Endocrine System: Hormones and Glands

Comparison of Nervous and Endocrine Systems

• The nervous system initiates responses rapidly, whereas the endocrine system does so slowly.
• The nervous system produces short-duration responses, but the endocrine system is responsible for long-duration ones.
• The nervous system acts via action potentials and neurotransmitters, while the endocrine system uses hormones released into the blood.
• The nervous system acts at specific locations determined by axon pathways, while the endocrine system acts at diffuse locations that can be anywhere blood reaches.
• Neurotransmitters act over very short distances, but hormones act over long distances.

Endocrine Organs

• The basic location of endocrine organs is scattered throughout the body, not grouped in specific areas.
• They secrete hormones (chemical messengers) that travel in blood or lymph.
• Lymph is a filtrate of blood that carries immune cells and chemicals..
• A hormone's chemical structure determines water solubility, method of transport, lifespan ("half-life"), and the receptors it acts upon.

Hormone Classification

• Amino acid-based hormones are water-soluble except for thyroxine.
• They cannot cross the plasma membrane so bind to a receptor in the plasma membrane.
• Steroids are lipid soluble and are synthesized from cholesterol.
• Steroid hormones cross the plasma membrane.
• Steroid hormones bind to a receptor inside the cell, directly activating genes to initiate a response.
• Amino acid based hormones trigger a second messenger molecule inside the cell, which mediates the response and takes longer than direct activation.

Hormone Activation Mechanisms

• Hormones only activate target cells that have appropriate receptors.
• Hormones can have different effects on different target cells.
• Hormones can produce changes by:
• Opening or closing ion channels, changing the membrane potential
• Stimulating the synthesis of enzymes or other proteins
• Activating or deactivating enzymes
• Inducing secretion of various substances
• Stimulating mitosis (cell division)

Stimuli Causing Hormone Release

• Synthesis and release of most hormones is regulated by negative feedback, which keeps blood hormone levels relatively constant.
• A stimulus triggers secretion, hormone levels rise and the desired effect on target organ, then inhibited hormone levels drop.
• Humoral stimuli are the simplest trigger (changing levels of ions or nutrients in blood).
• Examples include parathyroid hormone (blood Ca2+) and insulin (blood glucose).
• Neural stimuli involves the sympathetic nervous system.
• The adrenals stimulate the release of epinephrine or norepinephrine in response to stress.
• Hormonal stimuli is when glands secrete hormones in response to secretion of hormones by other glands.
• For example, the hypothalamus triggers the anterior pituitary gland release which triggers a target endocrine organ/gland.

Hormone Activity

• Binding to a specific receptor triggers a pre-programmed response inside the cell.
• The degree of target cell activation depends on:
• Blood levels of hormone (↑ hormone = ↑ activity)
• Number of receptors for hormone (↓ receptors = ↓ activity)
• Affinity strength of binding to receptors

Up-regulation

• Low hormone levels in blood cause target cells to make more receptors increases the number of available receptors.

Down-regulation

• High hormone levels in blood cause target cells to reduce the number of receptors limits the effects on target cell.
• It prevents overreacting to a sustained high hormone level, reducing the number of available receptors.
• Hormones circulate in the blood freely or bound to a protein carrier.
• Lipid-soluble hormones do not travel freely in the blood and must be attached to plasma proteins for transport.
• Hormones are broken down by enzymes in target cells, kidneys, or liver and then excreted.
• Half-life is the time required for blood concentration of hormone by half.
• Water-soluble hormones have a shorter half-life than lipid-soluble hormones.

Hormone Interaction with Target Cells

• Permissiveness is when one hormone needs another to exert full effects.
• Reproductive hormones must interact with thyroid hormone to ensure proper and timely development of reproductive system, without the 2nd hormone, effects are reduced
• Synergism is when two hormones work together on the target cell to produce amplified effects. The full effect is achieved when both hormones are present
• Glucagon and epinephrine together produce 150% more glucose than one acting alone, releasing stored glucose into blood.
• Antagonism is when one hormone opposes the action of another.
• Insulin lowers blood glucose, while glucagon raises it.

Pituitary Gland (Hypophysis)

• Location is the sella turcica of the sphenoid bone.
• Size and shape resembles a pea
• Connected to the hypothalamus by the infundibulum (stalk).
• Has two major lobes:
• Posterior: Composed of neural tissue and secretes hormones sent from the hypothalamus, having a neural connection called the hypothalamic-hypophyseal tract. It stores hormones, but does not manufacture them, and is made in hypothalamus
• Anterior: Composed of glandular tissue and manufactures and releases hormones and has a vascular connection to the hypothalamus but no neural connection.
• Allows for delivery of hormones from hypothalamus hypothalamus → ant. pituitary → target organ.

Posterior Pituitary Hormones

• Oxytocin functions to stimulate uterine contractions and initiates milk ejection.
• ADH (antidiuretic hormone) has the function of stimulating cells of the kidney tubule to reabsorb water from urine back into the blood stream
• The kidneys filter blood
• It initiates limits urination, conserving fluid.

Anterior Pituitary Hormones

• GH (growth hormone) is released in response to GHRH (growth hormone releasing hormone) secretion during sleep and affects all body cells, especially skeletal muscle and bone.
• TSH (thyroid stimulating hormone) is released in response to TRH (thyroid releasing hormone) secretion and affects the thyroid gland-stimulates release of thyroid hormones.
• ACTH (adrenocorticotropic hormone) is released in response to CRH (corticotropin releasing hormone) secretion and affects the adrenals: stimulating the release of glucocorticoids and androgens.
• FSH (follicle-stimulating hormone) is released in response to GnRH (gonadotropin releasing hormone) secretion.
• It has effects on the ovaries (ovarian follicle maturation and release of estrogen) as well as the testes stimulating sperm production.
• LH (luteinizing hormone) is released in response to GnRH (gonadotropin releasing hormone) secretion.
• In women it has effects on the ovaries (ovulation and release of estrogen and progesterone) and on the release of testosterone from Testes.
• PRL (prolactin) is released in response to decreased PIH (prolactin inhibiting hormone) secretion.
• This stimulates milk production in Breasts
• PIH inhibits PRL in males and non-lactating females.

Thyroid Gross Anatomy

• The largest endocrine gland in the body that's located in the anterior trachea, inferior to the larynx (voice box/vocal cords).
• Described as butterfly-shaped, two lobes connected by an isthmus.

Thyroid Histology

• It is composed of hollow, spherical follicles which are sac surrounded by cells.
• Follicular cells are cuboidal or squamous; form walls of follicles.
• Contains colloid (stored thyroid hormone secreted by follicular cells).
• Parafollicular cells are located between follicles and secrete calcitonin in response to high levels of blood Ca2+.
• Stimulus: ↑blood Ca2+
• Response: secretion of calcitonin humoral mechanism .

Thyroid Regulation

• A hormonal mechanism in which the hypothalamus releases TRH stimulating the anterior pituitary. Then, the #2 anterior pituitary releases TSH which stimulates the thyroid gland. Lastly #3 Thyroid gland releases thyroid hormone. If levels appropriate, then inhibits.
• TRH - thyrotropin-releasing hormone.
• TSH is a thyroid stimulating hormone.
• Thyroid hormone: - body's major metabolic hormone levels rise, hypothalamus & anterior pituitary inhibited is stimulated.

Thyroid Hormone

• The major metabolic hormone is lipid soluble and easily crosses plasma membrane
• Consists of two iodine-containing hormones known as T3 & Ty
• Follicles of the thyroid gland secrete mostly Ty which is converted to T3 at target organs/tissues.
• Its effects include that it increases basal metabolic rate and body heat; regulates tissue growth & development and maintains blood pressure
• The thyroid gland stores enough colloid to provide thyroid hormone for 2-3 months.

Parathyroid Gross Anatomy

• Has tiny and posterior to thyroid; usually four in number, but the location can vary slightly.
• Primary regulators of blood calcium levels (Ca2+ important for AP transmission, muscle contraction and blood clotting).
• PTH(parathyroid hormorie).

Parathyroid Histology

• Composed of primarily oxyphil cells (function unknown) and smaller parathyroid cells (which secrete PTH).
• It controls Ca2+ balance in blood (antagonist of calcitonin).
• Decreased blood (Ca2+) levels trigger PTH release while Increased blood (Ca2+) levels inhibit PTH release.

PTH Release Mechanism

• Has three target organs:
• Skeleton: stimulates osteoclasts to digest bone matrix, releasing calcium and phosphate into blood.
• Kidneys: enhances reabsorption of calcium from urine back into blood.
• Small Intestine: activates Vitamin D, which increases absorption of calcium from food.

Posterior Pituitary Hormones

• Hormone: Oxytocin

• Release Occurs: In response to stretching of uterine cervix or suckling of infant at breast

• Function: Stimulates uterine contractions; initiates milk ejection in the breasts.

• Hormone: ADH (Antidiuretic Hormone)

• Release Occurs: In response to increased blood solute or decreased blood volume; inhibited by appropriate hydration or alcohol intake.

• Function: Stimulates cells of the kidney tubule to reabsorb water from urine back into the blood stream.

Anterior Pituitary Hormones

• Hormone: GH (Growth Hormone)

• Release Occurs: In response to GHRH (growth hormone releasing hormone) secretion during sleep.

• Function: Affects all body cells, especially skeletal muscle and bone

• Hormone: TSH (Thyroid Stimulating Hormone)

• Release Occurs: In response to TRH (thyroid releasing hormone) secretion

• Function: Stimulates the thyroid gland to release thyroid hormones

• Hormone: ACTH (Adrenocorticotropic Hormone)

• Release Occurs: In response to CRH (corticotropin releasing hormone) secretion

• Function: Stimulate the Adrenals to release of glucocorticoids and androgens

• Hormone: FSH (Follicle Stimulating Hormone)

• Release Occurs: In response to GnRH (gonadotropin releasing hormone) secretion

• Function: Ovaries (ovarian follicle maturation and release of estrogen); testes (stimulates sperm production)

• Hormone: LH (Luteinizing Hormone)

• Release Occurs: In response to GnRH (gonadotropin releasing hormone) secretion

• Function: Ovaries (ovulation and release of estrogen and progesterone); testes (release of testosterone)

• Hormone: PRL (Prolactin)

• Release Occurs: In response to decreased PIH (prolactin inhibiting hormone) secretion.

• Function: Stimulates milk production in breasts

Adrenal Anatomy

• Paired organs on top of kidneys, encased in fibrous capsule and fat.
• Medulla- central portion of the sympathetic nervous system. fight or flight occurs here.
• Cortex- peripheral portion; makes up bulk of organ.
• The medulla and cortex produces different hormones mitigate stress

Adrenal Cortex

• produces corticosteroids (lipid soluble) longer lasting response than the hormones secreted by the adrenal medulla. Has three zones:
• Zona glomerulosa produces mineralocorticoids.
• Zona fasciculata - produces glucocorticoids.
• Zona reticularis produces gonadocorticoids.

Mineralocorticoids

• Regulate electrolyte concentrations in blood (particularly Nat & K+).

• Na+ concentration can influence blood volume and blood pressure, because (H2O attracted to Na+) where Na+ goes, H₂O follows

• K+ concentration influences membrane potentials to drive APS

• Aldosterone is the primary regulator of Nat and K+; targets kidney tubules to reabsorb Nat into blood (increases blood volume/pressure) and secrete K+ into urine →H2O follows prevents excess hyperpolarization

• Can alter acid-base balance of blood due to increased secretion of H+ after AP.

• Effects are brief (~20 min) allowing for precise control & continuous monitoring.

• Secretion is stimulated by decreased blood volume/blood pressure and increased blood kt levels.

Aldosterone Secretion Regulation

• Four factors influence aldosterone secretion (the first two are the most important).
• Renin-angiotensin-aldosterone mechanism.
• Plasma concentration of K+.
• Adrenocorticotropic hormone (ACTH).
• Atrial natriuretic peptide (ANP).
• ANP inhibits aldosterone release.
• Lowers blood volume/blood pressure by eliminating Na+ and H₂O in urine.

Glucocorticoids

• Influence glucose metabolism and help us resist stressors.
• Glucose is necessary for ATP production. helps to keep blood glucose levels stable & maintain blood

1. Cortisol is a primary glucocorticoid that converts fats and proteins to produce glucose to ATP.
2. Causes vasoconstriction to increase volume.
3. vessel diameter which increases pressure.
4. Because when volume pressure increases.

Gonadocorticoids

• Classified as weak androgens (male sex hormones) that are converted to testosterone (and sometimes to estrogen).
• The amount of gonadocorticoids produced by adrenal cortex much less than those produced by gonads.
• Contribute to:
• Axillary & pubic hair development.
• Sex drive in women.
• Post-menopausal estrogen production when estrogen is no longer produced by ovaries.

Adrenal Medulla

• This portion of the adrenal gland is part of the sympathetic nervous system is active during flight or flight
• Synthesizes both epinephrine and norepinephrine
• The adrenal medulla constricts blood vessels (raises blood pressure)
• Dilates airways
• Elicits very brief response and involved in fight or flight

Pineal Gland

• Tiny gland that hangs from the roof of 3rd ventrical in diencephalon, and consists of hypothalamus epithalamus.
• Gland secretes melatonin (which can make you sleepy.)
• Lowest levels secreted around noon
• Highest levels secreted during the night
• Melatonin receptors in hypothalamus help regulate sleep-wake cycle
• morning: Cortisol secretion wakes you up.

Pancreas

• Soft and tadpole shaped and is located partially posterior to the stomach.
• Exhibits both endocrine & exocrine and composed mainly of acinar cells which secrete digestive enzymes into the small intestine
• Pancreatic islets are clusters of two cell types:
• Alpha cells: fewer in number and secrete glucagon (increases blood glucose).
• Beta cells - more numerous and secrete insulin (which decreases blood glucose).

Glucagon/Insulin Regulation

• Glucagon breaks down glycogen in liver to release glucose into blood.
• Insulin transports glucose into body and muscle calls for storage
• This can be used to make : ATP. Stored as glycogen and is stored or converted to fat.

Gonads

• Ovaries and testes produce same steroid sex hormones as adrenals, but in higher quantity.
• Ovaries produce ova (eggs), estrogen and progesterone maturation of reproductive organs, appearance of secondary sex characteristics at puberty, regulation of menstrual cycle)
• Testes produce sprem and and testosterone maturation of reproductive organs, appearance of secondary sex characteristics at puberty, sex drive, sperm production)
• Placenta is a temorary endocrine organ which secretes estrogen, progesteone, and hCG.
• Temporary endocrine oragn can influence the course of pregnancy.