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Organismal SystemsA Summary of Biological Systems
Chemical Defense Systems Animal and plants have chemical
defenses to fight against foreign invaders.
The vertebrate immune system is one of the best studied of these systems.
Vertebrate Immune System The immune system
recognizes foreign invaders such as viruses, bacteria, fungi, parasites and other pathogens.
Two major modes of attack have evolved: Innate Immunity
(Nonspecific, Generalized Attack)
Acquired Immunity (Specific, Specialized Attack)
“Generalized Attack”: these cells wage an instant campaign of destruction against any pathogen while signaling other cells of the presence of intruders
“Specialized Attack”: these cells wage a more specific and enduring attack and are capable of producing lasting immunity against specific invaders.
Lines of Defense The immune system has 3 main lines of defense:
1st line: Physical barriers, chemical barriers, and mechanical barriers.
2nd line: Phagocytes, complement, inflammation, fever
3rd line: Cell-mediated and humoral
Innate and Nonspecific
Acquired and Specific
Innate vs. Acquired ImmunityInnate Immunity: is present at birth
(before exposure to pathogens), is nonspecific and consists of external barriers plus internal cellular and chemical defenses
Acquired Immunity: develops after exposure to foreign invaders and involves a very specific response to pathogens.
Fig. 43-2
INNATE IMMUNITY
Recognition of traitsshared by broad rangesof pathogens, using asmall set of receptors
•
•Rapid response
•Recognition of traitsspecific to particularpathogens, using a vastarray of receptors
•Slower response
ACQUIRED IMMUNITY
Pathogens(microorganisms
and viruses)
Barrier defenses:SkinMucous membranesSecretions
Internal defenses:Phagocytic cellsAntimicrobial proteinsInflammatory responseNatural killer cells
Humoral response:Antibodies defend againstinfection in body fluids.
Cell-mediated response:Cytotoxic lymphocytes defendagainst infection in body cells.
A Microbe Invading the Body will Encounter the Following Defenses:
1st Line of Defense:Barrier defenses:
Skin: physical barrier prevents entry into body: low pH prevents microbial growth
Mucous membranes of respiratory, urinary, and reproductive tracts: traps microbes, low pH of body fluids is hostile to microbes
Once past the 1st line: 2nd Line of Defense:
White blood cells are the key players in a series of increasingly specific attacks against invading microbes. White blood cells
(leukocytes): engulf a pathogen in the body and trap it within a vacuole. The vacuole then fuses with a lysosome to destroy the microbe (phagocytosis). Many cells are involved in this “Generalized Attack”
The Generalized Attack “On-the-ready” cells of the generalized attack:
Neutrophils: “eat” pathogens and send out distress signals.
Macrophages: arise from monocytes. They are the “big eaters”. They circulate through the lymph system looking for any foreign invader. Some reside permanently in the spleen and lymph nodes, lying in wait for microbes.
Eosinophils: release destructive enzymes to attack large invaders like parasitic worms. Also involved in the inflammatory response.
Basophils: contain histamines that are released during the inflammatory response.
Dendritic cells: arise from monocytes. They stimulate the development of acquired immunity.
Notice that all of the immune cells are derived from a multi-potent cell in the bone marrow known as a Hematopoietic stem cell.
(Dendritic cells not shown)
Proteins, Complement and InflammationThe remaining components of the 2nd
line of defense do not involve white blood cells. Peptides and proteins attack microbes
directly or impede their reproduction.Example: Interferon proteins provide
innate defense against viruses and help to activate macrophages.
Interferon produced by one infected cell can induce nearby cells to produce substances that interfere with viral reproduction. This limits the cell-to-cell spread of viruses
About 30 proteins make up the complement system, which causes lysis of invading cells and helps trigger inflammation
Inflammatory Responses Following an injury, mast cells release
histamine, which promotes changes in blood vessels; this is part of the inflammatory response
These changes increase local blood supply and allow more phagocytes and antimicrobial proteins to enter tissues.
Pus (a fluid rich in white blood cells, dead microbes, and cell debris) accumulates at the site of inflammation.
Fig. 43-8-1
Pathogen Splinter
Macrophage
Mast cell
Chemicalsignals
Capillary
Phagocytic cellRed blood cells
Fig. 43-8-2
Pathogen Splinter
Macrophage
Mast cell
Chemicalsignals
Capillary
Phagocytic cellRed blood cells
Fluid
Fig. 43-8-3
Pathogen Splinter
Macrophage
Mast cell
Chemicalsignals
Capillary
Phagocytic cellRed blood cells
Fluid
Phagocytosis
Symptoms of inflammation include redness, warmth, pain, and swelling.
Inflammation can be either local or systemic (throughout the body)
Fever is a systemic inflammatory response triggered by pyrogens released by macrophages, and toxins from pathogens.
Septic shock is a life-threatening condition caused by an overwhelming inflammatory response.
Natural Killer Cells The last component of the innate
immune system are the natural killer cells.
All cells in the body (except red blood cells) have a class 1 MHC protein on their surface. (major histocompatibility complex)
Cancerous or infected cells no longer express this protein
Natural killer (NK) cells attack these damaged cells, inhibiting further spread of the virus or cancer.
Evading the Innate Immune System
Some pathogens evade the innate immune attack by modifying their surface to prevent recognition or by resisting breakdown following phagocytosis.
Example: Tuberculosis (TB)—these bacterium are resistant to the enzymes inside the lysosomes. Thus, they can hide inside white blood cells without being digested. This disease kills more than a million people per year.
Acquired Immunity The 3rd Line of Defense:
White blood cells called lymphocytes recognize and respond to antigens (foreign molecules).
Lymphocytes that mature in the thymus above the heart are called T cells, and those that mature in bone marrow are called B cells.
These are the cells involved in the “Specialized Attack”
Lymphocytes contribute to immunological memory, an enhanced response to a foreign molecule encountered previously. This is what allows us to develop lifetime immunity to diseases like chickenpox.
The specialized attack usually occurs after being signaled by cells already involved in the generalized attack.Cytokines are secreted by
macrophages and dendritic cells to recruit and activate lymphocytes.
The Specialized AttackThe three stars of this more specialized
attack are the B cells, Helper T cells, and Killer T cells (cytotoxic T cells).B cells mature into plasma cells that
generate highly specific antibodies capable of lasting immunity.
Helper T cells play a central role in coordinating the attack
Killer T cells, once activated, destroy virus-infected cells.
B cells and T cells have receptor proteins that can bind to foreign molecules.
Each individual lymphocyte is specialized to recognize a specific type of molecule.
Antigen Recognition by Lymphocytes
An antigen is any foreign molecule to which a lymphocyte responds
A single B cell or T cell has about 100,000 identical antigen receptors.
A typical immune response to a virus is seen in the following diagram.
From the diagram, we can see that there are two types of specific responses: humoral response (involving B-cells) and cell mediated response (involving cytotoxic T-cells)
The helper T cells can initiate both responses.
Antigen RecognitionRecognition of the antigen begins when
a macrophage (as seen in the diagram), a B-cell, or a dendritic cell presents the foreign antigen by engulfing the invader, digesting the particle, and then presenting the antigen on the cell’s surface. MHC molecules (major
histocompatibility complex) are used to present the antigens
Fig. 43-12
Infected cell
Antigenfragment
Class I MHCmolecule
T cellreceptor
(a)
Antigenassociateswith MHCmolecule
T cellrecognizescombination
Cytotoxic T cell (b) Helper T cell
T cellreceptor
Class II MHCmolecule
Antigenfragment
Antigen-presentingcell
Microbe
1
11
2
22
Class I are found on body cells.Display antigens to cytotoxic T cells
Class II are on macrophages,dendritic cells or B cells. Displayto cytotoxic T cells and Helper T cells
Helper T cells then bind to the presented antigen and signal the production of more T and B cells by releasing cytokines.
This initiates both the humoral and the cell-mediated response.
The Humoral ResponseIn the humoral response, activated
B cells give rise to plasma cells, which secrete antibodies or immunoglobulins (Ig) specific to the antigen presented.
Memory B cells also form during the humoral response and persist long after the initial infection ends.
Fig. 43-14
B cells thatdiffer inantigen specificity
Antibodymolecules
Antigenreceptor
Antigen molecules
Clone of memory cells Clone of plasma cells
The Role of Antibodies By binding to a pathogen, antibodies
can neutralize the pathogen so that it can no longer infect a host cell.
By binding to a pathogen, antibodies can flag them so that they are more easily and quickly identified and destroyed by phagocytic cells.
Antibodies, together with proteins of the complement system generate a membrane attack complex and cell lysis.
Fig. 43-21
Viral neutralization
Virus
Opsonization
Bacterium
Macrophage
Activation of complement system and pore formation
Complement proteins
Formation ofmembraneattack complex
Flow of waterand ions
Pore
Foreigncell
The Cell-Mediated Response
In the cell-mediated response, cytotoxic T- cells target intracellular pathogens which B-cells cannot recognize.
Antibodies are not used.Cytotoxic T-cells and Natural
Killer cells detect and destroy altered or infected body cells.
Memory T-cells are also generated.
Fig. 43-16
Humoral (antibody-mediated) immune response
B cell
Plasma cells
Cell-mediated immune response
Key
Stimulates
Gives rise to
+
+
++
+
+
+Memory B cells
Antigen (1st exposure)
Engulfed by
Antigen-presenting cell
MemoryHelper T cells
Helper T cell Cytotoxic T cell
MemoryCytotoxic T cells
ActiveCytotoxic T cells
Antigen (2nd exposure)
Secretedantibodies
Defend against extracellular pathogens by binding to antigens,thereby neutralizing pathogens or making them better targetsfor phagocytes and complement proteins.
Defend against intracellular pathogensand cancer by binding to and lysing theinfected cells or cancer cells.
+
+ +
Primary and Secondary Responses
The first exposure to a specific antigen represents the primary immune response.
During this time, plasma cells are generated, T cells are activated, antibodies and memory B and T cells are produced.
In the secondary immune response, memory cells facilitate a faster, more efficient response.
Fig. 43-15
Antibodiesto A
Antibodiesto B
Secondary immune response toantigen A produces antibodies to A;primary immune response to antigenB produces antibodies to B.
Primary immune responseto antigen A producesantibodies to A.
An
tib
od
y co
nce
ntr
atio
n(a
rbit
rary
un
its)
Exposureto antigen A
Exposure toantigens A and B
Time (days)
104
103
102
101
100
0 7 14 21 28 35 42 49 56
Lymphocyte Development The acquired immune system has three
important properties Receptor diversity—our cells have an
amazing ability to rearrange genes to generate over 1 million different B cells and 10 million different T cells.
A lack of reactivity against host cells—as lymphocytes mature, any that exhibit receptors specific for the body’s own molecules are destroyed by apoptosis.
Immunological memory—there is an increase in cell number and behavior triggered by the binding of antigen that allows the immune system to “remember attackers”
Active and Passive Immunity Active immunity develops naturally in
response to an infection. It can also develop following
immunization, also called vaccination. Passive immunity provides
immediate, short-term protection It is conferred naturally when antibodies
cross the placenta from mother to fetus or from mother to infant in breast milk.
It can be conferred artificially by injecting antibodies into a non-immune person
Immune Rejection Cells transferred from one person to
another can be attacked by immune defenses
This complicates blood transfusions and organ transplants.
MHC molecules are different from person to person and this difference stimulates most organ rejections. Successful transplants try to match MHC
tissue types and utilize immunosuppressive drugs.
Blood Groups Antigens on red blood cells
determine whether a person has blood type A (A antigen), B (B antigen), AB (both A and B antigens), or O (neither antigen)
Antibodies to nonself blood types exist in the body
Transfusion with incompatible blood leads to destruction of the transfused cells
Recipient-donor combinations can be fatal or safe
Disruptions ofImmune System Function Allergies:
exaggerated responses to certain antigens called allergens
Anaphylactic shock: an acute, allergic, life-threatening reaction that can occur within seconds of allergen exposure
Autoimmune Diseases: the immune system loses tolerance for self and turns against certain molecules of the body. Examples: Lupus,
rheumatoid arthritis, and multiple sclerosis.
Acquired Immunodeficiency Syndrome (AIDS): Caused by human
immunodeficiency virus (HIV) Infects Helper T-cells Impairs both the humoral and the
cell-mediated immune responses.
HIV eludes the immune system because of antigenic variation and an ability to remain latent while integrated into host DNA.
People with AIDS are highly susceptible to opportunistic infections and cancers that take advantage of an immune system in collapse.
Cancer: The frequency of certain cancers increases when the immune response is impaired.
Two suggested explanations are: Immune system normally suppresses
cancerous cells Increased inflammation increases the
risk of cancer.
Eliminating Wastes and Obtaining Nutrients Organisms have a variety of
mechanisms for obtaining nutrients and eliminating wastes.
These mechanisms all contribute to maintaining homeostasis in living things.
Removal of Nitrogen Waste All animals must regulate the amount of,
and composition of, their body fluids. Examples:
Sponges: have no excretory organs,: nitrogen waste diffuses out across the body wall.
Flatworms: have a tubular excretory organ that delivers nitrogen waste in the form of ammonia to a special pore in the body surface.
Insects: convert ammonia to uric acid to reduce water loss.
Vertebrates: have a urinary system with two kidneys that filters the blood and adjusts its solute concentration.
Circulation and Gas Exchange In most organisms, circulation and gas
exchange play a critical role in carrying nutrients and oxygen to cells and assisting in the removal of wastes.
Circulation and Gas Exchange In most animals, the circulatory and
respiratory systems are closely linked. In small and/or thin animals, cells
can exchange materials directly with the surrounding medium
In other animals, transport systems connect the organs of exchange with the body cells.
Most complex animals have internal transport systems that circulate fluid.
Gastrovascular CavitiesSimple animals, such as
cnidarians and flatworms, have a body wall that is only two cells thick and encloses a gastrovascular cavity.
This cavity functions in both digestion and distribution of substances throughout the body.
Fig. 42-2
Circularcanal
Radial canalMouth
(a) The moon jelly Aurelia, a cnidarian The planarian Dugesia, aflatworm
(b)
MouthPharynx
2 mm5 cm
Open and Closed Circulatory Systems
More complex animals have either open or closed circulatory systems.
Both systems have three basic components:A circulatory fluid (blood or
hemolymph)A set of tubes (blood vessels)A muscular pump (the heart)
In insects, other arthropods, and most molluscs, blood bathes the organs directly in an open circulatory systemThere is no distinction between
blood and interstitial fluid, and this general body fluid is more correctly called hemolymph.
Vertebrate animals have a closed circulatory system in which blood is confined to vessels and is distinct from the interstitial fluid.Closed systems are more efficient.
Fig. 42-3
Heart
Hemolymph in sinusessurrounding organs
Heart
Interstitialfluid
Small branch vesselsIn each organ
Blood
Dorsal vessel(main heart)
Auxiliary hearts Ventral vessels
(b) A closed circulatory system(a) An open circulatory system
Tubular heart
Pores
Organization of Vertebrate Circulatory Systems The circulatory system is an example of a
homeostatic mechanism that supports the idea of common ancestry.
It carries nutrients and oxygen to cells and assists in the removal of wastes from cells.
All vertebrate circulatory systems are closed (common ancestry)
However, there are variations between fish, amphibians, and mammals that have evolved over millions of years (diversity)
Fish: heart has two chambers (one atrium and one ventricle) and blood flows through one circuit. It picks up oxygen in the capillary beds of the gills and delivers it to capillary beds in all body tissue.
Amphibians: heart has three chambers (two atria and one ventricle) and blood flows along two partially separated circuits. Oxygenated blood and oxygen-poor blood mix a bit in the ventricle.
Mammals and Birds: heart has four chambers (two atria and two ventricles) and blood flows through two fully separated circuits. One goes to the lungs and back and the second goes from the heart to all body tissues and back. This keeps oxygen rich blood completely separate from the oxygen poor blood.
Fig. 42-4
Artery
Ventricle
AtriumHeart
Vein
Systemic capillaries
Systemiccirculation
Gillcirculation
Gill capillaries
Fig. 42-5
Amphibians
Lung and skin capillaries
Pulmocutaneouscircuit
Atrium (A)
Ventricle (V)
Atrium (A)
Systemiccircuit
Right Left
Systemic capillaries
Reptiles (Except Birds)
Lung capillaries
Pulmonarycircuit
Rightsystemicaorta
Right LeftLeftsystemicaorta
Systemic capillaries
A A
VV
Systemic capillaries
Pulmonarycircuit
Systemiccircuit
Right Left
A A
VV
Lung capillaries
Mammals and Birds
Mammalian Circulation Mammals provide an excellent example
of double circulation. Blood begins its flow with the right
ventricle pumping blood to the lungs In the lungs, the blood loads O2 and
unloads CO2
Oxygen-rich blood from the lungs enters the heart at the left atrium and is pumped through the aorta to the body tissues by the left ventricle
The aorta provides blood to the heart through the coronary arteries
Blood returns to the heart through the superior vena cava (blood from head, neck, and forelimbs) and inferior vena cava (blood from trunk and hind limbs)
The superior vena cava and inferior vena cava flow into the right atrium
Four valves prevent backflow of blood in the heart. Two atrioventricular (AV) valves
separate each atrium and ventricle. The semilunar valves control blood
flow to the aorta and the pulmonary artery.
Fig. 42-6
Superiorvena cava
Pulmonaryartery
Capillariesof right lung
3
7
3
8
9
24
11
51
10
Aorta
Pulmonaryvein
Right atrium
Right ventricle
Inferiorvena cava
Capillaries ofabdominal organsand hind limbs
Pulmonaryvein
Left atrium
Left ventricle
Aorta
Capillariesof left lung
Pulmonaryartery
Capillaries ofhead andforelimbs
Fig. 42-7
Pulmonary artery
Rightatrium
Semilunarvalve
Atrioventricularvalve
Rightventricle
Leftventricle
Atrioventricularvalve
Leftatrium
Semilunarvalve
Pulmonaryartery
Aorta
The Mammalian Heart
A closer look at the mammalian heart provides a better understanding of double circulation.
The contraction, or pumping, phase is called systole.
The relaxation, or filling, phase is called diastole.
Fig. 42-8-1
Semilunarvalvesclosed
0.4 secAVvalvesopen
Atrial andventriculardiastole
1
Fig. 42-8-2
Semilunarvalvesclosed
0.4 secAVvalvesopen
Atrial andventriculardiastole
1
2
0.1 sec
Atrial systole;ventriculardiastole
Fig. 42-8
Semilunarvalvesclosed
0.4 secAVvalvesopen
Atrial andventriculardiastole
1
2
0.1 sec
Atrial systole;ventriculardiastole
3
0.3 sec
Semilunarvalvesopen
AV valvesclosed
Ventricular systole;atrial diastole
Vessels of the Circulatory System Three main blood vessels are: arteries, veins
and capillaries Arteries: carry oxygenated blood away
from heart; branch into arterioles and then to capillaries
Capillaries: form a network known as capillary beds; provide the site for chemical exchange between the blood and interstitial fluid.
Veins: carry deoxygenated blood back to heart; capillaries converge into venules and then into veins
The critical exchange of substances between the blood and interstitial fluid takes place across the thin endothelial walls of the capillaries
The difference between blood pressure and osmotic pressure drives fluids out of capillaries at the arteriole end and into capillaries at the venule end
Fig. 42-16
Body tissue
CapillaryINTERSTITIAL FLUID
Net fluidmovement out
Direction ofblood flow
Net fluidmovement in
Blood pressure
Inward flow
Outward flowOsmotic pressure
Arterial end of capillary Venous end
Pre
ss
ure
The lymphatic system returns fluid that leaks out in the capillary beds
This system aids in body defenseFluid, called lymph, reenters the
circulation directly at the venous end of the capillary bed and indirectly through the lymphatic system
The lymphatic system drains into veins in the neck
Lymph nodes are organs that filter lymph and play an important role in the body’s defense
Edema is swelling caused by disruptions in the flow of lymph
Gas ExchangeGas exchange supplies oxygen
for cellular respiration and disposes of carbon dioxide.
Gases like O2 and CO2, diffuse down pressure gradients in the lungs and other organs from where their partial pressures are higher to where they are lower.
Respiratory Media Animals can use air or water as a
source of oxygen. Obtaining oxygen from water actually
requires greater efficiency than air breathing since there is less oxygen available in water than in air.
Gas exchange takes place by diffusion across either the skin, gills, tracheae, or lungs.
All respiratory organs increase surface area for gas exchange.
GillsGills are out foldings of the bodyFish move water over their gills
and use a countercurrent exchange system, where blood flows in the opposite direction to water passing over the gills; blood is always less saturated with O2 than the water it meets.
Fig. 42-22
Anatomy of gills
Gillarch
Waterflow Operculum
Gillarch
Gill filamentorganization
Bloodvessels
Oxygen-poor blood
Oxygen-rich blood
Fluid flowthrough
gill filament
Lamella
Blood flow throughcapillaries in lamella
Water flowbetweenlamellae
Countercurrent exchange
PO2 (mm Hg) in water
PO2 (mm Hg) in blood
Net diffu-sion of O2
from waterto blood
150 120 90 60 30
110 80 20Gill filaments
50140
Tracheal Systems The tracheal system of insects
consists of tiny branching tubes that penetrate the body.
These tubes supply O2 directly to body cells.
The respiratory and circulatory systems are separate.
Larger insects must ventilate their tracheal system to meet O2 demands.
Fig. 42-23
Air sacs
Tracheae
Externalopening
Bodycell
AirsacTracheole
Tracheoles Mitochondria Muscle fiber
2.5 µmBody wall
Trachea
Air
Lungs Lungs are in-foldings of the body
surface. The circulatory system (open or
closed) transports gases between the lungs and the rest of the body.
Air inhaled through the nostrils passes through the pharynx to the larynx, trachea, bronchi, bronchioles, and alveoli.
Fig. 42-24
Pharynx
Larynx
(Esophagus)
Trachea
Right lung
Bronchus
Bronchiole
DiaphragmHeart SEM
Leftlung
Nasalcavity
Terminalbronchiole
Branch ofpulmonaryvein(oxygen-richblood)
Branch ofpulmonaryartery(oxygen-poorblood)
Alveoli
ColorizedSEM50 µm 50 µm
How animals breathAn amphibian ventilates its lungs by
positive pressure breathing, which forces air down the trachea.
Mammals ventilate their lungs by negative pressure breathing, which pulls air into the lungs.
In either case, the gas exchange must be coordinated with circulation.
Coordination of Circulation and Gas Exchange
Blood arriving in the lungs has a low partial pressure of O2 and a high partial pressure of CO2 relative to air in the alveoli
In the alveoli, O2 diffuses into the blood and CO2 diffuses into the air
In tissue capillaries, partial pressure gradients favor diffusion of O2 into the interstitial fluids and CO2 into the blood
Respiratory Pigments
Respiratory pigments, proteins that transport oxygen, greatly increase the amount of oxygen that blood can carry
Arthropods (other than insects) and many molluscs have hemocyanin with copper as the oxygen-binding component
Most vertebrates and some invertebrates use hemoglobin contained within erythrocytes
Carbon Dioxide Transport
Hemoglobin also helps transport CO2 and assists in buffering
CO2 from respiring cells diffuses into the blood and is transported either in blood plasma, bound to hemoglobin, or as bicarbonate ions (HCO3
–)
Developmental Regulation Many mechanisms control the
development of an organism. All cells in a multicellullular organism are
derived from the same fertilized egg and contain the same genes.
Differentiation is the process in which cells become specialized to express certain genes.
Example: all cells express genes that code for the enzymes of glycolysis: but only red blood cells express the genes that code for hemoglobin.
Most cells use less than 10% of their genes
Cells contain transcription factors (regulatory proteins) that turn on certain genes
Master Genes Master genes control other genes.
Example: Homeotic genes (a type of master gene)code for the transcription factors needed to express certain other genes. The products of these homeotic genes cause cells to differentiate into tissues that form specific structures like the head.
The products of these genes create gradients which affect other genes.
Thus, development of an embryo is controlled layer after layer by master genes.
Defect in a Master Gene (Antennapedia gene) in fruit flies causes legs to grow on the head.
Apoptosis Apoptosis
(programmed cell death) also plays a role in normal development.
Many cells must self destruct at a specific time in order to ensure proper development.
Example: Formation of human hand
Nervous SystemMany animals have a complex
nervous system that consists of:A central nervous system
(CNS): where integration takes place. Consists of brain and spinal cord.
A peripheral nervous system (PNS): brings information into and out of the CNS.
Introduction to Information Processing
Nervous systems process information in three stages: sensory input, integration, and motor output
Sensory receptors collect information from both outside and inside the body. Ex: rods and cones of the eyes; pressure receptors in the skin. They send this information along sensory neurons to the brain or ganglia.
Here interneurons connect sensory and motor neurons or make local connections in the brain and spinal cord.
Motor output leaves the brain or ganglia via motor neurons, which transmit signals to effectors, such as muscle cells and glands, to trigger a response.
Fig. 48-3
Sensor
Sensory input
Integration
Effector
Motor output
Peripheral nervoussystem (PNS)
Central nervoussystem (CNS)
Neurons Neurons = nerve cells
Use two types of signals: electrical signals (long distance) and chemical signals (short distance)
3 parts: Cell body—contains the nucleus and organelles Dendrites—short extensions: receive incoming
messages from other cells Axons—long extensions: transmit messages to
other cells. A Synapse is a junction between an axon and
another cell.
The synaptic terminal of one axon passes information across the synapse in the form of chemical messengers called neurotransmitters
Examples of neurotransmitters include acetylcholine, dopamine, and serotonin.
Information is transmitted from a presynaptic cell (a neuron) to a postsynaptic cell (a neuron, muscle, or gland cell)
Most neurons are nourished or insulated by cells called glia
Fig. 48-4
Dendrites
Stimulus
Nucleus
Cellbody
Axonhillock
Presynapticcell
Axon
Synaptic terminalsSynapse
Postsynaptic cellNeurotransmitter
Ion pumps and ion channels
Membrane potential (Voltage) is the difference in electrical charge across the plasma membrane of a cell.
Messages are transmitted as changes in membrane potential
The resting potential is the membrane potential of a neuron at rest.
Formation of the Resting Potential
In a mammalian neuron at resting potential, the concentration of K+ is greater inside the cell, while the concentration of Na+ is greater outside the cell
Sodium-potassium pumps use the energy of ATP to maintain these K+ and Na+ gradients across the plasma membrane
These concentration gradients represent chemical potential energy
2
EXTRACELLULAR
FLUID
[Na+]
high [K+]
low
[Na+] low [K+] high
Na+ Na+
Na+
Na+ Na+ Na+
CYTOPLAS
M
ATP AD
P
P
Na+ Na
+ Na+
P 3
K+
K+ 6
K+ K+
5 4
K+
K+
P P
1
Fig. 7-16-7
The opening of ion channels in the plasma membrane converts chemical potential to electrical potential
A neuron at resting potential contains many open K+ channels and fewer open Na+ channels; K+ diffuses out of the cell
Anions trapped inside the cell contribute to the negative charge within the neuron
Fig. 48-6a
OUTSIDECELL
[K+]5 mM
[Na+]150 mM
[Cl–]120 mM
INSIDECELL
[K+]140 mM
[Na+]15 mM
[Cl–]10 mM
[A–]100 mM
(a)
Fig. 48-6b
(b)
OUTSIDECELL
Na+Key
K+
Sodium-potassiumpump
Potassiumchannel
Sodiumchannel
INSIDECELL
The signals carried by axons are called Action Potentials.
An Action potential begins when gated ion channels opens or closes in response to stimuli which changes the membrane potential of the cell.
The potassium and sodium gates are the most important for stimulating action potentials.
The Generation of Action Potentials
When gated K+ channels open, K+ diffuses out, making the inside of the cell more negative
This is hyperpolarization If gated Na+ channels open and Na+
diffuses into the cell, then the inside of the cell is more positive
This is depolarization. Graded potentials are changes in
polarization where the magnitude of the change varies with the strength of the stimulus
Fig. 48-9a
Stimuli
+50
Mem
bra
ne p
ote
nti
al (m
V)
–50 Threshold
Restingpotential
Hyperpolarizations–100
0 2 3 4Time (msec)
(a) Graded hyperpolarizations
0
1 5
Hyperpolarization
Fig. 48-9b
Stimuli
+50
Mem
bra
ne p
ote
nti
al (m
V)
–50 Threshold
Restingpotential
Depolarizations–100
0 2 3 4Time (msec)
(b) Graded depolarizations
1 5
0
Depolarization
An action potential is a brief all-or-none depolarization of a neuron’s plasma membrane
It occurs if a stimulus causes the membrane voltage to cross a particular threshold
A neuron can produce hundreds of action potentials per second
The frequency of action potentials can reflect the strength of a stimulus
At resting potential1. Most voltage-gated Na+ and K+ channels are
closed, but some K+ channels (not voltage-gated) are open
Fig. 48-10-1
KeyNa+
K+
+50Actionpotential
Threshold
0
1
4
51
–50
Resting potential
Mem
bra
ne p
ote
nti
al
(mV
)
–100Time
Extracellular fluid
Plasmamembrane
Cytosol
Inactivation loop
Resting state
Sodiumchannel
Potassiumchannel
Depolarization
2
3
1
When an action potential is generated2. Voltage-gated Na+ channels open first and
Na+ flows into the cell3. During the rising phase, the threshold is
crossed, and the membrane potential increases
4. During the falling phase, voltage-gated Na+ channels become inactivated; voltage-gated K+ channels open, and K+ flows out of the cell
Fig. 48-10-2
KeyNa+
K+
+50Actionpotential
Threshold
0
1
4
51
–50
Resting potential
Mem
bra
ne p
ote
nti
al
(mV
)
–100Time
Extracellular fluid
Plasmamembrane
Cytosol
Inactivation loop
Resting state
Sodiumchannel
Potassiumchannel
Depolarization
2
3
2
1
Fig. 48-10-3
KeyNa+
K+
+50Actionpotential
Threshold
0
1
4
51
–50
Resting potential
Mem
bra
ne p
ote
nti
al
(mV
)
–100Time
Extracellular fluid
Plasmamembrane
Cytosol
Inactivation loop
Resting state
Sodiumchannel
Potassiumchannel
Depolarization
Rising phase of the action potential
2
3
2
1
3
Fig. 48-10-4
KeyNa+
K+
+50Actionpotential
Threshold
0
1
4
51
–50
Resting potential
Mem
bra
ne p
ote
nti
al
(mV
)
–100Time
Extracellular fluid
Plasmamembrane
Cytosol
Inactivation loop
Resting state
Sodiumchannel
Potassiumchannel
Depolarization
Rising phase of the action potential Falling phase of the action potential
2
3
2
1
3 4
5. During the undershoot, membrane permeability to K+ is at first higher than at rest, then voltage-gated K+ channels close; resting potential is restored
Fig. 48-10-5
KeyNa+
K+
+50Actionpotential
Threshold
0
1
4
51
–50
Resting potential
Mem
bra
ne p
ote
nti
al
(mV
)
–100Time
Extracellular fluid
Plasmamembrane
Cytosol
Inactivation loop
Resting state
Sodiumchannel
Potassiumchannel
Depolarization
Rising phase of the action potential Falling phase of the action potential
5 Undershoot
2
3
2
1
3 4
Saltatory Conduction Action potentials travel in one
direction only. Their speed is influenced by the myelin
sheath (insulating layer of glia cells) Action potential form only at the Nodes
of Ranvier (gaps in the myelin sheath) They jump from node to node in a
movement called, saltatory conduction, which greatly increases their speed.
Fig. 48-13
Cell body
Schwann cell
Depolarized region(node of Ranvier)
Myelinsheath
Axon
Synapses Nerve cells communicate with each
other at gaps known as synapses Most synapses are chemical synapses
and involve neurotransmitters. Action potentials cause the release of
neurotransmitters. They travel across the synaptic cleft
and affect the post synaptic cell. Neurotransmitters can be excitatory or
inhibitory.
Fig. 48-15
Voltage-gatedCa2+ channel
Ca2+12
3
4
Synapticcleft
Ligand-gatedion channels
Postsynapticmembrane
Presynapticmembrane
Synaptic vesiclescontainingneurotransmitter
5
6
K+Na+
Five Major Types of Neurotransmitters