Chapter 16 Flashcards
Endocrine system acts with …
nervous system to coordinate and integrate activity of body cells
- Influences metabolic activities via hormones transported in blood
- Responses slower but longer lasting than nervous system responses
Endocrinology:
study of hormones and endocrine organs
Nervous System
- initiates responses rapidly
- short-duration responses
- acts via action potentials and neurotransmitters
- acts at specific locations determined by axon pathways
- neurotransmitters act over very short distances
Endocrine System
- initiates responses slowly
- long-duration responses
- acts via hormones released into the blood
- acts at diffuse locations - targets can be anywhere blood reaches
- hormones act over long distances
Endocrine system controls and integrates:
- Reproduction
- Growth and development
- Maintenance of electrolyte, water, and nutrient balance of blood
- Regulation of cellular metabolism and energy balance
- Mobilization of body defenses
Exocrine glands:
- Produce nonhormonal substances (examples: sweat, saliva)
- Have ducts to carry secretion to membrane surface
Endocrine glands:
- Produce hormones
- Lack ducts
Endocrine glands include…
- pituitary
- thyroid
- parathyroid
- adrenal
- pineal glands
Hypothalamus is a … organ
neuroendocrine
Some have exocrine and endocrine functions …
- Pancreas
- Gonads
- Placenta
Other tissues and organs that produce hormones…
Adipose cells, thymus, and cells in walls of small intestine, stomach, kidneys, and heart
Chemical messenger comparison: Hormones
long-distance chemical signals that travel in blood or lymph throughout the body
Chemical messenger comparison: Autocrines
short-distance chemical signals that exert effects on same cells that secrete them
Chemical messenger comparison: Paracrines
short-distance chemical signals that act locally (same tissue) but affect cells other than those that secrete them
Two main classes of hormones:
Amino acid–based hormones
- Amino acid derivatives, peptides, and proteins
Steroids
- Synthesized from cholesterol
- Gonadal and adrenocortical hormones
Hormone action on target cells may be to:
- Alter plasma membrane permeability and/or membrane potential by opening or closing ion channels
- Stimulate synthesis of enzymes or other proteins
- Activate or deactivate enzymes
- Induce secretory activity
- Stimulate mitosis
Hormones act in one of two ways, depending on their chemical nature and receptor location…
- ) Water-soluble hormones (all amino acid–based hormones except thyroid hormone)
- Act on plasma membrane receptors
- Act via G protein second messengers
- Cannot enter cell - ) Lipid-soluble hormones (steroid and thyroid hormones)
- Act on intracellular receptors that directly activate genes
- Can enter cell
Plasma Membrane Receptors and Second-Messenger Systems: Cyclic AMP (cAMP) signaling mechanism
- ) Hormone (first messenger) binds to receptor
- ) Receptor activates a G protein
- ) G protein activates or inhibits effector enzyme adenylate cyclase
- ) Adenylate cyclase then converts ATP to cAMP (second messenger)
- ) cAMP activates protein kinases that phosphorylate (add a phosphate) other proteins
Intracellular Receptors and Direct Gene Activation…
- Lipid-soluble steroid hormones and thyroid hormone can diffuse into target cells and bind with intracellular receptors
- Receptor-hormone complex enters nucleus and binds to specific region of DNA
- Helps initiate DNA transcription to produce mRNA
- mRNA is then translated into specific protein (proteins synthesized have various functions / Ex: metabolic activities, structural purposes, or exported from cell)
Hormone release: Blood levels of hormones
- Controlled by negative feedback systems (Increased hormone effects on target organs can inhibit further hormone release)
- Levels vary only within narrow, desirable range
- Hormone release is triggered by Endocrine gland stimuli and
Nervous system modulation
Endocrine gland stimuli: Humoral stimuli
Changing blood levels of ions and nutrients directly stimulate secretion of hormones
- Ex: Ca2+ in blood
Declining blood Ca2+ concentration stimulates parathyroid glands to secrete PTH (parathyroid hormone). PTH causes Ca2+ concentrations to rise, and stimulus is removed
Endocrine gland stimuli: Neural stimuli
Nerve fibers stimulate hormone release
- Sympathetic nervous system fibers stimulate adrenal medulla to secrete catecholamines
Endocrine gland stimuli: hormonal stimuli
Hormones stimulate other endocrine organs to release their hormones
- Hypothalamic hormones stimulate release of most anterior pituitary hormones
- Anterior pituitary hormones stimulate targets to secrete still more hormones
- Hypothalamic–pituitary–target endocrine organ feedback loop (hormones from final target organs inhibit release of anterior pituitary hormones)
Nervous system modulation: Nervous system can make adjustments to …
hormone levels when needed
- Can modify stimulation or inhibition of endocrine glands
Nervous system modulation: Nervous system can override…
normal endocrine controls
- Ex: under severe stress, hypothalamus and sympathetic nervous system override insulin to allow blood glucose levels to increase
- Prepare body for “fight or flight”
Target cells must have specific…
receptors to which hormone binds
- ex: ACTH receptors are found only on certain cells of adrenal cortex, but thyroxin receptors are found on nearly all cells of body
Target cell activation depends on three factors:
- ) Blood levels of hormone
- ) Relative number of receptors on/in target cell
- ) Affinity (strength) of binding between receptor and hormone
Hormones circulate in blood either…
free or bound
- Steroids and thyroid hormone are attached to plasma proteins; others don’t require carriers
Concentration of circulating hormone reflects:
- ) Rate of release
2. ) Speed at which it is inactivated and removed from body
Hormones can be removed from blood by:
- Degrading enzymes or
- Kidneys or
- Liver
Half-life: time required for level of hormone in blood level to decrease by half
- Varies anywhere from fraction of a minute to a week, depending on hormone
Amount of hormone can influence number of receptors for that hormone …
- Up-regulation: target cells form more receptors in response to low hormone levels
- Down-regulation: target cells lose receptors in response to high hormone levels (Desensitizes the target cells to prevent them from overreacting to persistently high levels of hormone)
Multiple hormones may act on same target at same time …
- Permissiveness
- Synergism
- Antagonism
Permissiveness
One hormone cannot exert its effects without another hormone being present
- Ex: reproductive hormones need thyroid hormone to have effect
Synergism
More than one hormone produces same effects on target cell, causing amplification
- Ex: glucagon and epinephrine both cause liver to release glucose
Antagonism
One or more hormones oppose(s) action of another hormone
- Ex: insulin and glucagon
Hypothalamus is connected to pituitary gland via …
stalk called infundibulum
Pituitary secretes at least…
eight major hormones
It has two major lobes:
- Posterior pituitary: composed of neural tissue that secretes neurohormones
- Anterior pituitary: consists of glandular tissue
Posterior lobe is neural tissue derived from …
a downgrowth of brain
Posterior lobe secretes …
Two neurohormones (oxytocin and ADH)
- Hormones are stored in axon terminals in posterior pituitary and are released into blood when neurons fire
Oxytocin
- Strong stimulant of uterine contractions released during childbirth
- Also acts as hormonal trigger for milk ejection
- Both are positive feedback mechanisms
- Acts as neurotransmitter in brain
Antidiuretic hormone (ADH)
- Hypothalamus contains osmoreceptors that monitor solute concentrations
- If concentration too high, posterior pituitary triggered to secrete ADH
- Targets kidney tubules to reabsorb more water to inhibit or prevent urine formation
Anterior lobe secretes…
six hormones - all are peptide hormones
- All but two are tropic hormones (tropins) that regulate secretion of other hormones
- Growth hormone (GH)
- Thyroid-stimulating hormone (TSH) (tropic)
- Adrenocorticotropic hormone (ACTH) (tropic)
- Follicle-stimulating hormone (FSH) (tropic)
- Luteinizing hormone (LH) (tropic)
- Prolactin (PRL)
Growth hormone (GH)
Has direct actions on metabolism and indirect growth-promoting actions
- Direct actions on metabolism:
(Triggers liver to break down glycogen into glucose
Increases blood levels of fatty acids for use as fuel and encourages cellular protein synthesis) - Indirect actions on growth:
(GH stimulates most cells to enlarge and divide, but major targets are bone and skeletal muscle)
Hypersecretion of GH is usually caused by anterior pituitary tumor …
- In children results in gigantism
(Can reach heights of 8 feet) - In adults results in acromegaly
(Overgrowth of hands, feet, and face)
Hyposecretion of GH
- In children results in pituitary dwarfism (May reach height of only 4 feet)
- In adults usually causes no problems
Regulation of Thyroid hormone secretion…
Hypothalamus releases TRH (thyrotropin releasing hormone)
Anterior pituitary releases TSH (thyroid stimulating hormone)
Thyroid gland releases Thyroid hormones (T3/T4)
- The main functions of the thyroid hormones are to increase basal metabolic rate and body heat production.
- Low levels of TH can stimulate TRH release. TRH can also override the negative feedback in emergency situations, such as exposure to extreme cold (particularly in infants).
Thyroid Hormone (TH)
- Body’s major metabolic hormone
- Found in two forms:
T4 (thyroxine): major form secreted and in blood
Must be converted to T3 at tissue level
T3 (triiodothyronine): different structure; more active
10x more active than T4 at receptors.
- Hormones carried in blood by thyroxine-binding globulins (TBGs)
Thyroid Hormone (TH) Effects
- increasing basal metabolic rate and body heat production, by turning on transcription of genes concerned with glucose oxidation. this is the hormones calorigenic effect
- regulating tissue growth and development. TH is critical for normal skeletal and nervous system development and maturation and for reproductive capabilities
- maintaining blood pressure by increasing the number of adrenergic receptors in blood vessels
Ever wonder why most table salt is iodized?
- Lack of iodine decreases TH levels, which triggers increased TSH secretion, triggering thyroid to synthesize more and more unusable thyroglobulin
- Thyroid enlarges even though TH levels are low
Thyroid Disorders: Hyposecretion
Called ‘myxedema’
Symptoms:
- Low metabolic rate
- Lethargy
- Feeling cold/chilly
- Edema
- Dry skin
Overweight is often secondary to low metabolism.
Thyroid Disorders: Hypersecretion
Called ‘Graves’ disease’
Symptoms:
- Elevated metabolic rate
- Excess sweating
- Nervousness
- Irregular heartbeat
- Exopthalmos
Elevated metabolic rate causes weight loss despite adequate food.
Adrenocorticotropic hormone (ACTH)
- ACTH stimulates adrenal cortex to release corticosteroids
Regulation of ACTH release:
- Triggered by hypothalamic corticotropin-releasing hormone (CRH) in daily rhythm
- Highest levels in morning
- Internal and external factors that alter release of CRH include fever, hypoglycemia, and stressors
Adrenal Gland
Paired, pyramid-shaped organs atop kidneys
Structurally and functionally, it is two glands in one
- Adrenal cortex: synthesizes and secretes several different hormones
- Adrenal medulla: nervous tissue that is part of sympathetic nervous system
Cortex produces corticosteroids…
Three layers of cortical cells produce the different corticosteroids:
- Mineralocorticoids (glomerulosa cells)
- Glucocorticoids (fasciculata cells)
- Gonadocorticoids (reticularis cells)
Adrenal cortex: mineralocorticoids
Regulate electrolyte concentrations (primarily Na+ and K+) in ECF
- Importance of Na+: affects ECF volume, blood volume, blood pressure.
- Importance of K+: sets resting membrane potential of cells
Aldosterone: most potent mineralocorticoid
- Stimulates Na+ reabsorption by kidneys
- Results in increased blood volume and blood pressure
- Stimulates K+ elimination by kidneys
Aldosteronism
hypersecretion usually due to adrenal tumors
Results in two major problems:
- ) Hypertension and edema due to excessive Na+
- ) Excretion of K+, leading to abnormal low plasma levels and nonresponsive neurons and muscle
Glucocorticoid Overview
Hormone: Glucocorticoid (cortisol)
Regulation of Release: stimulated by ACTH, inhibited by feedback back inhibition exerted by cortisol
Target Organ and Effects: body cells; promote gluconeogenesis and hyperglycemia; mobilize fats for energy metabolism; stimulate protein catabolism; assist body to resist stressors; depress inflammatory and immune repsonses
Effects of Hypersecretion vs Hyposecretion: Hyper is cushing’s syndrome. Hypo is addison’s disease
Short term vs. long term stress
Short-term stressors cause SNS activation and include the release of E and NE from the adrenal medulla.
Long-term/prolonged/chronic stress also causes the endocrine system to exert more of an effect by causing CRH and ACTH release, which leads to mineralo- and gluco-corticoid release. Cortisol exerts negative feedback on CRH and ACTH release.
This long-term stress response can lead to chronic hypertension (due to increased blood volume and sodium levels) as well as diabetes (increased blood glucose levels) and increased risk of infection due to immunosuppression.
Short-term stress
the hypothalamus activates the adrenal medulla via neural stimuli
short-term stress response helps the body successfully respond to a short-term emergency (running from fire)
Long-term stress
the hypothalamus activates the adrenal cortex via hormonal stimuli. this greatly reinforces the continuing short-term stress response
long-term stress response helps the body successfully deal with some long-term stressors (starvation). if the stress response is prolonged, it can become detrimental. it may lead to high BP, muscle loss, altered immune function, etc.
Hypersecretion- Cushing’s syndrome/disease …
Depresses cartilage/bone formation and immune system; inhibits inflammation; disrupts neural, cardiovascular, and gastrointestinal function
- Causes: tumor on pituitary gland or adrenal cortex; overuse of corticosteroids
- Treatment: removal of tumor, discontinuation of drugs
Glucocorticoid drugs can control symptoms of many inflammatory diseases (arthritis, allergies) but can also cause undesirable effects.
Hyposecretion—Addison’s disease…
- Usually involves deficits in both glucocorticoids and mineralocorticoids
- Decrease in plasma glucose and Na+ levels
- Weight loss, severe dehydration, and hypotension are common
- Treatment: corticosteroid replacement
Early sign is characteristic bronzing of skin due to high levels of ACTH which triggers melanin production in melanocytes
Pancreas
Triangular gland located partially behind stomach
- Acinar cells (exocrine) produce enzyme-rich juice for digestion
- Pancreatic islets (islets of Langerhans) contain endocrine cells
- Alpha cells produce glucagon (hyperglycemic hormone)
- Beta cells produce insulin (hypoglycemic hormone)
Insulin
Secreted when blood glucose levels increase
Insulin lowers blood glucose levels:
- Enhances membrane transport of glucose into fat and muscle cells
- Inhibits breakdown of glycogen to glucose
- Inhibits conversion of amino acids or fats to glucose
Insulin also triggers cells to:
- Catalyze oxidation of glucose for ATP production: first priority
- Polymerize glucose to form glycogen
- Convert glucose to fat (particularly in adipose tissue)
Factors that influence insulin release…
pancreas beta cells secrete insulin when stimulated by:
- elevated blood glucose levels
- rising blood levels of amino acids and fatty acids
- acetylcholine released by parasympathetic nerve fibers
- hyperglycemia hormones, this effect is indirect and occurs because all of these hormones increase blood glucose levels
activation of sympathetic NS inhibits insulin release
Glucagon
Release triggered by decreased blood glucose levels, rising amino acid levels, or sympathetic nervous system
Raises blood glucose levels by targeting liver to:
- Break down glycogen into glucose (Glycogenolysis)
- Synthesize glucose from lactic acid and other noncarbohydrates (Gluconeogenesis)
- Release glucose into blood
Negative feedback loops regulating blood glucose levels are controlled by …
hormone secretion from the pancreas
Diabetes mellitus (DM) can be due to:
Hyposecretion of insulin: Type 1
Hypoactivity of insulin: Type 2
Three cardinal signs of DM:
Polyuria: large urine output
Glucose acts as osmotic diuretic
Polydipsia: excessive thirst
From water loss due to polyuria
Polyphagia: excessive hunger and food consumption
Cells cannot take up glucose and are “starving”
