Chapter 41

Biology Lecture Notes

The activities of the various specialized parts of an animal are coordinated by the two major systems of internal communication: the nervous system and the endocrine system.

*The nervous system is involved with high-speed messages.

*The endocrine system is slower and involves the production,

release, and movement of chemical messages.

I. A variety of chemical signals coordinate body funcitons: an

overview

All animals exhibit some coordination by chemical signals:

*Hormones produced by the endocrine system convey

information between organs of the body.

*Pheromones are chemical signals used to communicate

between different individuals.

*Other messengers, such as neurotransmitters, act

between cells on a localized scale.

A. Hormones

The endocrine system has 3 key components: hormones, endocrine glands, and target cells with their molecular receptors for hormones.

Hormone=Secreted by endocrine or neurosecretory cells, a chemical signal that travels in body fluids to target cells where it elicits a specific response.

Neurosecretory cell=neuron that receives signals from other nerve cells and responds by releasing hormones into body fluids or into a storage organ from which they are later released.

Target cell=A cell equippped to respond to a given hormone.

Endocrine glands=Ductless glands that secrete hormones into the body fluids for distribution through the body.

*Composed of the specialized cells which secrete

hormones.

*May comprise only a portion of organs that produce

other materials (e.g. pancreas).

Exocrine glands=Glands that produce a variety of substances (e.g. sweat, mucus, digestive enzymes) and convey their products by means of ducts.

*Many organs perform both exocrine and endocrine

functions.

*Exocrine glands are not a part of the endocrine system.

Hormones are grouped into two general classes according to similarities in chemical structure: steroid hormones and hormones derived from amino acids.

*Steroid hormones are lipid molecules fashioned from

cholesterol.

*Includes the sex hormones.

*Hormones derived from amino acids may be small or

large. Includes

*Amine hormones (modified versions of single

amino acids).

*Peptide hormones (short chains of amino acids).

*Protein hormones.

*Glycoproteins.

A hormone’s target cell possesses a hormone receptor to which the hormone molecule must bind.

*The hormone receptor is a protein within the cell or on

the plasma membrane of the target cell.

*The hormone and receptor are shape specific and must

bind together to elicit a response in the cell to the

hormone.

The actions of antagonistic hormones are important in maintaining homeostasis.

*Feedback mechanisms are usually involved in adjusting

hormone actions.

B. Pheromones

Pheromones=Chemical signals that functin between animals of the same species.

*Classified according to function (e.g. mate attractant,

territorial maker, alarm substance).

*Are small, volatile, easily dispersed molecules that are

active in minute amounts.

*Have an important role in social behavior.

C. Local Regulators

Local regulators are chemical messengers that affect target cells adjacent to or near their point of secretion. Includes synaptic signaling and paracrine signaling.

*Synaptic signaling is direct and involves the release of a

neuron’s transmitter into a synapse.

*Paracrine signaling is less direct but very localized.

*Regulatory substances are released into interstitial

fluids but affect only nearby target cells (i.e.

histamine and interleukins).

Local regulators may be gases produced by cells, growth factors, or prostaglandins.

1. Gases produced by cells. Many types of cells produce gases which can act as neurotransmitters and paracrine signals.

*Nitric oxide (NO) released by endothelial cells of

blood vessels makes the adjacent smooth muscle

cells relax, dilating the vessel.

*Nitric oxide released by white blood cells kills

certain cancer cells and bacteria in the body fluids.

2. Growth Factors

Growth factors=Peptides and proteins that regulate the behavior of cells in growing and developing tissues.

*Must be present in the extracellular environment

for certain cell types to grow and develop

normally.

*Studied mainly in cultures of mammalian cells but

also regulate development within the animal body.

*Binding of the growth factor to a specific receptor

protein on the outer membrane surface triggers

the target cell’s response – a change in gene

expression in the target-cell nucleus

3. Prostaglandins

Prostaglandins (PGs)=Modified fatty acids released into interstitial fluid that function as local regulators.

*Often derived from lipids of the plasma

membrane.

*Very subtle differences in their molecular structure

profoundly affect how these signals affect target

cells (e.g. antagonistic actions of PGE and PGF).

*PGs secreted by the placenta help induce labor

during childbirth by causing chemical changes in

the nearby uterine muscles.

*Other PGs help defend the body by inducing fever

and inflammation.

II. Hormone binding to specific receptors triggers signaling mechanisms at the cellular level

Hormones can act in very low concentration and can affect different target cells in an animal or different species of animals very differently.

*A hormone-induced change in a target organ cell results

from one of two possible signal transduction pathways.

*Signal transduction pathways include a series of steps

between hormone-receptor binding and target-cell

response.

A. Steroid Hormones and Gene Expression

Steroid hormones enter the target cell by passing through the target cell membrane and binding to a receptor protein in the cytoplasm. (See Campbell, Figure 41.3)

*The hormone-receptor complex then has the proper

configuration to bind to specific regulatory sites located

along the genome.

*Binding of the hormone-receptor complex to regulatory

sites may induce or suppress expression of specific

genes.

*When expression is stimulated, mRNA molecules are

produced, exit to the cytoplasm, and direct the synthesis

of new proteins.

*Two cell types can respond differently to a single

hormone since the hormone-receptor complex probably

binds with regulatory sites controlling different genes in

the two kinds of cells.

Steroid hormones primarily affect the synthesis of proteins.

B. Peptide Hormones and Signal Transduction

Peptide hormones and most hormones derived from amino acids are unable to pass through the target cell plasma membrane, they thus require a different mechanism of action.

*These hormones attach to specific receptor proteins

embedded in the target cell’s plasma membrane.

*The receptors are components of a signal-transduction

pathway that converts the extracellular signal of the

hormone to intracellular signals that alter the target-

cell’s behavior.

Peptide hormones usually affect the activity of enzymes and other proteins already present in the cell.

III. Many chemical signals are relayed and amplified by second messenger and protein kinases.

Earl W. Sutherland and his colleagues discovered the signal-transduction pathway used by nonsteriod hormones.

*They discovered that a second messenger transmits the signal from the plasma membrane to the metabolic machinery in the cytoplasm.

A. Cyclic AMP

Cyclic AMP (cyclic adenosine monophosphate) was discovered as the second messenger in Sutherland’s studies of how epinephrine stimulates glycogen hydrolysis in liver and muscle cells.

*Binding of a hormone with its receptor activates a G

protein, which binds with GTP (guanosine triphosphate),

an energy carrying molecule.

*Activated G protein binds to adebylyl cyclase, an effector

enzyme embedded in the plasma membrane.

*Adenylyl cyclase catalyzes the conversion of ATP to

cyclic AMP (cAMP).

*cAMP relays the signal from the membrane to the

metabolic machinery of the cytoplasm.

*A second type of G protein inhibits adenylyl cyclase,

when activated by and inhibitory hormone-receptor

complex. This reduces cAMP concentration in the

cytoplasm.

*Regulation of adenylyl cyclase by the opposing effects of

two G proteins enables a cell to respond to slight

changes in the proportions of antagonistic hormones.

B. Signal Amplification and Specificity

Many metabolic responses to hormones involve an enzyme cascade, where each step activates an enzyme that in turn activates the next enzyme in the series. For example, a cell’s response to epinephrine:

*cAMP activates a cAMP dependent protein kinase (an

enzyme that catalyzes that transfer of a phosphate group

from ATP to proteins = phosphorylation),which in turn

can activate phosphorylase kinase.

*Phosphorylase kinase adds a phosphate group to

glycogen phosphorylase which actually hydrolyzes

glycogen.

*The cascade thus amplifies response to the hormone

since the number of activated products increases with

each step.

In addition to working as a second messenger, cAMP has been found to inhibit or stimulate key enzyme components of the signal-transduction pathway of many growth factors.

C. Cytoplasmic Ca2+ and Signal Transduction

Many chemical messengers in animals (neurotransmitters, growth factors, some hormones) stimulate responses in their target cells by increasing the calcium ion concentration of the cytoplasm.

*The second messenger in this pathway is inositol triphosphate.

Inositol triphosphate (IP3) acts as follows: (See Campbell, Figure 41.7)

*The hormone (first messenger) binds to its receptor,

activating a G protein (different from those that function in the

cAMP system) that in turn stimulates an enzyme in the plasma

memrane called phospholipase C.

*Phospholipase C cleaves a membrane phospholipid into IP3

and diacylglycerol: both may function as second messengers.

*Diacylglycerol stimulates another membrane enzyme, protein

kinase C, which triggers various target cell responses by

phosphorylating specific proteins.

*IP3 released from the plasma membrane stimulates

movement of Ca2+ from the ER into the cytoplasm.

*The Ca2+ regulates cellular protein activity either by itself or

by first binding to calmodulin which inds to other proteins.

*The signal is thus transmitted to various enzymes and other

proteins by the increased concentration of Ca2+ in the

cytoplasm.

*The target cell then produces a specific response to the

original hormonal signal.

IV. Invertebrate control systems often integrate endocrine and nervous system functions

Invertebrates posses a diversity of hormones which function in homeostasis, reproduction, development and behavior.

In many cases, the hormone may stimulate one activity while inhibiting another.

*In Hydra, one hormone stimulates growth and budding

while inhibiting sexual reproduction.

*In the sea slug, a peptide hormone secreted by

specialized neurons stimulates egg laying while inhibiting

feeding and locomotion.

All arthropods have extensive endocrine systems that regulate growth and reproduction, water balance, pigment movement in the integument and eyes, and metabolism.

Hormones may act together such as in the case of insect development and molting.

*Ecdysone is secreted by a pair of prothoracic glands; it

triggers molting and favors development of adult

characteristics and metamorphosis.

*Ecdysone stimulates transcription of specific

genes, the same mechanism of action as

vertebrate steroid hormones.

*Brain hormone promotes development by stimulating

production of ecdysone.

*Brain hormone and ecdysone are balanced by juvenile

hormone (JH), which actively promotes retention of

larval characteristics; JH is secreted by the corpora

allata.

*When JH levels decrease, ecdysone-induced molting

produces a pupa; in the pupa, adult anatomy replaces

larval anatomy during metamorphosis.

V. The hypothalamus and pituitary integrate many functions of the verterate endocrine system.

Vertebrates possess a number of tissues and organs that secrete hormones. (See Campbell, Figure 41.9 and Table 41.1)

*Some hormones affect most tissues of the body (e.g. sex hormones).

*Some hormones affect only one or a few tissues.

*Some affect other endocrine glands; these are called tropic hormones.

A. Structural and Functional Relationships of the Hypothalamus and the Pituitary Gland

The hypothalamus is a region of the lower brain that receives information from nerves throughout the body and brain and initiates endocrine signals appropriate to the environmental conditions.

*Contains two sets of neurosecretory cells whose

secretions are stored in or regulate activity of the

pituitary gland.

The pituitary gland is an extension of the brain located at the base of the hypothalaus. It consists of two lobes and has numerous endocrine functions. (See Campbell, Figure 41.10)

1. The neurohypophysis, or posterior pituitary, stores and secretes two peptide hormones which are made by the hypothalamus: oxytocin and antidiuretic hormone.

*These hormones act directly on muscles of the

uterus and on the kidneys.

2. The adenohypophysis, or anterior pituitary, is composed of non-nervous tissues and synthesizes its own hormones.

*Controlled by two kinds of hormones secreted by

neurosecretory cells in the hypothalamus:

releasing hormones and inhibiting hormones.

*Releasing hormones stimulate the anterior

pituitary to secrete its hormones.

*Inhibiting hormones stop the anterior pituitary

from secreting its hormones.

B. Posterior Pituitary Hormones

The hypothalamic peptide hormones oxytocin and antidiuretic hormone are stored in and relased from the posterior pituitary.

*they are synthesized in the hypothalamus and secreted into the posterior pituitary.

Oxytocin induces uterine muscle contraction and cuases the mammary glands to eject milk during nursing.

Antidiuretic hormone (ADH) is part of the feedback mechanism that helps regulate blood osmolarity.

*It acts on the kidneys to increase water retention which

results in a decrease in urine volume.

*Osmoreceptors (specialized nerve cells) in the

hypothalamus monitor blood osmolarity.

*When plasma osmolarity increases, the osmoreceptors

shrink slightly (lose water by osmosis) and transmit

nerve impulses to certain hypothalamic neurosecretory

cells.

*These neurosecretory cells respond by releasing ADH

into the general circulation from their tips in the

posterior pituitary.

*The target cells for ADH are cells lining the collecting

ducts of nephrons in the kidneys.

*ADH binds to receptors on the target cells and activates

a signal-transduction pathway that increases the water

permeability of the collecting duct.

*Water retention is increased as water exits the collecting

ducts and enters nearby capillaries.

*The osmoreceptors also stimulate a thirst drive.

*Drinking water reduces blood osmolarity to the set

point.

*As more dilute blood (lower osmolarity) arrives at the

brain, the hypothalamus responds by reducing ADH

secretion and lowering thirst sensations.

*This prevents over compensation by stopping

hormone secretion and quenching thirst.

Note that this negative feedback scheme includes a hormonal

action and a behavioral response (drinking).

C. Anterior Pituitary Hormones

The adenohypophysis produces many different hormones and is regulated by releasing factors from the hypothalamus.

*Four are tropic hormones that stimulate other endocrine

glands to synthesize and release their hormones.

Growth hormone (GH) is a protein hormone which affects a wide variety of tissues.

*It promotes growth of some tissues directly and

promotes growth of others indirectly by stimulating the production of growth factors by other tissues.

*For example, GH stimulates the liver to secrete insulin-

like growth factors (IGFs) which stimulate bone and

cartilage growth.

Prolactin (PRL) is a protein hormone similar in structure to GH although their physiological roles are very different.

*PRL produces a diversity of effects in different

vertebrates:

*It stimulates mammary gland development and milk

synthesis in mammals.

*Regulates fat metabolism and reproduction in birds.

*Delays metamorphosis and may function as a larval

growth hormone in amphibians.

*Regulates salt and water balance in freshwater fish.

Follicle-stimulating hormone (FSH) is a tropic hormone which

affects the gonads (=gonadotropin).

*In males, it is necessary for spermatogenesis.

*In females, it stimulates ovarian follicle growth.

Luteinizing hormone (LH) is another gonadotropin.

*It stimulates ovulation and corpus luteum formation in

females and spermatogenesis in males.

Thyroid-stimulating hormone is a tropic hormone which stimulates the tyroid gland to produce and secrete its own hormones.

The remaining hormones from the anterior pituitary are formed by the cleaving of a single large protein, pro-opiomelanocortin, into short fragments.

*At least three of these fragments become active peptide

hormones:

*Adrenocorticotropin (ACTH) stimulates the adrenal

cortex to produce and secrete its steroid hormones.

*Melanocyte-stimulating hormone (MSH) regulates the

activity of pigment-containing skin cells in some

vertebrates.

*Endorphins inhibit pain perception.

D. Hypothalamic Hormones

Regulation of the anterior pituitary by the hypothalamus involves releasing hormones produced by neurosecretory cells in the hypothalamus into capillaries.

*The releasing hormones are secreted into capillaries that

drain into short portal vessels that subdivide into a

second capillary bed within the anterior pituitary.

*These hormones then travel through the second

capillary network within the anterior pituitary, where

they stimulate or inhibit release of specific hormones by

pituitary cells.

*Every hormone produced by the anterior pituitary is

controlled by at least one releasing hormone; some

have both a releasing and an inhibiting hormone.

VI. The verterbrate endocrine system coordinates homeostasis and regulaes growth, development, and reproduction

There are several endocrine glands that play major roles in the control of homeostasis, growth, development, and reproductive functions of vertebrates.

A. The Thyroid Gland

The thyroid gland consists of tow lobes located on the ventral surface of the trachea in mammals, on the two sides of the pharynx in other vertebrates.

*Produces two hormones, T3 (triiodothyronine) and T4

(thyroxine or tetraiodothyronine) derived from the

amino acid tyrosine.

*These differ in structure by only one iodine atom.

*Both have the same effects on their target although T3

is usually more reactive than T4 in mammals.

The thyroid gland plays a major role in vertebrate development and maturation.

*Thyroid hormones control metamorphosis in amphibians.

*Normal function of bone-forming cells and the branching

of nerve cells during embryonic brain development of

nonhuman animals also requires the presence of the

thyroid hormones.

The thyroid gland is critical for maintaining homeostasis in mammals.

*Help maintain normal blood pressure, heart rate, muscle

tone, digestion, and reproductive functions.

*T3 and T4 tend to increase the rate of oxygen

consumption and cellular metabolism.

*Serious metabolic disorders can result from a deficiency

or excess of thyroid hormones.

*Hyperthyroidism, excessive secretion of thyroid

hormones, causes high body temperature,

sweating, weight loss, irritability and high blood

pressure.

*Hypothyroidism, low secretion of thyroid

hormones, can cause cretinism in infants and

weight gain, lethargy and cold-intolerance in

adults.

*Goiter (enlarged thyroid) is caused by a dietary

iodine deficiency and thyroid hormones cannot by

synthesized.

Thyroid hormone secretin is regulated by the hypothalamus

and pituitary through a negative feedback system.

*The hypothalamus secretes TRH (TSH-releasing

hormone) which stimulates TSH (thyroid-stimulating

hormone) secretion by the anterior pituitary.

*When TSH binds to receptors in the thyroid, cAMP is

generated and triggers release of T3 and T4.

*High levels of T3, T4, and TSH inhibits TRH secretion.

The thyroid gland in mammals also produces and secretes calcitonin which is a peptide hormone that lowers blood calcium levels.

B. The Parathyroid Glands

Four parathyroid glands are embedded in the surface of the thyroid and funciton in the homeostasis of calcium ions.

*They secrete PTH (papathyroid hormone) which raises

blood Ca2+ levels (antagonistic to calcitonin) and needs

vitamin D to function.

*PTH stimulates Ca2+ reabsorption in the kidney and

induces osteoclasts to decompose bone and release

Ca2+ into the blood.

PTH and calcitonin (which have opposite affects) work in an antagonistic manner and their balance in the body maintains the proper blood calcium levels.

C. The Pancreas

The pancreas is composed primarily of exocrine tissue which produces digestive enzymes; however, scattered among the exocrine tissue are clusters of endocrine cells called the islets of Langerhans.

*Each islet is composed of alpha cells, which secrete the

peptide hormone glucagon, and beta cells which secrete

the hormone insulin.

Glucagon and insulin work together in an antagonistic manner to regulate the concentration of glucose in the blood. (See Campbell, Figure 41.13)

*Blood glucose levels must remain near 90 mg/mL in

humans for proper body function.

*At glucose levels above the set point, insulin is secreted

and lowers blood glucose concentration by stimulating

body cells to take up glucose from the blood.

*It also slows glycogen breakdown in the liver and

inhibits the conversion of amino acids and fatty

acids to sugar.

*When blood glucose levels drop below the set point,

glucagon is secreted and increases blood glucose

concentrations by stimulating the liver to increase the

hydrolysis of glycogen, convert amino acids and fatty

acids to glucose, and slowly release glucose into the

blood.

Glucose homeostasis is critical due to its function as the major fuel for cellular respiration and a key source of carbon for the synthesis of other organic compounds.

Serious conditions can result when glucose homeostasis is unbalanced. Diabetes mellitus is caused by a deficiency of insulin or a loss of response to insulin in target tissues. Diabetes occurs in two forms:

*Type I diabetes mellitus is an autoimmune disorder that appears in juveniles and is insulin dependent.

*Results from an immune system attack on the cells of the pancreas.

*Usually occurs suddenly during childhood and destroys the ability of the pancreas to produce insulin.

*Treated by insulin injection several times each day.

*Type II diabetes mellitus occurs most frequently in adults over 40.

*May be due to an insulin deficiency but more commonly results from reduced responsiveness in target cells because of changes in insulin receptors.

*Is non-insulin-dependent and can be treated with exercise and dietary controls.

Both types of diabetes mellitus will result in high blood sugar concentrations if untreated.

*Kidneys excrete glucose, resulting in higher concentrations in the urine.

*More water is excreted due to the high concentration of glucose (results in the symptoms of copious urine production accompanied by thirst).

*Fat must serve as the major fuel source for cellular respiration since glucose does not enter the cells.

*In severe cases, acidic metabolites formed during fat metabolism may lower the blood pH to a critical level.

D. The Adrenal Glands

Adrenal glands are located adjacent to kidneys.

*In mammals, each gland has an outer adrenal cortex and inner adrenal medulla which are composed of different cell types, have different functions, and are of different embryonic origin.

*Different arrangements of these same tissues found in other vertebrates.

The adrenal medulla synthesizes and secretes catecholamines (epinephrine and norepinephrine).

*Catecholamines are secreted in times of stress when nerve cells excited by stressful stimuli release the neurotransmitter acetylcholine in the medulla. Acetylcholine combines with cell receptors, stimulating release of epinephrine.

*Norepinephrine is released independently of epinephrine.

*Epinephrine and norepinephrine released into the blood results in rapid and dramatic effects on several targets:

*Glucose is mobilized in skeletal muscle and liver cells.

*Fatty acid release from fat cells is stimulated (may serve as extra energy sources).

*The rate and stroke volume of the heartbeat is increased.

*Blood is shunted away from the skin, gut, and kidneys to the heart, brain, and skeletal muscles by stimulation of smooth muscle contraction in some blood vessels and relaxation in others.

*Oxygen delivery to the body cells is increased by dilation of bronchioles in the lungs.

The adrenal cortex synthesizes and secretes corticosteroids. In humans the two primary types are: glucocorticoids (cortisol) and mineralocorticoids (aldosterone)

*Stressful stimuli cause the hypothalamus to secrete a releasing hormone that stimulates release of ACTH from the anterior pituitary.

*ACTH stimulates release of corticosteroids from the adrenal cortex.

*Glucocorticoids promote glucose synthesis from noncarbohydrate substances such as proteins.

*Skeletal muscle proteins are broken down and the carbon skeletons transported to the liver and kidneys.

*The liver and kidneys convert the carbon to glucose which is released into the blood to increase the fuel supply.

*Also have immunosuppressive effects and are used to treat inflammation.

*Mineralocorticoids affect salt and water balance.

*Aldosterone stimulates kidney cells to reabsorb sodium ions and water from the filtrate.

*This raises blood volume and blood pressure.

*Aldosterone from the RAAS (renin-angiotensin-aldosterone system), ADH, and ANF (atrial natriuretic factor) from the heart form a regulatory complex which influences the kidneys ability to maintain the blood’s ion and water concentrations.

*The RAAS regulates aldosterone secretion in response to changes in plasma ion concentrations.

*Severe stress also stimulates aldosterone secretion by causing the hypothalamus to secrete releasing hormones that increase ACTH secretion from the anterior pituitary that increases aldosterone secretion by the adrenl cortex.

Current evidence indicates glucocorticoids and mineralocorticoids are important to maintaining body homeostasis during extended periods of stress.

A third group of corticosteriods are sex hormones.

*Include androgens (male hormones) similar to testosterone and small amounts of estrogens and progesterone (female hormones).

*The roles f adrenal sex hormones are not well understood.

E. The Gonads

The testes of males and ovaries of females produce steroid hormones that affect growth and development as well as regulates reproductive cycles and behaviors.

The gonads of both males and females produce all three categories of gonadal steroids (androgens, estrogens, and progestins) although the proportions differ.

Androgens generally stimulate the development and maintenance of the male reproductive system.

*Produced in greater quantities in males than females.

*Primary androgen is testosterone.

*Androgens produced during early embryonic development determine whether the fetus will be male or female.

*High androgen concentrations at puberty stimulate development of male secondary sex characteristics.

Estrogens perform the same functions in females as androgens do in males.

*Estradiol is the primary estrogen produced.

*Maintain the female system and stimulate development of female secondary sex characteristics.

Progestins are primarily involved with preparing and maintaining the uterus for reproduction in mammals.

*Include progesterone.

The gonadotopins from the anterior pituitary (FSH and LH) control the synthesis of both androgens and estrogens.

*FSH and LH are in turn controlled by gonadotropin-releasing hormone (GnRH) from the hypothalamus.

F. Other Endocrine Organs

Many organs with primarily nonendocrine functions also secrete hormones.

*The digestive tract secretes at least eight hormones (e.g. gastrin, secretin).

*Kidneys secrete erythropoietin which stimulates red blood cell production.

*Two other important endocrine glands are the pineal gland and the thymus.

The pineal gland is a small mass of tissue near the center of the mammalian brain although it is closer to the surface in some other vertebrates.

*The pineal contains light sensitive cells or has nervous connections with the eyes.

*It secretes melatonin, which is a modified amino acid.

*Melatonin regulates functions related to light and to seasons marked by changes in day length, such as biological rhythms associated with reproduction.

*Melatonin is secreted only at night; larger amounts are thus secreted during the winter than the summer.

*The role of melatonin in regulating biological rhythms is not yet understood.

The thymus is located under the breast-bond in humans.

*It is large in children and reduced in adults.

*It secretes several messengers including thymosin which stimulates T lymphocyte development and differentiation after they leave the thymus.

VII. The endocrine system and the nervous system are structurally, chemically, and functionally related.

The endocrine system and nervous system interact and are often inseparable. Relationships between the two systems may be: structural, chemical, or functional.

Structural relationships result from the fact that many endocrine glands are either made of nerve tissue or evolved from the nervous tissue.

*The hypothalamus and posterior pituitary are nervous tissues that secret hormones into the blood.

*The adrenal medulla evolved from the same cells that produce certain ganglia of the nervous system.

Chemical relationships center around those vertebrate hormones that are used as signals by both systems.

*Norepinephrine function as both and adrenal hormone and as a neurtotransmitter.

Functional relationships relate to the coordination of physiological processes by both nervous and hormonal components arranged in series.

*Each system can affect the output of the other.

References:

Campbell, N. Biology. 4^th ed. Menlo Park, California:

Benjamin/Cummings, 1996.

Marieb, E.N. Human Anatomy and Physiology. 3^rd ed.

Redwood City, California: Benjamin/Cummings, 1995.