Chapter 45
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Chapter 45. Chemical Signals in Animals. Hormones. Hormones are chemical signals. The endocrine system consists of: Endocrine cells which are hormone-secreting cells and Endocrine glands which are hormone-secreting organs. Specific target cells respond to specific hormones.

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Chapter 45

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Chapter 45

Chemical Signals in Animals


  • Hormones are chemical signals.

    • The endocrine system consists of:

      • Endocrine cells which are hormone-secreting cells and

      • Endocrine glands which are hormone-secreting organs.

  • Specific target cells respond to specific hormones.

Primary Function of Hormones

  • Homeostasis

  • Growth and Development

  • Reproduction

  • Energy Metabolism

  • Behavior


glucose release from liver

glucagon from pancreas



blood glucose high

Negative Feedback Loop

blood glucose low

Biological Cycles


endocrine cell

receptor protein

target cell

Mechanism of Action on Target Cells



endocrine cell

intracellular receptor

target cell

Mechanism of Action on Target Cells


Tropic vs Nontropic Hormones

Tropic hormones- stimulate the production and secretion of hormones by other endocrine glands; ex. TSH

Nontropic hormones- stimulates cellular growth, metabolism, or other functions; ex. thyroxine

Invertebrate nervous systems clearly illustrate endocrine and nervous system interactions

  • Invertebrates have a wide variety of hormones that are involved in the regulation of homeostasis.

  • An example from hydra:

    • One hormone stimulate asexual reproduction.

    • Another hormone prevents sexual reproduction.

  • An example from a mollusk:

    • The hormone that regulates egg laying also inhibits feeding and locomotion.

    • Arthropods have extensive endocrine systems regulating molting in insects and crustaceans.

      • The regulation of molting in insects.

        • Ecdysone also promotes the development of adult features.

        • Neurosecretory cells in the brain produce brain hormone (BH), which regulates the secretion of ecdysone.

      • Juvenile hormone (JH) promotes the retention of larval characteristics.

    • Different signal-transduction pathways in different cells can lead to different responses to the same signal.

    Major Endocrine Organs

    pineal gland


    pituitary gland

    Endocrine Organs of the Brain

    Pineal Gland

    Produces melatonin (synthesized from seratonin, a derivative of tryptophan)

    • Secreted directly in CSF to blood

    • High levels at night make us sleepy; low level during day

    • Pineal gland is stimulated by darkness and inhibited by light

    • Function in regulating circadian rhythms (sleep, body temp, appetite)  biological clock


    anterior pituitary

    posterior pituitary

    Pituitary Gland

    Posterior Pituitary


    Posterior Pituitary

    Anterior Pituitary

    Hormones of the

    Posterior Pituitary

    Antidiuretic Hormone (ADH)

    Oxytocin (+ feedback)

    Anterior Pituitary


    Posterior Pituitary

    Anterior Pituitary

    Hormones of the

    Anterior Pituitary

    • Growth Hormone (GH)

      • Dwarfism

      • Gigantism & Acromegaly

    • Thyroid Stimulating Hormone (TSH)

    • Adrenocorticotropic Hormone (ACTH)

    • Gonadotropins (FSH, ICSH, LH)

    • Prolactin (PRL)

    • Melanocyte-stimulating Hormone (MSH)

    Growth Hormone (GH)

    • Acts on the liver, stimulating it to release several polypeptide hormones.

    • Stimulates amino acid uptake and protein synthesis in target cells.

    • Ultimately stimulates cell growth (cell size and number), especially in muscle and bone.

    • Also stimulates fat breakdown.

    strenuous exercise

    GH Levels



    Thyroid-Stimulating Hormone (TSH)

    • Acts on the thyroid gland, stimulating it to release T3 & T4

    • These thyroid hormones increase glucose catabolism and body heat production.

    • Negative feedback mechanism involved in regulating levels.

    Adrenocorticotropic Hormone (ACTH)

    • Acts on the adrenal cortex, stimulating it to secrete glucocorticoids (e.g., cortisol).

    • Glucocorticoids promote the synthesis of glucose from noncarbohydrate sources such as amino acids, and fatty acids

    • Negative feedback mechanism involved in regulating levels.

    Thyroid Gland




    Thyroid Follicles

    Thyroid Hormones

    Thyroid gland selectively uptakes iodine to produce T3 & T4

    • Thyroxine (T4)

    • Triiodothyronine (T3)

    • Both control metabolic rate and cellular oxidation

    • Calcitonin (from parafolicular cells)- lowers blood CA ++ levels and causes CA++ reabsorption in bone

    Thyroid Hormone Regulation

    Thyroid Disorders

    • Hyperthyroidism (Grave’s, Goiter)

    • Hypothyroidism (Cretinism, Myxedima)

    parathyroid glands




    Parathyroid Glands

    Parathyroid Hormone (PTH)

    • PTH release:

    • stimulates osteoclasts

    • enhances reabsorption of Ca++ by kidneys

    • increases absorption of Ca++ by intestinal mucosal cells

    • Hyperparathyroidism- too much Ca++ drawn out of bone; could be due to tumor

    • Hypoparathyroidism- most often follow parathyroid gland trauma or after removal of thyroid--- tetany, muscle twitches, convulsions; if untreatedrespiratory paralysis and death

    PTH Effects



    • Regulates glucose uptake by cells

    • Controlled via negative feedback: insulin & glucagon

    • Blood sugar level: 90 mg/mL

    Islets of Langerhan


    • Produced by the  cells of the Islets of Langerhan

    • Catalyze oxidation of glucose for ATP production

    • Lowers blood glucose levels by promoting transport of glucose into cells.

    • Stimulates glucose uptake by the liver and muscle cells.

    • Stimulates glycogen synthesis in the liver and muscle cells.

    • Also stimulates amino acid uptake and protein synthesis of muscle tissue


    • Produced by the  cells of the Islets of Langerhans

    • Stimulates change of glycogen to glucose in the liver.

    • Synthesis of glucose from lactic acid and non carbohydrate molecules such as fatty acids and amino acids

    • Causes  in blood glucose concentration

    hypoglycemic- low blood sugar; deficient in glucagon

    Regulation of Blood Sugar Levels

    Diabetes Melitus

    Type I Diabeteshyposecretion of insulininsulin dependantjuvenile onsetType II Diabeteslate onset (adult)insensitivity of cells to insulinmanage by exercise & diet

    adrenal cortex

    adrenal medulla

    Adrenal Glands

    Hormones of the Adrenal Medulla

    • Adrenalin (epinephrine): converts glycogen to glucose in liver

    • Noradrenalin (norepinephrine): increases blood pressure

  • (sympathetic nervous system)

    • Corticosteroids: glucose levels)

  • Hormones of the Adrenal Cortex

    • Glucocorticoids- cortisol

    • Decrease protein synthesis

    • Increase release and use of fatty acids

    • Stimulates the liver to produce glucose from non carb’s

    • Mineralcorticoids- aldosterone

    • Stimulates cells in kidney to reabsorb Na+ from filtrate

    • Increases water reabsorption in kidneys

    • Increases blood pressure

    • Sex Steroids- small amts (androgens)

    • Onset of puberty

    • Sex drive

    Cushing’s Syndrome

    Hypersecretion of cortisone; may be caused by an ACTH releasing tumor in pituitary

    Symptoms: trunkal obesity and moon face, emotional instability

    Treatment: removal of adrenal gland and hormone replacement

    Addison’s Disease

    Hyposecretion of glucocorticoids and mineral corticoids;

    Symptoms- wt loss, low levels of plasma glucose and Na+ levels, high levels of K+

    Treatment- corticosteroid replacement therapy


    Located anterior to the heart

    Produces- thymopoetin and thymosin helps direct maturation and specialization of T-lymphocytes (immunity)


    Ovaries- produce estrogen and progesteroneresponsible for maturation of the reproductive organs and 2ndary sex characteristics in girls at puberty

    Female Reproductive System


    Testes- produce sperm and testosterone (initiates maturation of male repro organs and 2ndary sex characteristics in boys at puberty)

    Male Reproductive System

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