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