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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