53.0 Introduction

1. Two Systems Regulate Homeostasis
1. Nervous and Endocrine Systems fig 53.1
2. Mechanism of Control
1. Release regulatory molecules
2. Bind to receptor proteins in cells of target organs

53.1 Regulation is often accomplished by chemical messengers

1. Types of Regulatory Molecules
1. Systems Differ in Means of Transportation, Not Chemical Nature
1. Nervous system control
1. Axons release neurotransmitters into synaptic cleft
2. Neurotransmitters bind to receptor proteins on membrane of postsynaptic cell
3. Transmission affects only single cell that receives neurotransmitter

2. Endocrine control
1. Endocrine glands secrete regulatory chemicals called hormones
2. Hormones transmitted through circulatory system
3. All cells exposed to hormones, only target cells respond

3. Some chemicals are found in both systems
1. Example: Norepinephrine
2. Released by sympathetic nerve endings
3. Secreted by adrenal gland

4. Neurohormones
1. Specialized neurons secrete chemicals into blood, not synaptic cleft
2. Brain neurons secrete chemicals, can be considered an endocrine gland

2. Other Regulatory Chemicals Are Released and Act within an Organ
1. Cells of an organ can regulate each other
2. Regulation is not endocrine, since chemicals not carried by bloodstream
3. Regulation similar to hormone regulation
4. Two forms of organ regulation
1. Autocrine: Similar cells regulate each other
2. Paracrine: Regulatory molecules produced, act on different tissues in same organ

5. Comparison of chemical regulatory molecules fig 53.2

2. Endocrine Glands and Hormones
1. Components of the Endocrine System fig 53.3
1. Includes organs that function exclusively as endocrine glands tbl 53.1
2. Includes organs that have functions in addition to secreting hormones
1. Endocrine glands lack ducts
2. Secrete products into surrounding capillaries

3. Pancreas has both exocrine and endocrine functions
1. Exocrine = produce pancreatic juice, via pancreatic duct, to intestine
2. Endocrine = secretes insulin and glucagon into blood

2. Chemical Categories of Hormones
1. Peptide hormones
1. Are chains of 100 or fewer amino acids
2. Includes insulin and antidiuretic hormone

2. Glycoproteins
1. Polypeptides longer than 100 amino acids connected to carbohydrates
2. Follicle-stimulating hormone, luteinizing hormone

3. Amines
1. Derived from tyrosine and tryptophan
2. Secreted by adrenal medulla, thyroid and pineal glands
3. Catecholamines are secreted by the adrenal medulla
1. Include epinephrine, norepinephrine
2. Derived from tyrosine

4. Thyroxine secreted by thyroid gland
1. Derived from tyrosine

5. Melatonin secreted by pineal gland
1. Derived from tryptophan

4. Steroid hormones
1. Lipids derived from cholesterol
2. Sex steroids
1. Secreted by testes, ovaries, placenta, adrenal cortex
2. Include testosterone, estradiol, progesterone

3. Corticosteroids
4. Secreted by only by adrenal cortex
5. Include cortisol (glucose balance), aldosterone (salt balance)

3. Neural and Endocrine Interactions
1. Secretory activity of endocrine gland often controlled by nervous system
2. Include adrenal medulla, posterior pituitary and pineal glands
3. Glands are derived from neural ectoderm, same tissue that forms nervous system
4. Major site for neural regulation is the anterior pituitary gland
1. Hypothalamus controls hormonal secretions of anterior pituitary
2. Anterior pituitary in turn regulates other endocrine glands

5. Other hormone activity is independent of neural control
1. Release of insulin by pancreas and aldosterone by adrenal cortex
2. Stimulated by increases in blood concentrations of glucose and potassium respectively

3. Autocrine and Paracrine Regulation
1. Autocrine and Paracrine Regulation Occurs in Organs and Immune System Cells
1. Regulatory molecules may be called cytokines
1. May regulate different cells of immune system
2. Lymphokines are produced by lymphocytes

2. Many paracrine regulators called growth factors
1. Promote growth and cell division in specific organs
2. Examples: Platelet-derived growth factor, epidermal growth factor, insulin-like growth factor
3. Nerve growth factors belong to neurotrophin family

3. Nitric oxide is a paracrine regulator
1. Also functions as neurotransmitter
2. When produced by endothelium of blood vessels, considered paracrine
3. Diffuses to smooth muscle layer of blood vessel, promotes vasodilation

4. Endothelium of blood vessels produces other paracrine regulators
1. Endothelin: Stimulates vasoconstriction
2. Bradykinin promotes vasodilation
3. Supplements vascular control by the autonomic nerves

2. Paracrine Regulation by Prostaglandins
1. Chemistry of prostaglandin
1. 20-carbon-long fatty acid with a five-carbon ring
2. Derive from arachidonic acid
3. Released from phospholipids in cell membrane by hormonal or other stimulation

2. Produced in almost every organ, participate in a variety of regulatory functions
3. Immune system
1. Promote aspects of inflammation, including pain and fever
2. Alleviate symptoms with drugs that inhibit its synthesis

4. Reproductive system
1. Play role in ovulation
2. Excess involved in premature labor, endometriosis, dysmenorrhea

5. Digestive system
1. Produced by stomach and intestines
2. Inhibit gastric secretions, influence intestinal motility, fluid absorption

6. Respiratory system
1. Some cause constriction of blood vessels in lungs, smooth muscle in bronchioles
2. Others cause dilation

7. Circulatory system
1. Needed for proper functioning of blood platelets in blood clotting

8. Urinary system
1. Produced in renal medulla, cause vasodilation
2. Result in increased renal blood flow, increased urine excretion

9. Function inhibited by nonsteroidal antiinflammatory drugs (NSAIDs)
1. Aspirin is most widely used, also indomethacin and ibuprofen
2. Drugs inhibit enzyme that produces prostaglandins from arachidonic acid
3. Inhibit inflammation and associated pain
4. May also produce gastric bleeding and prolonged clotting time

53.2 Lipophilic and polar hormones regulate their target cells by different means

1. Hormones that Enter Cells
1. Solubility of Hormones
1. Lipophilic hormones are lipid-soluble, polar hormones are water-soluble
2. Lipophilic molecules
1. Include steroids and thyroxine fig 53.4
2. Include regulatory molecules retinoids or vitamin A
3. Easily enter cells, lipid portion is not a barrier

3. Water-soluble hormones do not easily pass through membranes
4. Steroids are lipids, thus lipid-soluble
5. Thyroxine is lipophilic since it is derived from a nonpolar amino acid

2. Mechanism of Action of Steroid Hormones
1. Do not dissolve in plasma, attach to protein carriers
2. Dissociate from carrier at target cell, pass through plasma membrane fig 53.5
1. Some bind to receptor in cytoplasm, complex moves into nucleus
2. In others, receptor is in nucleus, hormone must enter first

3. Both types of steroids have same action when in nucleus
1. Complex binds to specific regions of DNA in nucleus, the hormone response elements
2. Initiates transcription of specific genes
3. Resulting messenger RNA directs synthesis of proteins
4. May be enzymes that alter metabolism of target cell

3. Mechanism of Action of Thyroid Hormone
1. Mechanism resembles that of steroid hormones
2. Thyroxine contains four iodines, called tetraiodothyronine, T₄
3. Thyroid also produces triiodothyronine, three iodines, T₃
4. Both enter cells, T₄ that enters is changed into T₃ fig 53.6
1. T₃ enters nucleus and binds to receptor protein
2. Complex binds to hormone response elements in DNA

2. Hormones that Do Not Enter Cells
1. Polar Molecules Cannot Cross Plasma Membrane of Target Cells
1. Include peptide and glycoprotein hormones, epinephrine, norepinephrine
2. Bind to receptor molecules on outer surface of plasma membrane
1. Requires second messenger within target cell to produce action
2. Include particular organic molecules and Ca⁺⁺

3. Increases concentration of second messengers in target cell cytoplasm
4. Binding is reversible and usually brief
1. Dissociates from receptor after second messenger activated
2. May be carried by blood to another target cell
3. Eventually degraded by enzymes in the liver

2. The Cyclic AMP Second Messenger System
1. Epinephrine binds to à- andá-adrenergic receptors each activates a different system
2. When epinephrine binds to á-adrenergic receptors on liver cell membrane fig 53.7
1. Cyclic AMP (adenosine monophosphate) is second messenger
1. First system described in early 1960s

2. Binding to receptor causes one G protein subunit to dissociate from other two
3. Released subunit diffuses within plasma membrane
4. Encounters adenyl cyclase, normally inactive membrane-bound enzyme
5. G protein subunit activates adenyl cyclase
6. Activated adenyl cyclase produces cAMP from ATP
7. cAMP leaves inner surface of membrane, diffuses within cytoplasm
8. Binds and activates protein kinase-A
9. Protein kinase-A adds phosphate groups to specific cellular proteins

3. Proteins phosphorylated by protein kinase-A vary by cell type
1. Variation results in diverse effects of epinephrin on different tissues
2. Liver cells: Activates phosphorylase, converts glycogen to glucose
1. Causes liver to secrete glucose, fight-or-flight reaction
2. Occurs when adrenal medulla stimulated by autonomic nervous system

3. Cardiac muscle cells: Activates proteins that cause heart to beat faster, harder

3. The IP₃/Ca⁺⁺ Second Messenger System
1. When epinephrine binds to à-adrenergic receptors
1. Works through a different G protein
2. Activates another membrane-bound enzyme, phospholipase C fig 53.8
3. Cleaves certain membrane phospholipids
4. Produces second messenger inositol triphosphate (IP₃)
5. Diffuses from membrane into cytoplasm
6. Binds to receptors on surface of endoplasmic reticulum
1. ER accumulates Ca⁺⁺ by actively transporting it out of cytoplasm
2. Other pumps transport Ca⁺⁺ from cytoplasm to extracellular fluid
3. Very steep concentration gradient between cytoplasm and inside of ER
4. Another steep gradient between cytoplasm and extracellular fluid

7. IP₃ binds to receptors on ER, stimulates it to release Ca⁺⁺
8. Ca⁺⁺ may also enter cytoplasm through opened membrane calcium channels
9. Ca⁺⁺ in cytoplasm binds to calmodulin, has regulatory functions like cAMP
10. Calmodulin activates another protein kinase to phosphorylate different proteins

2. Advantage of multiple second messenger systems
1. Example: Antagonistic actions of epinephrin and insulin on liver cells
2. Epinephrine uses cAMP as second messenger to convert glycogen to glucose
3. Insulin promotes conversion of glucose to glycogen
4. Thus, insulin cannot use cAMP as second messenger
5. Insulin may in part utilize IP₃/Ca⁺⁺ second messenger system

53.3 The hypothalamus controls the secretions of the pituitary gland

1. The Posterior Pituitary Gland
1. Structure of the Pituitary Gland
1. Pituitary gland is located in brain
1. Hangs by stalk below the hypothalamus fig 53.9
2. Posterior to optic chiasma

2. Composed of two independently functioning glands
1. Anterior pituitary appears glandular
2. Posterior pituitary appears fibrous

3. Different embryonic origins, secrete different hormones, regulated by different systems

2. The Posterior Pituitary Gland
1. Appears fibrous, contains axons
1. Cell bodies of axons originate in hypothalamus
2. Extend along stalk of pituitary as tract of fibers

2. Embryonic origins
1. Floor of third ventricle of brain forms hypothalamus
2. Part of neural tissue grows downward producing posterior pituitary
3. Hypothalamus and posterior pituitary connected by axons

3. Secretions of the posterior pituitary
1. Discovered in patient with a bullet lodged in the gland
1. Had to urinate every 30 minutes through the whole day
2. Removal of gland produces same symptoms
3. Discovered antidiuretic hormone

2. Antidiuretic hormone (ADH) = vasopressin fig 53.10
1. Stimulates kidney water retention
2. Damage or alcohol causes excessive urination

3. Oxytocin
1. Peptide hormone composed of nine amino acids
2. Stimulates contraction of smooth muscles around mammary glands
3. Initiates milk release with suckling
4. Stimulates uterine contraction during childbirth

4. Both hormones synthesized inside neuron cells in hypothalamus
1. Transported down axons to synapses in pituitary
2. Stored in posterior pituitary
3. Released into blood stream with nerve stimulus
4. Secretion of hormones as neuroendocrine reflexes

2. The Anterior Pituitary Gland
1. Origin and Secretions of the Anterior Pituitary Gland
1. Derived from pouch of epithelium that pinches off from roof of mouth
1. Not derived from brain tissue
2. Is a complete gland, produces the hormones it secretes

2. Many hormones stimulate growth of target organ, including other endocrine glands
1. Are called tropic hormones or tropins

3. When target gland is another endocrine gland, gland is stimulated by tropic hormone to secrete its own hormones
4. Summary of hormones secreted by anterior pituitary fig 53.11
1. Growth hormone (GH or somatotropin)
1. Promotes growth of bone, muscle, other tissues
2. Essential for proper metabolic regulation

2. Adrenocorticotropic hormone (ACTH or corticotropin)
1. Stimulates adrenal gland to produce corticosteroid hormones
2. Include cortisol and corticosterone to regulate glucose homeostasis

3. Thyroid-stimulating hormone (TSH)
1. Stimulates thyroid to produce thyroxin
2. Thyroxin stimulates oxidative respiration

4. Luteinizing hormone (LH)
1. Needed for ovulation, formation of corpus luteum in females
2. Stimulates testes to produce testosterone in males
3. Testosterone needed for sperm production, secondary sex characteristics

5. Follicle-stimulating hormone (FSH)
1. Required for development of ovarian follicles in females
2. Required for sperm development in males
3. FSH and LH are called gonadotropins

6. Prolactin (PRL)
1. Stimulates breasts to produce milk
2. Helps regulate kidney function
3. Production of "cropmilk" in some birds
4. In fish that travel between fresh and salt water, acts on gills of to promote sodium retention

7. Melanocyte-stimulating hormone (MSH)
1. Stimulates melanin pigment in fish, amphibians and reptiles
2. No known function in mammals
3. High levels of ACTH causes skin darkening, has MSH sequence in its structure

2. Growth Hormone
1. Anterior pituitary initially associated with growth disorders
1. Surgical removal corrects acromegaly
2. Tumors cause gigantism fig 53.12
3. Gigantism caused by excessive secretion of growth hormone (GH) in growing child
1. Stimulates protein synthesis, growth of muscles and connective tissues
2. Promotes bone elongation in epiphyseal growth plates
3. Stimulation is indirect
4. GH stimulates production of insulin-like growth factors by liver
5. Causes growth of bones and tissue

4. Causes acromegaly when skeletal growth plates are sealed in adults
5. Deficiency in childhood causes pituitary dwarfism

3. Other Anterior Pituitary Hormones
1. Prolactin
1. Acts on organs that are not endocrine glands (like GH)
2. Stimulates milk production in mammals and "crop milk" in birds
3. Has varied effects on electrolyte balance in kidneys, fish gills and salt glands of marine birds

2. Other anterior pituitary glands act on specific glands (unlike PRL and GH)
3. Thyroid-stimulating hormone (TSH) also called thyrotropin
1. Stimulates only thyroid gland

4. Adrenocorticotropic hormone (ACTH) stimulates only adrenal cortex
5. FSH and LH act only on gonads, gonadotropic hormones
1. Act on different target cells in both females and males

4. Hypothalamic Control of Anterior Pituitary Gland Secretion
1. Anterior pituitary not derived from brain, has no axon tract from hypothalamus
2. Control is via hormones not nerve impulses
3. Neurons in hypothalamus secrete releasing and inhibiting hormones
1. Carried by blood directly to anterior pituitary fig 53.13
2. Transported inside short blood vessels that connect two beds of capillaries
3. One bed in hypothalamus, other in anterior pituitary
4. Called hypothalamo-hypophyseal portal system
5. Travel in blood from hypothalamus directly to anterior pituitary

4. Each releasing factor is specific for one tropic hormone
1. Thyrotropin-releasing hormone (TRH) stimulates release of TSH
2. Corticotropin-releasing hormone (CRH) stimulates release of ACTH
3. Gonadotropin-releasing hormone (GnRH) stimulates FSH and LH
4. Growth-hormone-releasing hormone (GHRH) recently discovered
5. Prolactin releasing hormone postulated but not yet identified

5. Also secretes hormones that inhibit release of certain anterior pituitary hormones
1. Somatostatin inhibits secretion of GH
2. Prolactin-inhibiting hormone (PIH) inhibits secretion of prolactin
3. Melanotropin-inhibiting hormone (MIH) inhibits secretion of MSH

5. Negative Feedback Control of Anterior Pituitary Gland Secretion
1. Hypothalamus no longer considered to be "master gland"
1. Adrenal medulla and pancreas not controlled by this system
2. Hypothalamus and anterior pituitary are themselves controlled by hormones

2. End hormones feed back to regulate glands that control their release fig 53.14
3. This is an example of negative feedback inhibition
4. Example: Hormonal control of thyroid gland fig 53.15
1. TRH from hypothalamus stimulates anterior pituitary to secrete TSH
2. TSH stimulates tyroid to release thyroxine
3. Thyroxine acts on many target organs including
1. Hypothalamus to inhibits TRH secretion
2. Anterior pituitary to inhibit TRH secretion

5. Negative feedback inhibition necessary for homeostasis
6. Example: Insufficient dietary iodine
1. Thyroid cannot produce thyroxine which contains iodine
2. Blood thyroxine levels very low
3. Less feedback inhibition to hypothalamus and anterior pituitary
4. Causes increased secretion of TRH and TSH
5. Stimulates thyroid to grow, but without iodine still no thyroxine
6. Causes an enlarged thyroid, a goiter fig 53.16

7. Positive feedback rare, cannot maintain consistency of internal environment
1. Only one example with regard to hypothalamus and anterior pituitary
2. Positive feedback accentuates change, drives change in same direction
3. Example: Control of ovulation
1. Explosive event ends in expulsion of egg cell from ovary
2. Ovarian hormone estradiol stimulates secretion of LH

53.4 Endocrine glands secrete hormones that regulate many body functions

1. The Thyroid and Parathyroid Glands
1. The Thyroid Gland
1. Located in front of the neck, just below Adam's apple
2. Produces thyroxine, smaller amounts of triiodothyronine (T₃)
1. Stimulates oxidative respiration, helps set body's metabolic rate
2. In children, promotes growth and stimulates maturation of nervous system
1. Children with under active thyroids have stunted growth, mental retardation
2. Condition called cretinism

3. Can supplement deficiencies (hypothyroid) with oral thyroxine
1. Only thyroxine and steroid hormones available orally
2. Are nonpolar, pass through plasma membrane of intestine cells without digestion

4. Additional function in only amphibians
1. Required for metamorphosis into adults fig 53.7
2. Removal of thyroid halts development into adult
3. Immature tadpoles fed thyroid gland will develop early into miniature frog

5. Thyroid also produces calcitonin
1. Helps maintain proper level of Ca⁺⁺ in blood
2. If blood Ca⁺⁺ is too high, calcitonin stimulates its uptake into bones
3. Significance in human physiology controversial
4. Parathyroid control more important in regulating Ca⁺⁺

2. The Parathyroid Glands and Calcium Homeostasis
1. Four small glands attached to thyroid
1. First experimentation on dogs
2. Removal caused blood Ca⁺⁺ to plummet
3. Returned to normal with addition of parathyroid gland extract
4. Over administration caused levels to rise far above normal
5. Calcium phosphate in bones dissolved

2. Produces parathyroid hormone (PTH)
1. One of two hormones absolutely essential for survival (aldosterone is other)
2. Synthesized and released when Ca⁺⁺ levels in blood get low
1. Ca⁺⁺ required for muscle contraction
2. Extreme low levels cause muscle spasms
3. Important for functioning of muscles, heart, nervous and endocrine systems

3. Cause osteoclasts to dissolve bone with subsequent Ca⁺⁺ release fig 53.18
4. Stimulates kidneys to reabsorb calcium from urine
5. Activates vitamin D to absorb Ca⁺⁺ from intestine
1. Vitamin D is inactive form of a hormone
2. Activated with gain of two OH groups, one enzyme in liver, one in kidneys
3. Enzyme in kidneys stimulated by PTH
4. Produces 1,2,5-dihydroxy vitamin D hormone
5. Stimulates Ca⁺⁺ uptake in intestines
6. Vitamin D deficiency causes rickets, poor bone formation

2. The Adrenal Glands
1. Two Glands in One fig 53.19
1. Adrenal glands located above each kidney
2. Composed of inner adrenal medulla and outer adrenal cortex

2. The Adrenal Medulla
1. Receives neural input from axons of sympathetic division of autonomic nervous system
2. With stimulation secretes epinephrine and norepinephrine
3. Triggers alarm response similar to sympathetic division, prepare for fight or flight
4. Responses: Faster heartbeat, increased blood pressure,dilated bronchioles, increased blood sugar, reduced blood flow to skin and digestive organs
5. Supplement actions of norepinephrine released as sympathetic neurotransmitter

3. The Adrenal Cortex: Homeostasis of Glucose and Na⁺
1. Produces cortisol (hydrocortisone) and related steroids
1. Maintains glucose homeostasis, thus called glucocorticoids
2. Stimulate breakdown of muscle proteins into amino acids, carried to liver
1. Stimulates liver to produce enzymes to convert amino acids to glucose, gluconeogenesis
2. Important during fasting

3. Modulate some aspects of the immune response
1. Suppress immune system in immune disorders like rheumatoid arthritis
2. Reduces inflammation

2. Produces aldosterone, a mineralocorticoid, helps regulate mineral balance
1. Acts on kidney to promote uptake of Na⁺
1. Na⁺ needed for normal blood volume and pressure
2. Prevents excess loss of Na⁺ thus Cl^_ and water from the urine
3. Loss of salt and water causes fall in blood volume and pressure

2. Promotes secretion of K⁺ in urine
1. Low aldosterone levels, blood K⁺ rises to dangerous levels

3. Removal of adrenals is fatal without hormone therapy

3. The Pancreas
1. Structure and Function of the Pancreas
1. Located adjacent to stomach, connected to duodenum by pancreatic duct
2. Also secretes bicarbonate ions and various digestive enzymes
3. Though to be just exocrine until clusters of cells identified
1. Called islets of Langerhans
2. Surgical removal caused glucose to appear in urine
3. Diabetes mellitus resulted

4. Insulin produced by á cells of islets
1. Isolated in 1922 from an extract of purified beef pancreas
2. First success with insulin therapy

5. Two types of diabetes mellitus
1. Type I: Insulin dependent
1. Lack insulin secreting b cells
2. Treated with insulin injections (cannot be oral since a peptide hormone)
3. Insulin used to come from pigs or cattle
4. Now use human insulin from genetically engineered bacteria
5. Ongoing research with transplant of b cells is encouraging

2. Type II: Non-insulin dependent
1. Insulin levels normal or high
2. Too few receptors in target tissue
3. Must control diet and exercise

6. Glucagon produced by a cells of islets
1. Action is antagonistic to insulin

7. The two hormones interact to regulate glucose fig 53.20
1. Eating carbohydrates increases blood glucose levels
2. Stimulates b cells to produce insulin, inhibits a cell production of glucagon
3. Insulin promotes cellular uptake of glucose
1. Stored as glycogen by liver and muscle cells
2. Stored as fat in adipose cells

4. Concentration of blood glucose falls in between meals
5. Insulin secretion decreased ,glucagon produced
6. Promotes hydrolysis of glycogen in liver and fat in adipose tissue
7. Glucose and fatty acids released into blood stream, used for energy

4. Other Endocrine Glands
1. Sexual Development, Biological Clocks and Immune Regulation in Vertebrates
1. Ovaries and testes produce steroid sex hormones
1. Estrogen, progesterone secreted by ovaries
1. Regulate menstrual cycle
2. Also secreted by placenta during pregnancy

2. Testosterone, other androgens promote protein synthesis
1. Responsible for larger body mass of males compared to females
2. Illegal use in body-builders, causes liver disorders
3. Stimulates breast development in males, liver converts androgens to estrogens

2. Pineal gland fig 47.24
1. Located in roof of third ventricle of brain
2. Size of pea, shape of pine cone
3. Evolved from "third eye", responds to light in fish, amphibians and reptiles
4. Secretes melatonin
5. Blanches skin of lower vertebrates by reducing dispersal of melanin granules
6. Secretion stimulated by activity of hypothalamic suprachiasmatic nucleus (SCN)
1. May be involved in reproductive system control in some vertebrates
2. Associated with light/dark cycles, may be involved with daily biorhythms

3. Thymus produces T lymphocytes and hormones regulating the immune system
4. Right atrium of heart secretes atrial natriuretic hormone
1. Stimulates kidney to excrete salt and water in urine
2. Antagonistic to aldosterone

5. Kidneys produce erythropoietin
1. Stimulates bone marrow to produce red blood cells

6. Skin secretes vitamin D

2. Molting and Metamorphosis in Insects
1. Influenced by hormonal secretions
2. Prior to molting neurosecretory cells on brain surface secrete brain hormone
1. In turn stimulates prothoracic gland to make molting hormone, ecdysone fig 53.21
2. Juvenile hormone produced by corpora allata near brain
3. Both hormones present for molting

3. Juvenile hormone determines result of any particular molt
1. Levels high, molt produces another larva
2. At end of metamorphosis, levels decrease, molt produces pupa
3. Eventually adult insect produced