Chapter 18: The Endocrine System

the glands and parts of glands that secrete hormones that integrate and control the body's metabolic activity. Endocrine glands include the pituitary, thyroid, parathyroids, adrenals, pancreas, ovaries, and testes.

The Endocrine System Defined:

Endocrine System

• Regulates long-term processes:– growth– development– reproduction

• Uses chemical messengers to relay information and instructions between cells

What are the modes of intercellular communication used by the

endocrine and nervous systems?

Direct Communication

• Exchange of ions and molecules between adjacent cells across gap junctions

• Occurs between 2 cells of same type• Highly specialized and relatively rare

Paracrine Communication

• Paracrine• Uses chemical signals to transfer information

from cell to cell within single tissue• Most common form of intercellular

communication

Endocrine Communication

• Endocrine cells release chemicals (hormones) into bloodstream

• Alters metabolic activities of many tissues and organs simultaneously

Target Cells

• Are specific cells that possess receptors needed to bind and “read” hormonal messages

Hormones

• Stimulate synthesis of enzymes or structural proteins

• Increase or decrease rate of synthesis• Turn existing enzyme or membrane channel

“on” or “off”

Endocrine System

• Is unable to handle crisis management•

• split-second responsesNervous System

How do the cellular components of the endocrine system

compare with those of other tissues and systems?

Endocrine System

• Includes all endocrine cells and body tissues that produce hormones or paracrine factors

Endocrine Cells

• Glandular secretory cells that release their secretions into extracellular fluid (blood and lymph)

Exocrine Cells

• Secrete their products onto epithelial surfaces• Use ducts instead of circulation

What are the major structural classes of hormones?

Hormones

• Can be divided into 3 groups: – amino acid derivatives– peptide hormones – lipid derivatives

Amino Acid Derivatives

• Small molecules structurally related to amino acids

• Synthesized from the amino acids tyrosine and tryptophan

Tyrosine Derivatives

• Thyroid hormones• Compounds:– epinephrine (E)– norepinephrine (NE)– dopamine, also called catecholamines

Tryptophan Derivative

• Melatonin: – produced by pineal gland

Peptide Hormones

• Chains of amino acids• Synthesized as prohormones:– inactive molecules converted to active hormones

before or after secretion

2 Groups of Peptide Hormones

• Group 1: – glycoproteins: • more than 200 amino acids long, with carbohydrate side

chains:– thyroid-stimulating hormone (TSH) – luteinizing hormone (LH) – follicle-stimulating hormone (FSH)

2 Groups of Peptide Hormones

• Group 2: – all hormones secreted by:• hypothalamus• heart• thymus• digestive tract• pancreas• posterior lobe of pituitary gland• anterior lobe of pituitary gland

2 Classes of Lipid Derivatives

• Eicosanoids: – derived from arachidonic acid

• Steroid hormones: – derived from cholesterol

Eicosanoids

• Are small molecules with five-carbon ring at one end

• Are important paracrine factors• Coordinate cellular activities• Affect enzymatic processes in extracellular

fluids

Leukotrienes

• Are eicosanoids released by activated white blood cells, or leukocytes

• Important in coordinating tissue responses to injury or disease

Prostaglandins

• A second group of eicosanoids produced in most tissues of body

• Are involved in coordinating local cellular activities

Steroid Hormones

• Are lipids structurally similar to cholesterol• Released by:– reproductive organs (androgens by testes,

estrogens, and progestins by ovaries)– adrenal glands (corticosteroids)– kidneys (calcitriol)

Steroid Hormones

• Remain in circulation longer than peptide hormones

• Are absorbed gradually by liver• Are converted to soluble form• Are excreted in bile or urine

Figure 18–2

Classes of Hormones

Hormones

• Circulate freely or bound to transport proteins

Free Hormones

• Remain functional for less than 1 hour:– diffuse out of bloodstream:• bind to receptors on target cells

– are absorbed:• broken down by cells of liver or kidney

– are broken down by enzymes:• in plasma or interstitial fluids

Thyroid and Steroid Hormones

• Remain in circulation much longer • Enter bloodstream:– more than 99% become attached to special

transport proteins

Bloodstream

• Contains substantial reserve of bound hormones

What are the general mechanisms of hormonal action?

Hormone Receptor

• Is a protein molecule to which a particular molecule binds strongly

• Responds to several different hormones

Cells

• Different tissues have different combinations of receptors

• Presence or absence of specific receptor determines hormonal sensitivity

Catecholamines andPeptide Hormones

• Are not lipid soluble• Unable to penetrate cell membrane• Bind to receptor proteins at outer surface of

cell membrane (extracellular receptors)

Eicosanoids

• Are lipid soluble• Diffuse across membrane to reach receptor

proteins on inner surface of membrane (intracellular receptors)

Hormone

• Binds to receptors in cell membrane• Cannot have direct effect on activities inside

target cell• Uses intracellular intermediary to exert effects

Intracellular Intermediaries

• First messenger:– leads to second messenger– may act as enzyme activator, inhibitor, or cofactor – results in change in rates of metabolic reactions

Important Second Messengers

• Cyclic-AMP (cAMP): – derivative of ATP

• Cyclic-GMP (cGMP): – derivative of GTP

• Calcium ions

G Protein

• Enzyme complex coupled to membrane receptor

• Involved in link between first messenger and second messenger

• Activated when hormone binds to receptor at membrane surface

Figure 18–3

G Proteins and Hormone Activity

G Protein

• Changes concentration of second messenger cyclic-AMP (cAMP) within cell

• Increased cAMP level accelerates metabolic activity within cell

Increased cAMP Levels (1 of 2)

1. Activated G protein:– activates enzyme adenylate cyclase

2. Adenylate cyclase:– converts ATP to cyclic-AMP

Increased cAMP Levels (2 of 2)

3. Cyclic-AMP (second messenger):– activates kinase

4. Activated kinases affect target cell:– depends on nature of proteins affected

Figure 18–3

G Proteins and Hormone Activity

Lower cAMP Levels

1. Activated G protein stimulates PDE activity2. Inhibits adenylate cyclase activity3. Levels of cAMP decline4. cAMP breakdown accelerates; cAMP

synthesis is prevented

Figure 18–3

G Proteins and Hormone Activity

G Proteins and Calcium Ions (1 of 2)

• Activated G proteins trigger:– opening of calcium ion channels in membrane– release of calcium ions from intracellular stores

G Proteins and Calcium Ions (2 of 2)

1. G protein activates enzyme phospholipase C (PLC)

2. Enzyme triggers receptor cascade:– production of diacylglycerol (DAG) and inositol

triphosphate (IP3) from membrane phospholipids

Steroid Hormones

• Cross cell membrane• Bind to receptors in cytoplasm or nucleus,

activating or inactivating specific genes

Figure 18–4a

Steroid Hormones

Steroid Hormones

• Alter rate of DNA transcription in nucleus:– change patterns of protein synthesis

• Directly affect metabolic activity and structure of target cell

Thyroid Hormones

Figure 18–4b

Thyroid Hormones

• Cross cell membrane:– primarily by transport mechanism

• Bind to receptors in nucleus and on mitochondria: – activating specific genes– changing rate of transcription

How are endocrine organs controlled?

Endocrine Reflexes

• Functional counterparts of neural reflexes• In most cases, controlled by negative feedback

mechanisms

Simple Endocrine Reflex

• Involves only 1 hormone• Controls hormone secretion by:– Heart (ANF)– Pancreas (insulin, glucagon)– parathyroid gland (calcitonin)– digestive tract

Complex Endocrine Reflex

• Involves:– 1 or more intermediary steps– 2 or more hormones• Stimulating, releasing, inhibitory hormones

Figure 18–1

Endocrine System

Figure 18–5

Hypothalamus

Hypothalamus (1 of 2)

• Integrates activities of nervous and endocrine systems in 3 ways:

1. Secretes regulatory hormones: – special hormones control endocrine cells in

pituitary gland

Hypothalamus (2 of 2)

2. Acts as an endocrine organ 3. Contains autonomic centers:– exert direct neural control over endocrine cells

of adrenal medullae• Neuroendocrine response

Neuroendocrine Reflexes

• Pathways include both neural and endocrine components

Complex Commands

• Issued by changing:– amount of hormone secreted– pattern of hormone release • (continuous, pulsed)

Pulses

• Hypothalamic and pituitary hormones released in sudden bursts

• Frequency varies response of target cells

Where is the pituitary gland located, and

what is its relationship to the hypothalamus?

Figure 18–6

Pituitary Gland

Pituitary Gland

• Also called hypophysis• Lies within sella turcica• Hangs inferior to hypothalamus:– connected by infundibulum

Diaphragma Sellae

• Locks pituitary in position• Isolates it from cranial cavity

Pituitary Gland

• Releases 9 important peptide hormones• Hormones bind to membrane receptors:– use cAMP as second messenger

Anterior Lobe

• Also called adenohypophysis:

Median Eminence

• Swelling near attachment of infundibulum• Where hypothalamic neurons release

regulatory factors:– into interstitial fluids– through fenestrated capillaries

Figure 18–7

Median Eminence

Portal Vessels

• Blood vessels link 2 capillary networks • Entire complex is portal system

Hypophyseal Portal System

• Ensures that regulatory factors reach intended target cells before entering general circulation

What are the hormones produced by the anterior lobe, and what are the functions of those hormones?

2 Classes of Hypothalamic Regulatory Hormones

1. Releasing hormones 2. Inhibiting hormones

Figure 18–8a

Hypothalamic Regulatory Hormones

• Rate of secretion is controlled by negative feedback

Figure 18–8b

Hypothalamic Regulatory Hormones

Releasing Hormones (RH)

• Stimulate synthesis and secretion of 1 or more hormones at anterior lobe

Inhibiting Hormones (IH)

• Prevent synthesis and secretion of hormones from anterior lobe

Figure 18–8a

Anterior Lobe

• Hormones “turn on” endocrine glands or support other organs

Thyroid-Stimulating Hormone (TSH)

• Also called thyrotropin• Triggers release of thyroid hormones

Adrenocorticotropic Hormone (ACTH)

• Also called corticotropin • Stimulates release of steroid hormones by

adrenal cortex • Targets cells that produce glucocorticoids

Gonadotropins

• Regulate activities of gonads (testes, ovaries)• Follicle-stimulating hormone • Luteinizing hormone

Follicle-Stimulating Hormone (FSH) (1 of 2)

• Also called follitropin • Stimulates follicle development and estrogen

secretion in females

Follicle-Stimulating Hormone (FSH) (2 of 2)

• Stimulates sustentacular cells in males: – promotes physical maturation of sperm

• Production inhibited by inhibin: – peptide hormone released by testes and ovaries

Luteinizing Hormone (LH)

• Also called lutropin • Causes ovulation and progestin production in

females• Causes androgen production in males

FSH and LH Production

• Stimulated by gonadotropin-releasing hormone (GnRH) from hypothalamus:– GnRH production inhibited by estrogens,

progestins, and androgens

Prolactin (PRL)

• Also called mammotropin• Stimulates development of mammary glands

and milk production• Production inhibited by prolactin-inhibiting

hormone (PIH)

Prolactin (PRL)

• Stimulates PIH release• Inhibits secretion of prolactin-releasing factors

(PRF)

Figure 18–8b

Prolactin (PRL)

Growth Hormone (GH)

• Also called somatotropin • Stimulates cell growth and replication • Production regulated by:– growth hormone–releasing hormone (GH–RH)– growth hormone–inhibiting hormone (GH–IH)

Melanocyte-Stimulating Hormone (MSH)

• Also called melanotropin • Stimulates melanocytes to produce melanin • Inhibited by dopamine

Melanocyte-Stimulating Hormone (MSH)

• Secreted during:– fetal development– early childhood– pregnancy– certain diseases

What hormones are produced by the posterior lobe, and

what are their functions?

Posterior Lobe

• Also called neurohypophysis • Contains unmyelinated axons of hypothalamic

neurons• Supraoptic and paraventricular nuclei

manufacture:– antidiuretic hormone (ADH)– oxytocin (OT)

Antidiuretic Hormone

• Decreases amount of water lost at kidneys• Elevates blood pressure• Release inhibited by alcohol

Oxytocin

• Stimulates contractile cells in mammary glands

• Stimulates smooth muscles in uterus• Secretion and milk ejection are part of

neuroendocrine reflex

Where is the thyroid gland located, and

what is its structure?

Thyroid Gland

• Lies anterior to thyroid cartilage of larynx • Consists of 2 lobes connected by narrow

isthmus

Thyroid Gland

Figure 18–10a, b

Thyroid Follicles

• Hollow spheres lined by cuboidal epithelium• Cells surround follicle cavity that contains

viscous colloid

Thyroid Follicles

• Surrounded by network of capillaries that:– deliver nutrients and regulatory hormones– accept secretory products and metabolic wastes

Thyroid Follicles

Figure 18–11a, b

Thyroglobulin

• Globular protein• Synthesized by follicle cells• Secreted into colloid of thyroid follicles• Molecules contain amino acid tyrosine

Figure 18–10c

Thyroglobulin

What hormones are produced by the thyroid gland?

Thyroxine (T4)

• Also called tetraiodothyronine• Contains 4 iodide ions

Triiodothyronine (T3)

• Contains 3 iodide ions

Rate of Thyroid Hormone Release

• Major factor: – TSH concentration in

circulating blood

Figure 18–11b

Thyroid-Binding Globulins (TBGs)

• Transport proteins• Attach to most T4 and T3 molecules

Transthyretin

• Also called thyroid-binding prealbumin (TBPA)• Is a transport protein• Attaches to most remaining T4 and T3

molecules

Unbound Thyroid Hormones

• Diffuse out of bloodstream and into other tissues

• Disturb equilibrium• Carrier proteins release more thyroid

hormones until new equilibrium is reached

Thyroid-Stimulating Hormone (TSH)

• Absence causes thyroid follicles to become inactive:– neither synthesis nor secretion occur

• Binds to membrane receptors• Activates key enzymes in thyroid hormone

production

What are the functions of thyroid hormones, and what are

the results of abnormal levels of thyroid hormones?

Thyroid Hormones

• Enter target cells by transport system• Affect most cells in body• Bind to receptors in:– cytoplasm– surfaces of mitochondria– nucleus

Calorigenic Effect

• Cell consumes more energy resulting in increased heat generation

Thyroid Hormones

• In children, essential to normal development of:– skeletal, muscular, and nervous systems

Thyroid Gland

• Is responsible for strong, immediate, and short-lived increase in rate of cellular metabolism

Thyroid Gland

Table 18–3

Iodide Ions

• Are actively transported into thyroid follicle cells:– stimulated by TSH

• Reserves in thyroid follicles• Excess removed from blood at kidneys• Deficiency limits rate of thyroid hormone

production

C (Clear) Cells

• Produce calcitonin (CT): – helps regulate concentrations of Ca2+ in body fluids

Where are the parathyroid glands?

Parathyroid Glands

• Embedded in posterior surface of thyroid gland

Figure 18–12

What is the function of the hormone produced by the parathyroid glands?

Parathyroid Hormone (PTH)

• Produced by chief cells • In response to low concentrations of Ca2+

4 Effects of PTH1. It stimulates osteoclasts:– accelerates mineral turnover– releases Ca2+ from bone

2. It inhibits osteoblasts:– reduces rate of calcium deposition in bone

3. It enhances reabsorption of Ca2+ at kidneys, reducing urinary loss

4. It stimulates formation and secretion of calcitriol at kidneys

Calcitriol

• Effects complement or enhance PTH• Enhances Ca2+, PO4

3— absorption by digestive tract

Parathyroid Glands

• Primary regulators of blood calcium I levels in adults

Figure 18–13

What are the location, structure, and general functions of the

adrenal gland?

Adrenal Glands

• Lie along superior border of each kidney• Subdivided into superficial adrenal cortex and

an inner adrenal medulla

Adrenal Cortex

• Subdivided into 3 regions: 1. zona glomerulosa2. zona fasciculate3. zona reticularis

What hormones are produced by the adrenal glands, and

what are their functions?

Zona Glomerulosa

• Outer region of adrenal cortex• Produces mineralocorticoids: – e.g., aldosterone

Aldosterone

• Stimulates:– conservation of sodium ions– elimination of potassium ions

• Increases sensitivity of salt receptors in taste buds

Aldosterone

• Secretion responds to:– Rise in blood Na+ – drop in blood K+ concentration– blood volume– blood pressure

Zona Fasciculata

• Produces glucocorticoids• Endocrine cells are larger and contain more

lipids than zona glomerulosa

Zona Fasciculata

• Secretes cortisol (hydrocortisone) with corticosterone:– liver converts cortisol to cortisone

Glucocorticoids

• Secretion regulated by negative feedback• Have inhibitory effect on production of:– corticotropin-releasing hormone (CRH) in

hypothalamus– ACTH in anterior lobe

Glucocorticoids

• Accelerate glucose synthesis and glycogen formation

• Show anti-inflammatory effects:– inhibit activities of white blood cells and other

components of immune system

Zona Reticularis

• Network of endocrine cells• Forms narrow band bordering each adrenal

medulla• Produces androgens under stimulation by

ACTH

Adrenal Medullae

• Secretory activities controlled by sympathetic division of ANS

• Produces epinephrine (adrenaline) and norepinephrine

• Metabolic changes persist for several minutes

Where is the pineal gland, and what are the functions of

the hormone it produces?

Pineal Gland

• Lies in posterior portion of roof of third ventricle

• Contains pinealocytes: – synthesize hormone melatonin

Functions of Melatonin

• Inhibiting reproductive functions• Protecting against damage by free radicals• *Setting circadian rhythms – (your 24 hr cycle)

Where is the pancreas, and what is its structure?

Pancreas

• Lies between:– inferior border of stomach– and proximal portion of small intestine

• Contains exocrine and endocrine cells

Figure 18–15

Pancreas

Endocrine Pancreas

• Cells form clusters: – pancreatic islets, or islets of Langerhans

What hormones are produced by the pancreas, and

what are their functions?

4 Types of Cells in Pancreatic Islets

• Alpha cells:– produce glucagon

• Beta cells:– secrete insulin

Blood Glucose Levels

• When levels rise: – beta cells secrete insulin, stimulates transport of

glucose across cell membranes• When levels decline: – alpha cells secrete glucagons, stimulates glucose

release by liver

Insulin

• Is a peptide hormone released by beta cells• Affects target cells

5 Effects of Insulin

1. Accelerates glucose uptake2. Accelerates glucose utilization and enhanced

ATP production3. Stimulates glycogen formation4. Stimulates amino acid absorption and

protein synthesis5. Stimulates triglyceride formation in adipose

tissue

Glucagons

• Released by alpha cells • Mobilize energy reserves• Affects target cells

3 Effects of Glucagons

1. Stimulates breakdown of glycogen in skeletal muscle and liver cells

2. Stimulates breakdown of triglycerides in adipose tissue

3. Stimulates production of glucose in liver

What are the functions of hormones produced by the kidneys, heart,

thymus, testes, ovaries, and adipose tissue?

Intestines

• Produce hormones important to coordination of digestive activities

Kidneys

• Produce the hormones calcitriol and erythropoietin

• Produce the enzyme (hormone?) renin

Calcitriol

• Stimulates calcium and phosphate ion absorption along digestive tract

Figure 18–17a

Effects of Calcitriol on Calcium Metabolism

• Stimulates formation and differentiation of osteoprogenitor cells and osteoclasts

• Stimulates bone resorption by osteoclasts

Effects of Calcitriol on Calcium Metabolism

• Stimulates Ca2+ reabsorption at kidneys• Suppresses parathyroid hormone (PTH)

production

Erythropoietin (EPO)

• Stimulates red blood cell production by bone marrow: – increased RBCs elevate blood volume

Renin

• Converts angiotensinogen to angiotensin I in liver

• Angiotensin I is converted to angiotensin II (in lungs via ACE)

Angiotensin II

1. Stimulates adrenal production of aldosterone

2. Stimulates pituitary release of ADH 3. Promotes thirst4. Elevates blood pressure

The Renin–Angiotensin System

Figure 18–17b

Heart

• Produces atrial natriuretic peptides (ANP and BNP):– when blood volume becomes excessive

Natriuretic Peptide

• Action opposes angiotensin II• Resulting in reduction in blood volume and

blood pressure

Thymus

• Produces thymosin hormones: – that helps develop and maintain normal immune

defenses– Promotes T cell maturation

Testes

• Produce androgens in interstitial cells: – testosterone:• is most important male hormone

• Secrete inhibin in sustentacular cells: – support differentiation and physical maturation of

sperm

Ovaries

• Produce estrogens:– principle estrogen is estradiol

• After ovulation, follicle cells:– reorganize into corpus luteum– release estrogens and progestins, especially

progesterone

Adipose Tissue Secretions

1. Leptin: – feedback control for appetite– controls normal levels of GnRH, gonadotropin

synthesis

2. Resistin: – reduces insulin sensitivity

Chapter 16

Autonomic Nervous System

Autonomic Nervous System (ANS)

• Operates without conscious instruction• Part of peripheral nervous system• Coordinates systems functions:– cardiovascular– respiratory– digestive– urinary– reproductive

Visceral Motor Neurons

• In brain stem and spinal cord, are known as preganglionic neurons

• (Ganglia- clusters of neuronal cell bodies and their dendrites)

Preganglionic Fibers

• Axons of preganglionic neurons

• Leave CNS and synapse on ganglionic neurons

Autonomic Ganglia

• Peripheral ganglia• Contain many ganglionic

neurons• Ganglionic neurons

innervate visceral (pertaining

to the internal organs) effectors: – cardiac muscle– smooth muscle– glands– adipose tissues

Postganglionic Fibers

• Axons of ganglionic neurons

• Begin at autonomic ganglia:– extend to peripheral

target organs

What are the divisions and functions of the ANS?

Sympathetic Division

• “Kicks in” only during exertion, stress, or emergency

Parasympathetic Division• Controls during resting

conditions

Divisions of the ANS

• 2 divisions may work together or independently:

Sympathetic Division

• Preganglionic fibers (thoracic and superior lumbar) synapse in ganglia near spinal cord

• Preganglionic fibers are short• Postganglionic fibers are long

Sympathetic Chain

• 3 cervical ganglia• 10–12 thoracic ganglia• 4–5 lumbar ganglia• 4–5 sacral ganglia• 1 coccygeal ganglion

7 Responses to Increased Sympathetic Activity (Fight or Flight)

1. Heightened mental alertness2. Increased metabolic rate3.Reduced digestive and urinary functions4.Energy reserves activated

1. 5. Increased respiratory rate and respiratory passageways dilate

5. 6. Increased heart rate and blood pressure6. 7. Sweat glands activated

Parasympathetic Division

• Preganglionic fibers originate in brain stem and sacral segments of spinal cord

• Synapse in ganglia close to (or within) target organs

• Preganglionic fibers are long• Postganglionic fibers are short

Parasympathetic-Rest and Reposeor Breed, Feed, & Read

• Parasympathetic division stimulates visceral activity

• Conserves energy and promotes sedentary activities

Pattern of Responses to Increased Levels of Parasympathetic Activity

• Decreased: – metabolic rate– heart rate and blood pressure

• Increased: – salivary and digestive glands secretion– motility and blood flow in digestive tract– Urination and defecation stimulation

Figure 16–3

ANS: Sympathetic Division

Structure of the Sympathetic Division

• Preganglionic neurons located between segments T1 and L2 of spinal cord

• Ganglionic neurons in ganglia near vertebral column

Figure 16–4

Ganglionic Neurons

• Occur in 3 locations:– sympathetic chain ganglia– collateral ganglia– adrenal medullae

Collateral ganglia

Figure 16–4c

The Adrenal Medullae

Modified Sympathetic Ganglion• At the center of each adrenal gland in area

known as adrenal medulla • Very short axons• When stimulated, release neurotransmitters

into bloodstream (not at synapse)• Functions as hormones affect target cells

throughout body

The Adrenal Medullae

What are the mechanisms of neurotransmitter release in the

sympathetic division?

Neuroendocrine Cells of Adrenal Medullae

• Secrete hormones epinephrine (E) and norepinephrine (NE)

Epinephrine

• Also called adrenaline • Is 75–80% of secretory output• Remaining is noradrenaline (NE)

Distribution

• Bloodstream carries hormones through body• Causing changes in metabolic activities of

different cells

Differences from Sympathetic Postganglionic Fiber Stimulation

• Cells not innervated by sympathetic postganglionic fibers

• Effects last longer: – hormones continue to diffuse out of bloodstream

Sympathetic Division

• Can change activities of tissues and organs by:– releasing NE at peripheral synapses– distributing E and NE throughout body in

bloodstream

Crisis Mode

• Entire division responds (sympathetic activation)

• Are controlled by sympathetic centers in hypothalamus

• Effects are not limited to peripheral tissues• Alters CNS activity as well

Cholinergic Synapses

• Use ACh as transmitter• Excitatory effect on ganglionic neurons

Adrenergic Neurons

• Use NE as neurotransmitter

NE Released by Varicosities

• Affects targets until reabsorbed or inactivated• 50–80% of NE is reabsorbed by varicosities:– is reused or broken down by MAO

• The rest diffuses out or is broken down by COMT

Duration of Effects on Postsynaptic Membrane

• NE persist for a few seconds• ACh only for 20 msec

Effects of Sympathetic Stimulation

• Primarily from interactions of NE and E with adrenergic membrane receptors

2 Classes of Sympathetic Receptors

• Alpha receptors• Beta receptors

Norepinephrine

• Stimulates alpha receptors to greater degree than beta receptors

Epinephrine

• Stimulates both classes of receptors

Localized Sympathetic Activity

• Involves release of NE at varicosities• Primarily affects alpha receptors near active

varicosities

Generalized Sympathetic Activation

• Release of Epinephrine by adrenal medulla • Affect alpha and beta receptors throughout

body

Alpha and Beta Receptors

• G proteins

What are the effects of sympathetic neurotransmitters on

target organs and tissues?

Stimulation of Alpha (a) Receptors

• Activates enzymes on inside of cell membrane

Alpha Receptors

• Alpha-1 (1)

• Alpha-2 (2)

Beta Receptors

• Two types:– Beta-1 (b1)

– Beta-2 (b2)

Beta-1 (b1)

• Increases metabolic activity

Beta-2 (b2)

• Causes inhibition• Triggers relaxation of smooth muscles along

respiratory tract

Beta-3 (b3)

• Is found in adipose tissue• Leads to lipolysis, the breakdown of

triglycerides in adipocytes• Releases fatty acids into circulation

Sympathetic Postganglionic Fibers

• Mostly adrenergic (release NE)• A few cholinergic (release ACh)• Innervate sweat glands of skin and blood

vessels of skeletal muscles and brain• Stimulate sweat gland secretion and dilates

blood vessels

ACh

• Released by parasympathetic division• Body wall and skeletal muscles are not

innervated by parasympathetic division• Both NE and ACh needed to regulate visceral

functions

What are the structures and functions of the parasympathetic

division of the autonomic nervous system?

Figure 16–7

ANS: The Parasympathetic Division

Autonomic Nuclei

• Are contained in the mesencephalon, pons, and medulla oblongata:– associated with cranial nerves III, VII, IX, X

• In lateral gray horns of spinal segments S2–S4

Ganglionic Neurons in Peripheral Ganglia

• Preganglionic fiber synapses on 6–8 ganglionic neurons:– terminal ganglion:• near target organ• usually paired

– intramural ganglion: • embedded in tissues of target organ• interconnected masses• clusters of ganglion cells

Pattern of Parasympathetic Division

• All ganglionic neurons in same ganglion• Postganglionic fibers influence same target

organ• Effects of parasympathetic stimulation more

specific and localized

What are the mechanisms of neurotransmitter release in the

parasympathetic division?

Parasympathetic Preganglionic Fibers

• Leave brain as components of cranial nerves:– III (oculomotor)– VII (facial)– IX (glossopharyngeal)– X (vagus)

Figure 16–8

Parasympathetic Innervation

Vagus Nerve

• Preganglionic parasympathetic innervation to structures in:– neck– thoracic and abdominopelvic cavity– distal portion of large intestine

Vagus Nerve

• Provides 75% of all parasympathetic outflow• Branches intermingle with fibers of

sympathetic division

Sacral Segments of Spinal Cord

• Preganglionic fibers carry sacral parasympathetic output

• Do not join ventral roots of spinal nerves

Pelvic Nerves

• Innervate intramural ganglia in walls of:– kidneys– urinary bladder– portions of large intestine– sex organs

What are the effects of parasympathetic

neurotransmitters on target organs and tissues?

Parasympathetic Activation

• Centers on relaxation, food processing, and energy absorption

• Localized effects, last a few seconds at most

10 Effects of Parasympathetic Activation

1. Constriction of pupils:– restricts light entering eyes

2. Secretion by digestive glands:– exocrine and endocrine

10 Effects of Parasympathetic Activation

3. Secretion of hormones4. Changes in blood flow and glandular activity:– associated with sexual arousal

10 Effects of Parasympathetic Activation

5. Increases smooth muscle activity:– along digestive tract

6. Defecation:– stimulation and coordination

10 Effects of Parasympathetic Activation

7. Contraction of urinary bladder:– during urination

8. Constriction of respiratory passageways

10 Effects of Parasympathetic Activation

9. Reduction in heart rate:– and force of contraction

10. Sexual arousal:– stimulation of sexual glands

Anabolic System

• Stimulation increases nutrient content of blood

• Cells absorb nutrients

Parasympathetic Neurons

• All release ACh as neurotransmitter• Effects vary widely

ACh

• Inactivated by by enzyme acetylcholinersterase (AChE) at synapse

• Ach is also inactivated by pseudocholinesterase in surrounding tissues

2 Types of ACh Receptors on Postsynaptic Membranes

• Nicotinic receptors• Muscarinic receptors

Nicotinic Receptors

• On surfaces of ganglion cells (sympathetic and parasympathetic)

• At neuromuscular junctions of somatic nervous system

Action of Nicotinic Receptors

• Exposure to ACh causes excitation of ganglionic neuron or muscle fiber

• Open chemically gated channels in postsynaptic membrane

Dangerous Environmental Toxins

• Muscarine:– binds to muscarinic receptors– targets parasympathetic neuromuscular or

neuroglandular junctions

Summary: Parasympathetic Division

• Ganglionic neurons located in or next to target organs

Summary: Parasympathetic Division

• Innervates areas served by:– cranial nerves– organs in thoracic – organs in abdominopelvic cavities

Summary: Parasympathetic Division

• All neurons are cholinergic:– ganglionic neurons have nicotinic receptors,

excited by ACh– muscarinic receptors at neuromuscular or

neuroglandular junctions produce either excitation or inhibition

Summary: Parasympathetic Division

• Effects of stimulation are brief and restricted to specific organs and sites

Comparing Sympathetic and Parasympathetic Divisions

• Sympathetic:– widespread impact– reaches organs and tissues throughout body

• Parasympathetic:– innervates only specific visceral structures

Figure 16–9

Differences between Sympathetic and Parasympathetic Divisions

What is the relationship between the two divisions of the autonomic nervous

system, and the significance of dual innervation?

Dual Innervation

• Most vital organs receive instructions from both sympathetic and parasympathetic divisions

• 2 divisions commonly have opposing effects

Anatomy of Dual Innervation

• Parasympathetic postganglionic fibers accompany cranial nerves to peripheral destinations

• Sympathetic innervation reaches same structures by traveling directly from superior cervical ganglia of sympathetic chain

Structure: Autonomic Plexuses

• Nerve networks in the thoracic and abdominopelvic cavities:– are formed by mingled sympathetic

postganglionic fibers and parasympathetic preganglionic fibers

• Travel with blood and lymphatic vessels that supply visceral organs

6 Autonomic Plexuses

1. Cardiac plexus2. Pulmonary plexus3. Esophageal plexus4. Celiac plexus5. Inferior mesenteric plexus6. Hypogastric plexus

Why is autonomic tone important?

Autonomic Tone

• Is an important aspect of ANS function:– if nerve is inactive under normal conditions, can

only increase activity– if nerve maintains background level of activity,

can increase or decrease activity

Autonomic Tone and Dual Innervation

• Significant where dual innervation occurs:– 2 divisions have opposing effects

• More important when dual innervation does not occur