Endocrine System PDF

ENDOCRINE SYSTEM CHARACTERISTICS OF THE ENDOCRINE SYSTEM Composed of endocrine glands and specialized endocrine cells Secrete small amounts of chemical messengers called hormones ○ Hormones diffuse into the bloodstream going to their effectors or target tissues NERVOUS SYSTEM...

ENDOCRINE SYSTEM CHARACTERISTICS OF THE ENDOCRINE SYSTEM Composed of endocrine glands and specialized endocrine cells Secrete small amounts of chemical messengers called hormones ○ Hormones diffuse into the bloodstream going to their effectors or target tissues NERVOUS SYSTEM ENDOCRINE SYSTEM Short duration responses Long duration responses Uses Neurotransmitters Uses hormones Some Neurons secrete hormones Affect receptors through G Proteins called neuropeptides Hormones circulate in the blood Affect receptors through G Proteins Responds fast Neurotransmitters are released Remains active for a substantial directly on their target cell length of time Responds faster Amplitude moderated signals Response duration lasts for as long as action potentials are sent Frequency modulated signals a. Amplitude-modulated system - The concentration of the hormone determines the strength of the signal and the magnitude of the response. For most hormones, a small concentration of a hormone is a weak signal and produces a small response, whereas a larger concentration is a stronger signal and results in a greater response. b. Frequency-modulated system - The strength of the signal depends on the frequency, not the size, of the action potentials. All action potentials are the same size in a given tissue. A low frequency of action potentials is a weak stimulus, and a higher frequency is a stronger stimulus.  Autocrine - Can signal itself within a local area Paracrine - Can signal nearby tissue Neurotransmitters - Secreted primarily by neurons through the synaptic cleft Endocrine - Secreted into the blood HORMONES: GENERAL CHARACTERISTICS Stability ○ Concept of half-life (exponential decrease) Communication - Hormones are able to communicate to its target cell Distribution HORMONES: CHEMICAL COMPOSITION LIPID SOLUBLE ○ Nonpolar ○ Includes steroid hormones, thyroid hormones and fatty acid derivative hormones ○ Travel in the bloodstream bound to proteins ○ Removed from the circulation through CONJUGATION Enzymes in the liver attach water-soluble molecules (sulfate/ glucuronic acid) where kidneys and liver can easily excrete them WATER SOLUBLE HORMONES ○ Polar molecules ○ Includes protein hormones, peptide hormones, most amino acid derivative hormones ○ Circulate as free hormones  ○ Have short half-lives due to hydrolytic enzymes called PROTEASES ○ Some modifications are made to protect them from being destroyed: 1) glycoproteins, 2) modified terminal end 3) binding proteins HORMONE SECRETION PATTERN DESCRIPTION Chronic Results in relatively constant blood levels over long periods of time e.g. Thyroid hormones Acute Concentration changes suddenly and irregularly e.g. Epinephrine Episodic Secreted at fairly predictable intervals and concentrations e.g. reproductive hormones CONTROL OF SECRETION HUMORAL STIMULI NEURAL STIMULI HORMONAL STIMULI Stimulation by Following action Hormones stimulate metabolites and potentials, the secretion of other molecules in the neurotransmitters are hormones bloodstream released into e.g. tropic hormones e.g. Calcium, Sodium, synapses of (released by anterior Glucose hormone-producing pituitary hormones) Release of companion cells Inhibiting hormones hormone that opposes e.g.releasing prevent secretion of the effects of the hormones other hormones e.g. secreted hormone (neuropeptides thyroid hormones e.g. glucagon and released by the inhibiting their own insulin hypothalamus) tropic hormones Use of inhibitory neurotransmitter e.g. excitatory and inhibitory neurotransmitters ] HORMONE RECEPTORS AND MECHANISMS OF ACTION Hormones exert their actions by binding to target cell proteins called RECEPTORS RECEPTOR SITE – specific portion of each receptor molecule where a hormone binds to SPECIFICITY – tendency for each type of hormone to bind to one type of receptor and not to others AGONISTS AND ANTAGONISTS E.G. MORPHINE NALOXONE Agonists - Increases or produce the desired effect (MORPHINE) Antagonists - Stop the effect (NALOXONE) DECREASE AND INCREASE IN RECEPTOR NUMBER DECREASE ○ Responses of target tissues rapidly decrease over time through DESENSITIZATION Occurs when the number of receptors rapidly decreases after exposure to certain hormones (DOWN REGULATION) E.g drinking coffee INCREASE ○ UP-REGULATION happens when a target tissue periodically increase sensitivity and increase the rate of receptor synthesis in target cells Lipid Soluble - Straight directly into the dna Water Soluble - Bind to the receptors and activate the G - Proteins INTRACELLULAR RECEPTORS Lipid-soluble hormones bind with intracellular receptors ○ The receptor-hormone complex diffuses into the nucleus where it activates genes mRNA is produced mRNA initiates the production of certain proteins (enzymes) that produce the response of the target cell to the hormone Intracellular receptor mechanisms are slow-acting because time is required to produce the mRNA and the protein ENDOCRINE GLANDS MAIN REGULATORY FUNCTIONS OF THE ENDOCRINE SYSTEM 1. Regulation of Metabolism 2. Control of food intake and digestion 3. Modulation of tissue development 4. Control of ion levels 5. Control of water balance 6. Changes in heart rate and BP 7. Control of blood glucose and nutrients 8. Control of reproductive functions 9. Stimulation of uterine contractions and milk 10. Modulation of immune function HYPOTHALAMUS ROSTRAL/ PREOPTIC AREA - Contains key integrative circuitry for thermoregulation, fever, electrolyte balance, wake-sleep circadian rhythms and sexual behavior TUBERAL HYPOTHALAMUS (Middle) – contains circuitry for feeding, sexual behavior, aggressiveness and autonomic and endocrine responses POSTERIOR AREA – provides output to the arousal system and hippocampus (role in regulating wakefulness and stress response) PITUITARY GLAND/ HYPOPHYSIS Rests at the sella turcica of the sphenoid bone and roughly the size of a pea Has 2 lobes: 1) POSTERIOR PITUITARY GLAND (neurohypophysis) and 2) ANTERIOR PITUITARY GLAND (adenohypophysis) INFUNDIBULIM – stalk of tissue that connects the pituitary gland and the hypothalamus PITUITARY GLAND POSTERIOR PITUITARY GLAND ○ Continuous with the hypothalamus in the brain ○ Its hormones are called neuropeptides ANTERIOR PITUITARY GLAND ○ Develops from an out-pocketing of the roof of the embryonic oral cavity ○ Par intermedia – not functional in adults ○ Produces traditional hormones PITUITARY GLAND AND THE HYPOTHALAMUS The hypothalamus regulates the pituitary gland through: ○ Anterior Pituitary - through specialized set of blood vessels called the hypothalamohypophyseal portal system Releasing hormones – stimulate hormone secretion Inhibiting hormones – decrease hormone secretion ○ Posterior Pituitary – through specialized neural pathway called the hypothalamohypophyseal tract  Posterior pituitary doesn’t release and produce the hormones, rather just use it as storage. GROWTH HORMONE GH stimulates growth in most tissues and is a regulator of metabolism ○ GH stimulates The uptake of amino acids and their conversion into proteins The breakdown of fats and the synthesis of glucose The production of somatomedins (with GH they promote bone and cartilage growth) ○ GH secretion increases in response to low blood glucose, stress, and an increase in certain amino acids ○ GH is regulated by two hypothalamic hormones Growth hormone-releasing hormone (GHRH) Growth hormone-inhibiting hormone (GHIH) THYROID GLAND The largest endocrine gland Located in the anterior neck and consists of two lateral lobes connected by a median tissue mass called the isthmus Composed of follicles that produce the glycoprotein thyroglobulin Other endocrine cells, the parafollicular cells, produce the hormone calcitonin THYROID HORMONES Consist of two related iodine-containing compounds ○ Triiodothyronine (T3): has two tyrosines with three bound iodine atoms (90%) ○ Tetraiodothyronine (thyroxine or T4): has two tyrosine molecules plus four bound iodine atoms (10%) EFFECTS OF THYROID HORMONES T3 and T4 ○ Increase the rate of glucose, fat, and protein metabolism in many tissues ○ Increase body temperature T3 and T4 play a role in ○ Maintaining blood pressure ○ Regulating tissue growth ○ Developing skeletal and nervous systems ○ Maturation and reproductive capabilities REGULATION OF THYROID HORMONE SECRETION Thyrotropin-releasing hormone (TRH) and thyroid-stimulating hormone (TSH) regulate T3 and T4 secretion ○ TRH from the hypothalamus increases TSH secretion Increases as a result of chronic exposure to cold Decreases as a result of food deprivation, injury, and infection ○ Increased TSH from the anterior pituitary increases T3 and T4 secretion ○ T3 and T4 inhibit TSH and TRH secretion (negative feedback) CALCITONIN Parafollicular cells secrete calcitonin Directly regulated by blood Ca2+ levels ○ Blood Ca2+ levels drop = calcitonin levels drop ○ Blood Ca2+ levels rise = calcitonin levels rise Calcitonin targets the skeleton to ○ Inhibit osteoclast activity and the release of calcium from the bone matrix ○ Stimulate calcium uptake and incorporation into the bone matrix PARATHYROID GLAND Tiny glands embedded in the posterior aspect of the thyroid Secrete a polypeptide hormone called parathyroid hormone (PTH) PTH is essential in regulating calcium balance in the blood (much more important than calcitonin) PTH increases the release of Ca2+ from bones into blood by increasing the number of osteoclasts PARATHYROID HORMONE PTH also ○ Promotes Ca2+ reabsorption by the kidneys and the formation of active vitamin D by the kidneys ○ Active vitamin D increases calcium absorption by the intestine A decrease in blood Ca2+ levels stimulates PTH secretion ADRENAL GLANDS Paired, pyramid-shaped organs that sit on top of the kidneys Divided into two parts ○ Adrenal medulla (inner area) Arises from the same cells that give rise to postganglionic sympathetic neurons ○ Adrenal cortex (outer area) Glandular tissue derived from embryonic mesoderm Composed of three layers Zona glomerulosa Zona fasciculata Zona reticularis Structurally and functionally, they are four glands in one HORMONES OF THE ADRENAL MEDULLA Approximately 80% of the hormones released is epinephrine (adrenaline) and 20% is norepinephrine Secretion of these hormones prepares the body for physical activity by: Increasing blood glucose levels Increasing the use of glycogen and glucose by skeletal muscle Increasing heart rate and force of contraction Causes vasoconstriction in the skin and viscera Causes vasodilation in skeletal and cardiac muscle Released by the sympathetic division of the ANS in response to Emotions Injury Stress HORMONES OF THE ADRENAL CORTEX Synthesizes and releases steroid hormones called corticosteroids Different corticosteroids are produced in each of the three layers ○ Zona glomerulosa: mineralocorticoids (chiefly aldosterone) ○ Zona fasciculata: glucocorticoids (chiefly cortisol) ○ Zona reticularis: gonadocorticoids (chiefly androgens) ZONA GLOMERULOSA Mineralocorticoids ○ Regulate electrolytes in extracellular fluids ○ Aldosterone: most important mineralocorticoid Maintains Na+ balance by reducing excretion of sodium from the body Stimulates reabsorption of Na+ by the kidneys Decreases K+ and H+ levels in the blood ○ Aldosterone secretion is stimulated by: Rising blood levels of K+ Low blood Na+ Decreasing blood volume or pressure ZONA FASCICULATA Glucocorticoids (especially cortisol) ○ Help the body resist stress by Keeping blood sugar levels relatively constant Maintaining blood volume and preventing water shift into tissue ○ Cortisol provokes Gluconeogenesis (formation of glucose from non-carbohydrates) Rises in blood glucose, fatty acids, and amino acids ○ Excessive levels of glucocorticoids Depress cartilage and bone formation Inhibit inflammation Depress the immune system Promote changes in cardiovascular, neural, and gastrointestinal function ZONA RETICULARIS Gonadocorticoids (Sex Hormones) ○ Most gonadocorticoids secreted are androgens (male sex hormones), and the most important one is testosterone ○ Androgens contribute to: The onset of puberty The appearance of secondary sex characteristics Sex drive in females Androgens can be converted into estrogens after menopause PANCREAS A triangular gland, which has both exocrine and endocrine cells, located behind the stomach Acinar cells produce an enzyme-rich juice used for digestion (exocrine product) Pancreatic islets (islets of Langerhans) produce hormones (endocrine products) The islets contain two major cell types: ○ Alpha (α) cells that produce glucagon ○ Beta (β) cells that produce insulin INSULIN Target tissues ○ Liver ○ Adipose tissue ○ Muscle ○ Satiety center in the hypothalamus Nervous system relys on blood glucose levels maintained by insulin Increases the uptake of glucose and amino acids by cells ○ Glucose Is used for energy Stored as glycogen Converted into fats ○ Amino acids are used to synthesize proteins Low levels of insulin promote the formation of ketone bodies by the liver GLUCAGON Target tissue is mainly the liver Causes the breakdown of glycogen to glucose Stimulates the synthesis of glucose from amino acids Liver releases glucose into the blood HORMONAL REGULATION OF NUTRIENTS After a meal, the following events take place ○ High glucose levels stimulate insulin secretion but inhibit glucagon, cortisol, GH, and epinephrine secretion ○ Insulin increases the uptake of glucose, amino acids, and fats, which are used for energy or are stored ○ Sometime after the meal, blood glucose levels drop Insulin levels decrease and glucagon, GH, cortisol, and epinephrine levels increase Glucose is released from tissues ○ The liver releases glucose into the blood, and the use of glucose by most tissues, other than nervous tissue, decreases ○ Adipose tissue releases fatty acids and ketones, which most tissues use for energy HORMONE REGULATION OF NUTRIENTS During exercise, the following events occur ○ Sympathetic activity increases epinephrine and glucagon secretion, causing a release of glucose from the liver into the blood ○ Low blood sugar levels, caused by the uptake of glucose by skeletal muscles, stimulate epinephrine, glucagon, GH, and cortisol secretion Causes an increase in fatty acids and ketones in the blood, all of which are used for energy TESTES & OVARIES Testes ○ Secrete testosterone Initiates maturation of male reproductive organs Causes appearance of secondary sexual characteristics and sex drive Is necessary for sperm production Maintains sex organs in their functional state Ovaries ○ Secrete estrogens and progesterone Maturation of the reproductive organs Appearance of secondary sexual characteristics Breast development and cyclic changes in the uterine mucosa PINEAL BODY Small, pinecone -shaped structure located superior and posterior to the thalamus Secretory product is melatonin Melatonin ○ Can inhibit reproductive maturation ○ May regulate sleep -wake cycles THYMUS Lobulated gland located deep to the sternum Major hormonal products are thymopoietins and thymosins These hormones are essential for the development of the T lymphocytes (T cells) of the immune system OTHER ENDOCRINE GLANDS Gastrointestinal tract ○ Produces gastrin, secretin, and cholecystokinin, which regulate digestive functions Kidneys ○ Produce erythropoietin, which stimulates red blood cell production Placenta ○ Secretes human chronic gonadotropin, which is essential for the maintenance of pregnancy HORMONE-LIKE SUBSTANCES Autocrine agents ○ Chemical signals that locally affect cells of the same type as the cell producing the autocrine agent ○ Prostaglandins, thromboxanes, prostacyclins, and leukotrienes Paracrine agents ○ Chemical signals that locally affect cells of a different type than the cell producing the paracrine agent ○ Growth factors, clotting factors, and histamine Autocrine and paracrine chemical signals differ from hormones in that ○ They are not secreted from discrete endocrine glands ○ They have local effects rather than systemic effects ○ They have functions that are not understood in all cases DEVELOPMENTAL ASPECTS Endocrine glands derive from all three germ layers. Those derived from mesoderm produce steroid hormones; the others produce the amino acid–based hormones. The natural decrease in function of the female’s ovaries during late middle age results in menopause. All endocrine glands gradually become less efficient as aging occurs. This change leads to a generalized increase in the incidence of diabetes mellitus and a lower metabolic rate