Download Keshara Senanayake Audesirk Chapter 33

Keshara Senanayake
Audesirk Chapter 33 - Chemical Control of the animal body: The endocrine system
-in all multicellular organisms individual cells must be in continuous communication with on another
-in some specialized tissues -> (heart muscle) their are GAP JUNCTIONS that directly link the insides of
the cells, allowing the flow of ions and electrical signals.
>commonly cells release chemical signaling molecules sometimes called "messenger molecules" that affect
other cells, either nearby or distant -> directed to specific "target cells"
>to ensure chemical message reaches target cells -> cells have receptors -> specialized protein molecules
that bind only to specific chemical messengers
>receptors may be located either on the plasma membrane or inside target cells -> upon binding to its
receptor, the chemical triggers some type of change within the target cell
>(3) general classes of messenger molecules -> each utilize a different distribution system:
-Local hormones diffuse through the extracellular fluid to cells in the immediate vicinity
-endocrine hormones are released into the blood -> distributed to nearby/distant cells
-neurotransmitters are released across the synaptic cleft between specialized regions of the neuron and its
target
>local hormones called prostaglandins
"How cells communicate"
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Communication
Chemical messengers
Examples
Direct
ions, small molecules
ions, flowing between cardiac muscle cells
Paracrine
local hormones
prostaglandins
Endocrine
hormones
insulin
Synaptic
neurotransmitters
acetylcholine
Mechanism of transmission:
Direct - direct movement through gap functions linking cytoplasms of adjacent cells
Paracrine - diffusion through extracellular fluid to nearby cells bearing receptors
Endocrine - carried in the bloodstream to near or distant cells bearing receptors
Synaptic - diffusion from a neuron across a narrow synaptic cleft to a cell bearing receptors
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Local hormones diffuse to nearby target cells
>most (not all) cells secrete local hormones into their immediate vicinity (such as cytokines that allow
immune cells to communicate)
--> prostaglandins -> modified fatty acids synthesized by cells from membrane phospholipids -> another
type of local hormone --> unlike most hormones (which are synthesized by a limited # of specialized cells)
-> produced by many (if not all) cells of the body --> many uses --> the use of local hormones to
communicate with nearby cells is called "Paracrine communication" whereas "endocrine communication"
utilizes chemicals that travel within the bloodstream often over considerable distances
-hormones of the endocrine are transported by the circulatory system
Keshara Senanayake
>endocrine hormones are chemical messages produced by specialized cells -> released in response to some
stimulus -> carried by circulatory system and influence target cells bearing specific receptors for those
hormones -> changed induced by hormonal messaged may be prolonged/irreversible
>induced changes are transient and reversible -> helping control/regulate the interrelated physiological
systems that compose the animal body -> regulation of body required communication -> hormones provide
communication
>the endocrine system -> consisting of hormones and the various cells that secrete and receive them move
information and instructions between cells that may be some distance apart
>hormones are released by cells of major endocrine glands/organs
>(3) classes of vertebrate endocrine hormones
1) peptide hormones -> made from chains of amino acids
2) amino acid based hormones -> synthesized from single amino acids
3) steroid hormones -> resemble cholesterol (from which most steroid hormones are synthesized)
*****
Hypothalamus - produces ADH and oxytocin, regulatory hormones for anterior pituitary
Pineal gland - melatonin
Pituitary gland:
anterior pituitary: ACTH, TSH, GH, PRL, FSH, LH, and MSH
Posterior gland:
release of oxytocin and ADH
Thyroid gland: thyroxine, calcitonin
Parathyroid glands: (on posterior surface of thyroid gland) parathyroid hormone
Thymus gland: (atrophies during adulthood) thymosins
Heart: atrial natriuretic peptide
Kidneys: erythropoietin
Digestive tract: numerous hormones
Adrenal glands (one at each kidney):
medulla: epinephrine, norepinephrine
Cortex: glucocorticoids (cortisol), aldosterone, testosterone
Pancreas islet cells: insulin, glucagons
Gonads:
testes (male): androgens, especially testosterone
ovaries (female): estrogens, progesterone
Hormones bind to specific receptors on target cells
->hormone released into bloodstream will reach nearly every cell in the body -> hormones act only on
target cells bearing receptors for that particular hormone molecule, thus exerting precise control
>cells that lack appropriate receptor will not respond to the hormonal message --> given hormone may
have several different effects depending on the nature of the receptor on the target cell it contacts
-receptor for hormones are found in (2) locations on target cell: 1) on plasma membrane and inside the cell
2) within the cytoplasm or the nucleus
>many peptide and amino acid-based hormones are soluble in water but not in lipids -> these hormones
cannot penetrate plasma membranes composed of phosolipids -> instead hormones bind to protein
receptors on the target cell's plasma membrane
>"membrane receptors" span the plasma membrane so a hormone binding to the external portion of the
receptor can cause a shape change on the portion of the receptor protein that protrudes into the cell -->
triggers production of a "second messenger" inside cell -> transfers the signal from the first messenger (the
hormone) to other molecules within the cell --> initiating a series of biochemical reactions
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>many molecules act as 2nd messengers -> many cases hormones binding to receptors result in the
conversion of ATP to cyclic AMP (cAMP) a nucleotide that regulates many cellular activities
>cyclic AMP (in its role as 2nd messenger) -> starts a chain of reactions inside the cell -> each reaction
involves an increasing number of molecules amplifying the original signal
>end result varies within the target cell -> channels may be opening in the plasma membrane -> substances
may be synthesized or secreted
>hormones like steroid is lipid soluble so it can pass through the plasma membrane
-hormone release is regulated by feedback mechanism
-the secretion of a hormone stimulates a response in target cells; the response then inhibits further secretion
of the hormone
>focused on endocrine functions of the hypothalamus-pituitary complex, the thyroid and the parathyroid
glands, the pancreas, the sex organs, and the adrenal gland
>2 basic types of glands: exocrine and endocrine
>exocrine glands produce secretions that are released outside the body or into the digestive tract
>exocrine glands secretions are released through tubes or openings called ducts
>exocrine glands include sweat/oil glands on skin (sebaceous glands), tear producing (lacrimal) glands or
eye and the milk producing (mammary) glands -> as well as glands that produce digestive secretions, such
as the salivary glands and some cells of the pancreas
>endocrine glands: called ductless glands
>release their hormones within the body
>an endocrine gland consists of clusters of hormone-producing cells embedded within a network of
capillaries -> cells secrete their hormones into the extracellular fluid surrounding the capillaries ->
hormones enter the capillaries by diffusion and are carried throughout the body by the bloodstream
>the hypothalamus controls the secretions of the pituitary gland
>the hypothalamus is a part of the brain that contains clusters of specialized nerve cells called
neurosecetory cells -> synthesize peptide hormones -> store them -> release them when stimulated
>the pituitary is a pea size gland that dangles from the hypothalamus by a stalk
>anatomically the pituitary gland consist of two distinct parts: the anterior pituitary and the posterior
pituitary
>hypothalamus controls the release of hormones from both parts
--> anterior pituitary is a true endocrine gland, composed of several types of hormone-secreting cells
enmeshed in a network of capillaries
>posterior pituitary is derived from an out-growth of the hypothalamus
>hypothalamic hormones control the anterior pituitary
>neurosecretory cells of the hypothalamus produce at least nine peptide hormones that regulate the release
of hormones from the anterior pituitary
>these peptides are called releasing hormones or inhibiting hormones
>depending on whether they stimulate or inhibit the release of a particular pituitary hormone
>releasing/inhibiting hormones are synthesized in nerve cells in the hypothalamus secreted into the
capillary bed in the lower portion of the hypothalamus ->blood vessel -> 2nd capillary bed that surrounds
the endocrine cells of the anterior pituitary -> there the releasers and inhibitors diffuse out of the capillaries
and influence pituitary hormone secretion
1) neurosecretary cells of hypothalamus produce releasing and inhibiting hormones 2) releasing or
inhibiting hormone secreted into capillaries feeding anterior lobe or pituitary 3) hormone is secreted into
the blood by endocrine cells of anterior pituitary
-anterior pituitary produces and releases a variety of hormones
>the anterior pituitary produces several peptide hormones
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>4 of these regulate hormone production in other endocrine glands
>follicle-stimulating hormone (FSH) and luteinizing hormone (LH) stimulate the production of sperm and
testosterone in males, and of eggs, estrogen, and progesterone in females
>thyroid-stimulating hormone (TSH) stimulates the thyroid gland to release its hormones
>adrenocorticotropic hormone (ACTH) stimulates the adrenal cortex causing the release of the hormone
cortisol from the adrenal cortex
>prolactin (w/ other hormones) stimulate mammary gland development during pregnancy
>melanocyte-stimulating hormone (MSH) -> stimulates the synthesis of the skin pigment melanin
>growth hormone regulates the body's growth by acting on nearly all the body's cells increasing protein
synthesis/fat utilization/storage of carbs -->too little or too much of hormone can cause dwarfism or
gigantism respectively
>posterior pituitary releases hormones produced by cells in the hypothalamus
>the posterior pituitary contains the endings of the two types of nuerosecretory cells whose cell bodies are
located in the hypothalamus
>these neurosecretory cell endings are enmeshed in a capillary bend into which they release hormones to be
carried into the bloodstream
>2 peptide hormones are synthesized in the hypothalamus and released from the posterior pituitary:
antidieretic hormone (ADH) and oxytocin
-lying in front of the neck (just below the larynx) is the thyroid gland
>produces 2 major hormones: thyroxine and calcitonin
>thyroxine is often referred to as "thyroid hormone" is an iodine-containing modified amino acid that raises
the metabolic rate of most body cells
>calcitonin is a peptide important in calcium metabolism
-thyroxin influences most of the cells in the body by elevating their metabolic rate and stimulating the
synthesis of enzymes that break down glucose and provide energy --> in adults thyroxine shows overall
metabolic rate (resting rate of cellular metabolism)
>requires for mental alertness --> low thyroxine leads to mental and physical lethargy -> decrease appetite
and intolerance to cold
>excess thyroxine leads to restlessness and irritability -> increase appetite and intolerance to heat
-> in juvenile animals thyroxine helps regulate growth by stimulating both metabolic rate and the
development of the nervous system --> under secretion of thyroid hormone early in life causes cretinism
(condition characterized by retardation [mental/physical])
>over secretion of thyroxine in developing vertebrates can trigger precocious development
>levels of thyroxine in the bloodstream are finely tuned by negative feedback -> thyroxine released is
stimulated by thyroid-stimulating hormone (TSH) from the anterior pituitary -> which is stimulated by a
releasing hormone from the hypothalamus
>amount of TSH release from the pituitary is regulated by negative feedback -> adequate levels of
thyroxine circulating in the bloodstream inhibit the secretion of both the releasing hormone (via
hypothalamus) and TSH (via anterior pituitary) -> inhibiting further release of thyroxine from the thyroid
gland
>iodide deficient diet can reduce production of thyroxine *can trigger feedback mechanism that acts to
restore normal hormone levels by dramatically increasing the # if thyroxine-producing cells* -> leads to
excessive growth of thyroid gland -> causes condition called goiter
>(4) small disks of the parathyroid glands are embedded in the back of the thyroid gland
>parathyroid secretes parathyroid hormone, which along with calcitonin controls the concentration of
calcium in the blood and other body fluids -> calcium in needed for many processes (nerve and muscle
function)
>calcium concentrations -> must be contained
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-> parathyroid hormone and calcitonin regulate calcium absorption -> release by the bones -> serves as
bank into which calcium can be deposited or withdrawn as necessary
-> response to low blood calcium -> parathyroid release parathyroid hormone which causes release of
calcium from bones
>if blood calcium levels become too high -> thyroid releases calcitonin which inhibits the release of
calcium from bones
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1) neurosecretory cells of the hypothalamus secrete TSH-releasing hormones 2) Releasing hormones causes
anterior pituitary to secrete thyroid-stimulating hormone (TSH) 3) TSH causes thyroid to secrete thyroxin,
which increases cellular metabolic rate throughout the body 4) Thyroxine inhibits TSH-releasing hormone
and TSH release by negative feedback
*********
>pancreas is a gland that produces both exocrine/endocrine secretion
>exocrine portion synthesizes digestive secretions that are released into the pancreatic duct and flow into
the small intestine --> endocrine portion consists of clusters of cells called islet cells
>produce peptide hormones -> one type of cell produces insulin another produces glucagons
>work in opposite ways to regulate carbohydrate and fat metabolism -> insulin reduces the blood glucose
level; glucagons increases it
>keeps blood glucose level constant -> when blood glucose rises insulin is released and causes body cells
to take up glucose and either metabolize it for energy or covert it to fat or glycogen (a polysaccharide made
of long chains of glucose molecules) -> when blood glucose levels drop-> glucagons is released ->
glucagons activates an enzyme in the liver that breaks down glycogen -> releasing glucose into the blood ->
glucagons also promotes lipid breakdown which releases fatty acids that can be metabolized for energy
>lack of insulin production of the failure of target cells to respond to insulin results in diabetes mellitus
>sex organs secrete steroid hormones
>sex organs do far more than produce sperm or eggs -> the testes in males and ovaries in females are
important endocrine organs -> testes secrete several hormones called androgens (like testosterone) -> the
ovaries secrete two types of steroid hormones: estrogen and progesterone
>sex hormones -> play role in puberty -> phase of life which the reproductive systems of both sexes
become mature and functional -> accompanied by behavioral change --> begins when hypothalamus starts
to secrete increasing amounts of releasing hormones -> stimulate anterior pituitary to secrete more
lutenizing hormone (LH) and follicle-stimulating hormone (FSH) into the bloodstream
>both LH and FSH stimulate target cell in testes/ovaries to produce higher levels of sex hormones ->
affects tissue throughout the body that have the needed receptors ->testosterone in males stimulate male
secondary sexual characteristics and estrogen in female
>the adrenal glands have two parts that secrete different hormones
>adrenal gland act in conjunction with your sympathetic nervous system
>the adrenal glands are two glands in one; the adrenal medulla and adrenal cortex
>adrenal medulla is located in the center of each gland consists of secretory cells derived during
development from nervous tissue, and its hormone secretion is controlled directly by the nervous system
>produces 2 hormones --> epinephrine and in smaller quantities norepinephrine in response to stress -->
prepares body for emergency action --> causes changes (dilation of eyes, opening air sacs, moving blood
from digestive tract to brain and muscles)
>the outer layer of the adrenal gland forms the adrenal cortex which secretes three types of steroids
hormones collectively called glucocorticoids -> of these cortisol is secreted in largest quantity --> these
hormones are stimulated by ACTH from the anterior pituitary in response to releasing hormone from the
hypothalamus -> hormone levels are controlled by negative feedback; circulating glucocorticoids inhibit
the release of both the hypothalamic releasing hormone and ACTH -> cortisol is released when the body is
stressed -> helps body cope with short-term stressors by raising blood glucose levels by stimulating glucose
production and promoting use of fats instead of glucose for energy production --> inhibits immune
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response
>although most body cells can produce energy from fats/proteins/carbs brain cells can only burn glucose
>adrenal cortex also secretes the hormone aldosterone which regulates the sodium content of the blood.
Sodium ions are most abundant positive ion in blood and extracellular fluid -> sodium ion gradient across
plasma membranes is a factor in many cellular events, including the production of electrical signals by
nerve cells -> if blood sodium falls the adrenal cortex releases aldosterone which causes the kidneys and
sweat glands to retain sodium, this enables salt and other sources of dietary sodium to raise blood sodium
levels, shutting off further aldosterone secretion (negative feedback)
>adrenal cortex also produces testosterone in small quantities in men and women
-the pineal gland located between the two hemispheres of the brain (just above and behind the
hypothalamus) produces the hormone melatonin -> secretes in a daily rhythm and is regulated by the eyes
>thymus is located in the chest cavity behind the breastbone (also called sternum) -> in addition to
producing the hormone thymosin which stimulates the development of specialized white blood cells (T
cells) that play an important role in the immune system, the thymus is extremely large in infants but under
the influence of sex hormones begins to increase with size during puberty
>kidneys are also important for maintaining body fluid homeostasis -> when oxygen content of the blood
drops, the kidney produce the hormone erythropoietin which increase red blood cell production -> kidneys
also produce a second hormone renin in response to low blood pressure such as that caused by bleeding -->
renin is an enzyme that catalyzes the production of the hormone angiotensin from proteins in the blood ->
raises blood pressure by constricting arterioles -> it also stimulates the release of aldosterone by the
adrenal cortex, causing the kidneys to retain sodium, which in turn increases blood volume
>atrial natriuretic peptide (ANP) is released by the cells of the atria when blood volume increases causing
extra distension of the heart --> ANP then reduces blood volume by decreasing the release of ADH and
aldosterone
>stomach and small intestine produce a variety of peptide hormones that regulate digestion
>hormones include --> gastrin, secretin, and cholecystokinin
>leptin is released by fat cells -> cause loss of weight in mice --> not in humans but it has variety of
benefits