturation
Leydig cells: hormone-producing cells
Primary Testicular Hormones
Androgens including testosterone, androstenedione, and dihyrdotestosterone
Sperm production
Maintenance of genial tract and accessory sex organs
Support secondary sex characteristics; like a deep voice, musculature, etc.
Influence metabolism
Sexual, aggressive, and other social behaviors
Ovaries
Female gonads
3 subunits:
Follicles: contain developing egg.
Corpora lutea: structures that develop from follicles after egg is released.
Stroma: supporting tissue.
Prior to ovulation: FSH (tropic hormone) stimulates follicle development, and mature follicles secrete estrogens
After ovulation: LH (tropic hormone) follicles to enlarge and differentiate into luteal cells and combine with theca cells to form the corpus luteum which secretes progestins
Primary Ovarian Hormones
Estrogens (i.e. estradiol): Affect genital tract Induce secondary sex characteristics Water metabolism & retention (bloating) Calcium metabolism Sexual and maternal behavior
Progestins:
Maintaining pregnancy
Sexual behavior
A Cautionary Note
Sex hormones, they should not be considered strictly male or female.
The two sexes differ in the concentrations of circulating androgens and estrogens.
The brain can convert androgens into estrogens
The ovaries make a small but an important amount of androgens
The testes make a small but an important amount of estrogens
Classes of Hormones
Proteins and Peptides
Steroids
Monoamines (class of neurotransmitter and hormones)
Lipid-based (prostaglandins)
Steroid Hormone Synthesis
Main sources: adrenal, gonads and brain
Structure: three 6-carbon rings plus one conjugated 5-carbon ring.
This chemical structure is identical among all vertebrate species
Different steroid hormones vary in the number and types of atoms attached to these rings
All steroid hormones are derived from cholesterol that is made from acetate by the liver
Prohormone
a molecule that can act as a hormone itself or can be converted into another hormone
Sometimes one hormone is a precursor for another hormone, and sometimes they can convert back and forth.
Aromatization conversion of T to estrogens through by the enzyme aromatase
Testosterone is a prohormone for estradial
Steroid Hormones
Fat soluble (i.e. can easily move through cell membranes, don’t need carries or channels)
NOT soluble in water thus require carrier proteins for transport through the blood to their target tissue
Receptors located inside the cells (cytosol or nucleus)
Hormone receptors form dimers, and translocate to the nucleus.
Hormone-receptor complexes bind to hormone response elements on DNA, regulate transcription
Have slow but lasting effects
Parking spot on the DNA: HRE’s (hormone response elements)
Non-genomic effects of steroid hormones
Steroids can also have a brief, rapid effect on some neurons without affecting gene expression
Alterations in neuronal excitability
Involves receptors found in the membrane
Receptor Isoforms
A version of a hormone receptor with slight differences in structure that give it different functional properties
Estradiol binds equally well to both receptors
Estrone binds preferentially to the alpha receptor
Estriol binds preferentially to the beta receptor
Protein & Peptide Hormones
Made up of individual amino acids
Peptide (small)
Protein/polypeptide (large)
Includes: Neurohormones of the hypothalamus Tropic hormones of the anterior pituitary Posterior pituitary hormones Parathyroid hormone Calcitonin Gastrointestinal hormones Ghrelin Leptin Pancreatic hormones
