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VT 106
Comparative Anatomy and Physiology
Endocrine System
ENDOCRINE SYSTEM – regulates body functions
endocrine glands – ductless glands; secrete products into interstitial fluid
hormones – products of endocrine glands and cells
usually diffuse into capillaries and circulate in blood
major endocrine glands – pituitary, thyroid, parathyroid, adrenals, pineal
other organs containing endocrine tissues
hypothalamus, thymus, pancreas, ovaries, testes, kidneys, stomach,
small intestine, placenta, others
COMPARISON OF NERVOUS AND ENDOCRINE REGULATION
Nervous System
synaptic communication - neurotransmitter released at a specific site
targets a specific muscle, gland, or neuron
rapid onset of response (milliseconds)
short duration – local inactivation of neurotransmitter
Endocrine System
endocrine communication - hormones released in bloodstream
targets any tissues in the body with the correct receptors
onset of response may take seconds to hours
longer duration – hormone inactivated by liver or excreted by kidneys
paracrine communication (local hormones) – hormones released in
interstitial fluid target neighboring cells with correct receptors
HORMONE FUNCTION
hormone receptors – receptor proteins on or in specific target cells
bind specific hormones and alter cell activity in some way
Chemical Classes of Hormones
steroid hormones – derived from cholesterol
sex hormones – androgens, estrogens
adrenal cortex hormones – mineralocorticoids, glucocorticoids
calcitriol
amino acid derivatives
thyroid hormones, epinephrine, norepinephrine, melatonin
peptide or protein hormones
insulin, growth hormone, oxytocin, many others
fatty acid derivatives (usually act as local hormones)
prostaglandins
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Mechanisms of Hormone Action – various target cells can respond differently to
the same hormone depending on the receptor present
turn genes on or off – alters gene expression (synthesis of a protein)
enzyme – alters target cell function
structural protein – alters target cell structure
turn enzymes on or off = alters cell function
Target Cell Response – depends on 3 factors:
1) hormone concentration
2) number of hormone receptors on target cell
down-regulation - decrease number of receptors; decreases sensitivity
up-regulation – increase number of receptors; increases sensitivity
3) influence of other hormones
antagonistic effect – one hormone opposes the action of another
net result depends on balance of opposing hormones
synergistic effect – effect of 2 hormones is greater than sum of those
hormones acting alone
permissive effect – the action of one hormone is required for the proper
function of another hormone
Regulation of Hormone Secretion
neural stimuli –endocrine cell stimulated by a neuron (neuroglandular junction)
humoral stimuli – endocrine cell stimulated by changes in composition of blood
or interstitial fluid
hormonal stimuli – presence or absence of another hormone stimulates endocrine
cell
negative feedback – most endocrine cells are inhibited as their products or effects
increase (eg. high thyroid hormone levels inhibit thyroid gland secretion)
HYPOTHALAMUS AND PITUITARY GLAND – function together to
regulate most aspects of growth, development, metabolism, and homeostasis
HYPOTHALAMUS – integrates nervous and endocrine systems
Neurosecretory cells – specialized neurons which produce hormones
hormones stored by posterior pituitary
antidiuretic hormone (ADH)
oxytocin
produced by neuron cell bodies in hypothalamus
hormone vesicles travel down axons to posterior pituitary
regulatory hormones – regulate anterior pituitary
releasing hormones – stimulate anterior pituitary
inhibiting hormones – inhibit anterior pituitary
produced in hypothalamus and released in interstitial space
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pituitary portal system – system of 2 capillary networks
delivers hormones secreted by the hypothalamus to the
anterior pituitary
Autonomic motor neurons
sympathetic stimulation of adrenal medulla
secretes epinephrine and norepinephrine as hormones
PITUITARY GLAND (HYPOPHYSIS) – ventral to hypothalamus in pituitary fossa
of sphenoid bone
infundibulum – stalk attaching pituitary to hypothalamus
Hormones of Anterior Pituitary (adenohypophysis)
tropins – hormones that regulate other endocrine glands or tissues
1) thyroid-stimulating hormone (TSH) – (thyrotropin)
stimulates thyroid to produce and secrete thyroid hormones (T3 and T4)
promoted by a releasing hormone from hypothalamus
inhibited by negative feedback of T3 and T4
2) adrenocorticotropic hormone (ACTH) - (corticotropin)
stimulates adrenal cortex to produce and secrete glucocorticoids
promoted by a releasing hormone (during stress)
inhibited by negative feedback of glucocorticoids
3) follicle-stimulating hormone (FSH) – (gonadotropin)
female – stimulates ovaries to produce egg follicles and estrogen
male – stimulates testes to produce sperm
promoted by a releasing hormone
inhibited by negative feedback of sex hormones
4) luteinizing hormone (LH) - (gonadotropin)
female – stimulates ovaries to ovulate and maintain pregnancy
male – stimulates testes to secrete testosterone
promoted by gonadotropin-releasing hormone (GnRH)
inhibited by negative feedback of sex hormones
5) prolactin (PRL) – (mammotropin)
stimulates mammary glands to produce and secrete milk
permissive and synergistic effects of other hormones required:
estrogens, progestins, glucocorticoids, GH, T4, insulin, oxytocin
promoted by a releasing hormone
inhibited by an inhibiting hormone (hypothalamus)
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6) growth hormone (GH) – (somatotropin)
stimulates cells in most body tissues and organs
causes cell growth and division, mobilization of energy stores,
increased protein synthesis
promoted by a releasing hormone
inhibited by an inhibiting hormone
gigantism – hypersecretion of GH during youth
pituitary dwarfism – hyposecretion of GH during youth
acromegally – hypersecretion of GH during adulthood
7) melanocyte-stimulating hormone (MSH)
associated with color changes in reptiles and amphibians
function unknown in mammals
Hormones of Posterior Pituitary – produced by hypothalamus
neuroendocrine reflexes – hormones are released due to neural impulses
1) oxytocin
stimulates uterine wall muscles to contract during parturition
stimulates mammary glands to eject milk (milk let-down)
promoted by sensory stimuli:
stretch of the cervix
suckling of young
positive feedback loop – hormone release continues until sensory
stimulus stops (birth, or young stop suckling)
2) antidiuretic hormone (ADH) – prevents diuresis (increased urine output)
promoted by:
high blood osmotic pressure (high solute concentration)
low blood pressure or volume (baroreceptors)
inhibited by negative feedback
target organs:
kidneys – less urine produced
sweat glands – decrease sweating
arterioles constrict = increases blood pressure
diabetes insipidus – hyposecretion of ADH
polyuria – excess urine output
polydipsia – excess drinking
THYROID GLAND
Anatomy of the Thyroid
adjacent to trachea, caudal to larynx
thyroid follicles – contain colloid (large quantity of thyroid hormones, T4
& T3, precursor molecule)
follicular cells – simple cuboidal epithelium that secrete T3 and T4)
parafollicular cells (C cells) – produce calcitonin
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Thyroid Hormones (T4 and T3)
T4 has 4 iodines (tetraiodothyronine) [thyroxine]
T3 has 3 iodines (triiodothyronine)
(thyroid hormones cannot be synthesized without adequate dietary iodine)
target – most cells of body
increases basal metabolic rate
increase cellular metabolism – more ATP production, protein synthesis,
more sodium/potassium pumps, mobilization of energy stores
calorigenic effect – body temperature rises
participate in growth – especially nervous tissue
Regulation of Thyroid Hormones
promoted by TSH from anterior pituitary
inhibited by negative feedback of T3
Thyroid Disorders
hypothyroidism – reduced secretion
in young – small size and mental retardation
in adult – low heart rate and body temperature, weakness, weight
gain, alopecia (hair loss)
hyperthyroidism – increased secretion
high heart rate and blood pressure, elevated body temperature,
nervousness, weight loss, increased appetite
signs of increased GI and urinary activity
goiter – enlarged thyroid
dietary iodine deficiency – hyperplasia of follicular cells
CALCITONIN – from parafollicular (C) cells of thyroid
lowers blood calcium (increases bone density)
targets
osteoclasts – inhibits; decreases bone resorption
kidneys – increases excretion of calcium ions in urine
promoted by high blood calcium
inhibited by low blood calcium
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PARATHYROID GLANDS
Anatomy of the Parathyroids
2-4 small glands attached to or within thyroid gland
produce parathyroid hormone (PTH) [parathormone]
Parathyroid Hormone – major regulator of calcium level in blood
increases blood calcium
targets:
osteoclasts – stimulates resorption of bone, which releases calcium
into blood
kidneys – decreases calcium excretion in urine
stimulates production of calcitriol – increases absorption of
dietary calcium
promoted by low blood calcium
inhibited by high blood calcium
hypocalcemia – low blood calcium; causes spontaneous nervous depolarization
and muscle weakness
hypoparathyroidism – insufficient PTH
milk fever (dairy cattle) – excess calcium loss in milk
eclampsia (cats and dogs) – high fetal calcium demands or milk production
hypercalcemia – high blood calcium
hyperparathyroidism – excess PTH
primary – tumor
secondary – insufficeint dietary calcium or calcitriol (Vit. D)
when chronic, can cause fragile bones
ADRENAL GLANDS
Anatomy of Adrenals – cranial and medial to each kidney
2 functional regions of adrenal gland:
adrenal cortex – outer region
produces 3 types of hormones:
mineralocorticoids
glucocorticoids
sex hormones
adrenal medulla – central region
modified sympathetic ganglion
neuroendocrine cells – produce epinephrine and norepinephrine
promoted by sympathetic ANS (fight-or-flight effects)
(review effects of sympathetic nervous system)
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Adrenal Cortical Hormones
1) Mineralocorticoids – regulate electrolyte and water homeostasis
aldosterone – main mineralocorticoid produced
targets kidneys – decreases excretion of Na+ and water in urine
increases excretion of K+ in urine
(increases blood pressure and blood Na+, decreases blood K+ )
promoted low blood Na+ or high blood K+
Also regulated by renin-angiotensin-aldosterone (RAA) pathway
1) decreased blood volume and pressure
2) kidneys secrete renin (enzyme) into blood
3) renin converts angiotensinogen (plasma protein from liver) into
angiotensin I
4) enzyme in lung capillaries converts angiotensin I to angiotensin II
5) angiotensin II promotes aldosterone secretion by adrenal cortex
6) aldosterone promotes Na+ and water reabsorption in kidneys
7) blood volume and pressure increase
2) Glucocorticoids – increases blood glucose and helps resist stress
cortisol (hydrocortisone) – main glucocorticoid
target – most cells of the body
promoted by ACTH from anterior pituitary (stress)
inhibited by negative feedback of glucocorticoids
Effects of Glucocorticoids (stress hormones)
resistance to stress – mobilize energy sources; increase blood
pressure, increase appetite
protein breakdown (muscles) mobilizes amino acids
lipolysis – breakdown and metabolism of fats
glucose synthesis and storage in liver
anti-inflammatory effects – inhibit white blood cells, reduce
swelling and pain, slows healing
depression of immune responses
Cushing’s syndrome – hyperadrenocorticism
loss of muscle, redistribution of fat (pendulous abdomen), poor
healing, infections, weakness, polyphagia (increased appetite)
high blood pressure – polyuria/polydipsia
Addison’s disease – hypoadrenocorticism
glucocorticoid drugs – frequently given to decrease inflammation and
reduce immune responses
may cause iatrogenic Cushing’s
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3) Sex hormones
little effect as amount is low compared to that produced in gonads
THE STRESS RESPONSE
stressor – physical or emotional disturbance that threatens equilibrium
regulated mainly by the hypothalamus
alarm phase – immediate sympathetic nervous response initiated by
hypothalamus
rapid energy mobilization to brain, heart, skeletal muscles
reduced non-essential functions
resistance phase – longer-lasting hormonal response initiated by hypothalamic
releasing hormones
CRH – ACTH – cortisol
GHRH – GH
TRH – TSH – T4 & T3
1) mobilizes lipids and proteins for energy
2) glucose-sparing for use by nervous tissues
3) synthesis of glucose from other molecules
4) maintains blood volume and pressure – supplies nutrients to tissue
PANCREATIC ISLETS
pancreas – between stomach and sm. intestine
99% exocrine glands producing digestive enzymes
1% pancreatic islets (islets of Langerhans) – produce hormones
alpha (A) cells – secrete glucagon (increases blood glucose)
beta (B) cells – secrete insulin (decreases blood glucose)
delta (D) cells – secrete somatostatin
inhibits glucagon and insulin release; slows nutrient absorption
from GI tract
Glucagon – increases blood glucose
main target - liver
glycogenolysis (glycogen stores broken down to glucose)
gluconeogenesis (lactic acid and amino acids converted to glucose)
other target – adipocytes
stimulates lipolysis (fat breakdown for energy)
decreases glucose consumption
promoted by hypoglycemia (low blood glucose) and sympathetic impulses
inhibited by glucose feedback, insulin, somatostatin from D cells
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Insulin – decreases blood glucose
targets – most body cells
causes uptake of glucose by body cells by stimulating synthesis
of glucose transporter proteins
stimulates use of glucose for ATP synthesis
also stimulates cellular synthesis and energy storage
promoted by hyperglycemia (high blood glucose) and parasympathetic n.s.
inhibited by negative feedback
Pancreatic Islet Disorders
Diabetes mellitus – inability to produce or respond to insulin
cells can’t take in or use glucose for energy
Signs and symptoms:
hyperglycemia
glucosuria – glucose in urine
polyuria / polydipsia – water follows glucose into urine
polyphagia – animal is starving
lipolysis – break down fat stores for energy
ketoacidosis – high blood ketones from fatty acids
insulin shock – hypoglycemia due to overdose of insulin or not eating
lack of glucose to brain causes convulsions, loss of consciousness,
and death
Pineal Gland – part of diencephalon; regulated by light and darkness
melatonin – secreted mainly in darkness
functions:
circadian rhythm – sleep and awakening cycles
timing of seasonal reproductive cycles
Thymus – lies over base of heart and distal trachea
participates in immune system
hormones promote maturation of lymphocytes
atrophies with age
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Other Endocrine Tissues
Stomach – gastrin – stimulates gastric functions
Kidneys
erythropoietin (EPO) – stimulates red blood cell formation
calcitriol (vitamin D) – stimulates dietary calcium absorption
Small intestine – secretin and cholecystokinin – stimulate exocrine pancreatic functions
and inhibit gastric functions
Placenta – various hormones that support pregnancy and parturition
Ovaries and Testes – sex hormones – androgens, estrogens, progestins, others
stimulate sexual characteristics, breeding behaviors, and pregnancy
prostaglandins (PGs) – various local hormones in may tissues
derived from membrane fatty acids
PGEs – released by damaged cells and inflammatory cells
stimulate inflammation, fever, and pain
NSAIDS (non-steroidal anti-inflammatory drugs) – drugs that
inhibit PGE synthesis
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