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Organismal Systems A 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 Immunity Innate 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 Pathogens (microorganisms and viruses) INNATE IMMUNITY • Recognition of traits shared by broad ranges of pathogens, using a small set of receptors • Rapid response ACQUIRED IMMUNITY • Recognition of traits specific to particular pathogens, using a vast array of receptors • Slower response Barrier defenses: Skin Mucous membranes Secretions Internal defenses: Phagocytic cells Antimicrobial proteins Inflammatory response Natural killer cells Humoral response: Antibodies defend against infection in body fluids. Cell-mediated response: Cytotoxic lymphocytes defend against 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 Inflammation The 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 Chemical Macrophage signals Mast cell Capillary Red blood cells Phagocytic cell Fig. 43-8-2 Pathogen Splinter Chemical Macrophage signals Mast cell Capillary Red blood cells Phagocytic cell Fluid Fig. 43-8-3 Pathogen Splinter Chemical Macrophage signals Mast cell Capillary Red blood cells Phagocytic cell 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 Attack The 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 Recognition Recognition 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 Microbe Antigenpresenting cell 1 Antigen associates with MHC molecule Antigen fragment Antigen fragment 1 Class I MHC molecule 1 T cell receptor (a) 2 2 T cell receptor 2 T cell recognizes combination Cytotoxic T cell Class I are found on body cells. Display antigens to cytotoxic T cells Class II MHC molecule (b) Helper T cell Class II are on macrophages, dendritic cells or B cells. Display to 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 Response In 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 Antigen molecules B cells that differ in antigen specificity Antigen receptor Antibody 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 Opsonization Activation of complement system and pore formation Bacterium Complement proteins Virus Formation of membrane attack complex Flow of water and ions Macrophage Pore Foreign cell 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 Cell-mediated immune response Key Antigen (1st exposure) + Engulfed by Gives rise to Antigenpresenting cell + Stimulates + + B cell Helper T cell + Cytotoxic T cell + Memory Helper T cells + + + Antigen (2nd exposure) Plasma cells Memory B cells + Memory Cytotoxic T cells Active Cytotoxic T cells Secreted antibodies Defend against extracellular pathogens by binding to antigens, thereby neutralizing pathogens or making them better targets for phagocytes and complement proteins. Defend against intracellular pathogens and cancer by binding to and lysing the infected 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 Antibody concentration (arbitrary units) Primary immune response to antigen A produces antibodies to A. Secondary immune response to antigen A produces antibodies to A; primary immune response to antigen B produces antibodies to B. 104 103 Antibodies to A 102 Antibodies to B 101 100 0 7 Exposure to antigen A 14 21 28 35 42 Exposure to antigens A and B Time (days) 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 of Immune System Function Allergies: exaggerated responses to certain antigens called allergens Anaphylactic shock: an acute, allergic, lifethreatening 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 Cavities Simple 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 Circular canal Mouth Pharynx Mouth Radial canal (a) The moon jelly Aurelia, a cnidarian 5 cm 2 mm (b) The planarian Dugesia, a flatworm 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 system There 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 sinuses surrounding organs Pores Heart Blood Interstitial fluid Small branch vessels In each organ Dorsal vessel (main heart) Tubular heart (a) An open circulatory system Auxiliary hearts Ventral vessels (b) A closed circulatory system 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 Gill capillaries Artery Gill circulation Ventricle Heart Atrium Vein Systemic circulation Systemic capillaries Fig. 42-5 Amphibians Reptiles (Except Birds) Mammals and Birds Lung and skin capillaries Lung capillaries Lung capillaries Pulmocutaneous circuit Atrium (A) Right systemic aorta Atrium (A) Ventricle (V) Left Right Systemic circuit Systemic capillaries Pulmonary circuit A V Right Pulmonary circuit A A V Left Systemic capillaries Left systemic aorta A V V Right Left Systemic circuit Systemic capillaries 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 Superior vena cava Capillaries of head and forelimbs 7 Pulmonary artery Pulmonary artery Capillaries of right lung Aorta 9 3 Capillaries of left lung 3 2 4 11 Pulmonary vein Right atrium 1 Pulmonary vein 5 Left atrium 10 Right ventricle Left ventricle Inferior vena cava Aorta 8 Capillaries of abdominal organs and hind limbs Fig. 42-7 Pulmonary artery Aorta Pulmonary artery Right atrium Left atrium Semilunar valve Semilunar valve Atrioventricular valve Atrioventricular valve Right ventricle Left ventricle 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 Semilunar valves closed AV valves open 1 Atrial and ventricular diastole 0.4 sec Fig. 42-8-2 2 Atrial systole; ventricular diastole Semilunar valves closed 0.1 sec AV valves open 1 Atrial and ventricular diastole 0.4 sec Fig. 42-8 2 Atrial systole; ventricular diastole Semilunar valves closed 0.1 sec AV valves open 1 Atrial and ventricular diastole 0.4 sec Semilunar valves open 0.3 sec AV valves closed 3 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 INTERSTITIAL FLUID Capillary Net fluid movement out Net fluid movement in Direction of blood flow Pressure Blood pressure Inward flow Outward flow Osmotic pressure Arterial end of capillary Venous end The lymphatic system returns fluid that leaks out in the capillary beds This system aids in body defense Fluid, 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 Exchange Gas 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. Gills Gills are out foldings of the body Fish 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 Fluid flow through gill filament Oxygen-poor blood Anatomy of gills Oxygen-rich blood Gill arch Lamella Gill arch Gill filament organization Blood vessels Water flow Operculum Water flow between lamellae Blood flow through capillaries in lamella Countercurrent exchange PO2 (mm Hg) in water 150 120 90 60 30 Gill filaments Net diffusion of O2 from water to blood 140 110 80 50 20 PO2 (mm Hg) in blood 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 External opening Tracheoles Mitochondria Muscle fiber Body cell Air sac Tracheole Trachea Air Body wall 2.5 µm 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 Branch of pulmonary vein (oxygen-rich blood) Branch of pulmonary artery (oxygen-poor blood) Terminal bronchiole Nasal cavity Pharynx Larynx Alveoli (Esophagus) Left lung Trachea Right lung Bronchus Bronchiole Diaphragm Heart SEM 50 µm Colorized SEM 50 µm How animals breath An 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 System Many 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 Sensory input Integration Sensor Motor output Effector Peripheral nervous system (PNS) Central nervous system (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 Cell body Axon hillock Presynaptic cell Axon Synapse Synaptic terminals Postsynaptic cell Neurotransmitter 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 Fig. 7-16-7 EXTRACELLULAR FLUID [Na+] high [K+] low Na Na + + Na + Na Na + + Na Na + + Na + CYTOPLASM 1 Na + [Na+] low [K+] high 2 P ADP ATP P 3 P 6 5 4 P 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 OUTSIDE [K+] CELL 5 mM INSIDE [K+] CELL 140 mM (a) [Na+] [Cl–] 150 mM 120 mM [Na+] 15 mM [Cl–] 10 mM [A–] 100 mM Fig. 48-6b Key Na+ K+ OUTSIDE CELL INSIDE CELL (b) Sodiumpotassium pump Potassium channel Sodium channel The Generation of Action Potentials 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. 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 Hyperpolarization Membrane potential (mV) +50 0 –50 Threshold Resting potential –100 Hyperpolarizations 0 1 2 3 4 5 Time (msec) (a) Graded hyperpolarizations Fig. 48-9b Stimuli Depolarization Membrane potential (mV) +50 0 –50 Threshold Resting potential –100 Depolarizations 0 1 2 3 4 5 Time (msec) (b) Graded depolarizations 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 potential 1. Most voltage-gated Na+ and K+ channels are closed, but some K+ channels (not voltagegated) are open Fig. 48-10-1 Key Na+ K+ Membrane potential (mV) +50 Action potential –50 Plasma membrane Cytosol Inactivation loop 1 Resting state 2 –100 Sodium channel 4 Threshold 1 Resting potential Depolarization Extracellular fluid 3 0 Time Potassium channel 5 1 When an action potential is generated 2. 3. 4. Voltage-gated Na+ channels open first and Na+ flows into the cell During the rising phase, the threshold is crossed, and the membrane potential increases 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 Key Na+ K+ Membrane potential (mV) +50 Action potential –50 2 Plasma membrane Cytosol Inactivation loop 1 Resting state 2 –100 Sodium channel 4 Threshold 1 Resting potential Depolarization Extracellular fluid 3 0 Time Potassium channel 5 1 Fig. 48-10-3 Key Na+ K+ 3 Rising phase of the action potential Membrane potential (mV) +50 Action potential –50 2 Plasma membrane Cytosol Inactivation loop 1 Resting state 2 –100 Sodium channel 4 Threshold 1 Resting potential Depolarization Extracellular fluid 3 0 Time Potassium channel 5 1 Fig. 48-10-4 Key Na+ K+ 3 4 Rising phase of the action potential Membrane potential (mV) +50 Action potential –50 2 Plasma membrane Cytosol Inactivation loop 1 Resting state 2 –100 Sodium channel 4 Threshold 1 Resting potential Depolarization Extracellular fluid 3 0 Time Potassium channel 5 1 Falling phase of the action potential 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 Key Na+ K+ 3 4 Rising phase of the action potential Falling phase of the action potential Membrane potential (mV) +50 Action potential –50 2 2 4 Threshold 1 1 5 Resting potential Depolarization Extracellular fluid 3 0 –100 Sodium channel Time Potassium channel Plasma membrane Cytosol Inactivation loop 5 1 Resting state Undershoot 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 Schwann cell Depolarized region (node of Ranvier) Cell body Myelin sheath 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 5 Synaptic vesicles containing neurotransmitter Voltage-gated Ca2+ channel 1 Ca2+ Synaptic cleft Presynaptic membrane Postsynaptic membrane 4 2 3 Ligand-gated ion channels 6 K+ Na+ Five Major Types of Neurotransmitters