Anatomy - Endocrine System

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Endocrine System

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-regulatory system
-involved in maintaining homeostasis
-endocrine + nervous system = 2 regulatory systems in the body

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Difference b/w Hormone and Neurotransmitter

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Neurotransmitter - released from axonal terminals into synaptic cleft
hormone - released from endocrine glands into extracellular fluid - interstitial fluid, or blood

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Hormones

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released from endocrine glands in response to changes in the environment(internal or external) termed stimuli
-these changes of stimuli lead to homeostatic imbalances and hormones are released to correct these stimuli
--hormones work through negative feedback mechanism

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Hormone receptor characteristics

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1) all hormone receptors are proteins - globular
2) they bind to hormones in a reversible manner
3) they bind to specific hormones w/ a high affinity and specificity - for their specific receptors
4) 2 families of hormone receptors - depends on calsses of hormones they bind
--cell surface receptors(membrane receptors)
--intracellular receptors
----cytoplasmic receptors
----nuclear receptors

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Polar Hormones

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biogenic amine, peptide/protein/glycoprotein hormones bind to cell surface receptors - transmembrane receptors
-span the plasma membrane and are exposed to the interior/exterior of the membrane
-polar hormones can't cross lipid bilayer
--hence cell surface receptors

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Intracellular receptors

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located in cytoplasm(cytoplasmic receptors) or in nucleus(nuclear receptors)
-steroid hormones derived from lipid cholesterol are all nonpolar
--can cross lipid bilayer to bind to intracellular receptors
--all steroid hormone receptors are intracellular

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How Hormones Interact w/ their target cells

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1)classical endocrine interaction
2)paracrine interaction
3)justacrine interaction
4)autocrine interaction

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Classical endocrine interaction

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gland releases hormone into bloodstream which transports the hormone to the target cells located a distance away
-ex. thyroid stimulationg hormone released from anterior pituitary travels through blood stream to reach its target organ - thyroid organ
TSH stimulates thyroid gland to secrete 2 thyroid hormones - T3 n T4

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Paracrine Interaction

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the endocrine cells releasing the hormones are neighboring cells to the target cells -> endocrine cells + target cells in same organ
-ex. in pancreas - both a pancreatic cells(secrete glucogon) and B pancreatic cells(secrete insulin)
when insulin is released one of the action of it is to bind to insulin receptors on the neighboring a cells to inhibit glucogon release

Insulin acts to stimulate cellular uptake of glucose from blood, glucogon acts to stimulate release of glucose from cells into blood - when released it binds to receptors on B cells to inhibit insulin release

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Justacrine interaction

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the endocrine cells and target cells in close contact - juxtaposed
--the hormone binds to receptors on target cells w/o first getting into extracellular fluid
ex. in closely packed cells - placenta -> fetal-maternal exchange unit

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Autocrine interaction

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the endocrine cells releasing the hormone also acts as target cells for the hormone
ex. B pancreatic cells release insulin after meals when blood glucose levels are high(hyperglycemia) - insulin binds to insulin receptors on B cells to stimulate more release of insulin

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Hormonal Interrelationship

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how hormones affect other hormones release in the body
5 types
1)agonism
2)antagonism
3)permissiveness
4)cooperativity
5)synergism

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Agonism

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a hormone released binds to the receptors of another hormone to stimulate the biological effects of the absent hormone
ex. hA binds to hB receptors -> effects of hB

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Antagonism

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a hormone released binds to the receptors of another hormone, blocking that hormone from accessing its own receptors
-biological effects of blocked hormone NOT observed

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Permissiveness

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a hormone released binds to its own receptors and elicits its biological effects which include stimulation of another hormone - increase in release or increase in receptors of that hormone
ex. hA -> ^release of hB and ^ #receptors of hB hence hA increases biological effects of hB

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Cooperativity

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at least 2 hormones work in tandem to bring about a desired biological effect

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Synergism

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a group of hormones work together to bring about a biological response greater than the sum of thei individual effects
ex. hA -> 2 units of bio effect
hB -> 3 units
hC -> 4 units
hA+B+C = 20 units instead of 9

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Hypothalamus

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"master gland" - the 9 hormones produced by it directly or indirectly affect the actions/release of other hormones
-9 hormones
-antidiuretic hormone, oxytocin, growth hormone releasing hormone, growth hormone inhibiting hormone, prolactin releasing hormone, prolactin inhibiting hormone, corticotropic releasing hormone, gonadotropic releasing hormone, thyrotropic releasing hormone

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Antidiuretic Hormone

Oxytocin

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ADH = vasopressin

These 2 are delivered via hypothalamic hypophyseal tract by anterograde transport to the posterior pituitary for storage and release in response to appropriate stimuli

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Growth hormone releasing hormone

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stimulates cells in anterior pituitary to secrete growth hormone

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Growth hormone inhibiting hormone

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stimulates same cells in anterior pituitary to inhibit growth hormone release - somatostatic

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Prolactin releasing hormone

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stimulates cells in the anterior pituitary to secrete prolactin - stimulates milk production in breasts

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prolactin inhibiting hormone

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dopamin - inhibits same cells from secreting prolactin

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corticotropic releasing hormone

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stimulates cells in anterior pituitary to produce the hormone adrenocorticotropic hormone - stimulates the adrenal cortex to secrete the glucocorticoids - these regulate glucose levels - major one in humans is cortisol
--overproduction of cortisol -> cushings syndrome

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Gonadotropic releasing hormone

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stimulates cells in the anterior pituitary to secrete gonadotropics
-follicle stimulating hormone
-luteinizing hormone
--these stimulate the gonads(testes/ovaries) to produce sex steroid hormones
----testosterone in males, estrogen/progesterone in females
------required for the formation of sex cells - gametes -- sperm in males, ova in females

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Thyrotropic releasing hormone

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stimulates cells in the anterior pituitary to produce thyroid stimulating hormone - stimulates thyroid gland to produce 2 thyroid hormones T3, T4

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negative feedback

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the biological effects of hormones negate/eliminate the stimuli that caused the release of the hormones

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stimuli for hormone release

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3 types
-humoral
-hormonal
-neural

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Humoral stimuli

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changes in the levels of chemicals in the body’s humors ( bodily fluids) stimulate endocrine glands to release hormones

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Neural Stimuli

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activation of the nervous system stimulates endocrine glands to release hormones

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Hormonal stimuli

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released hormones stimulate endocrine glands to release other hormones

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Posterior Pituitary gland

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may/may not be an endocrine gland - based of definition of endocrine gland
-if they need to produce the hormones then NO
-if they need to release hormones - then YES