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Chapter 48

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Chapter 48

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  1. Chapter 48 Nervous Systems

  2. Overview: Command and Control Center • The human brain • Contains an estimated 100 billion nerve cells, or neurons • Each neuron • May communicate with thousands of other neurons

  3. Figure 48.1 • Functional magnetic resonance imaging • Is a technology that can reconstruct a three-dimensional map of brain activity

  4. The results of brain imaging and other research methods • Reveal that groups of neurons function in specialized circuits dedicated to different tasks

  5. Concept 48.1: Nervous systems consist of circuits of neurons and supporting cells • All animals except sponges • Have some type of nervous system • What distinguishes the nervous systems of different animal groups • Is how the neurons are organized into circuits

  6. Nerve net Figure 48.2a (a) Hydra (cnidarian) Organization of Nervous Systems • The simplest animals with nervous systems, the cnidarians • Have neurons arranged in nerve nets

  7. Radialnerve Nervering Figure 48.2b (b) Sea star (echinoderm) • Sea stars have a nerve net in each arm • Connected by radial nerves to a central nerve ring

  8. Eyespot Brain Nerve cord Transversenerve Figure 48.2c (c) Planarian (flatworm) • In relatively simple cephalized animals, such as flatworms • A central nervous system (CNS) is evident

  9. Brain Brain Ventralnerve cord Ventral nervecord Segmentalganglia Segmentalganglion Figure 48.2d, e (d) Leech (annelid) (e) Insect (arthropod) • Annelids and arthropods • Have segmentally arranged clusters of neurons called ganglia • These ganglia connect to the CNS • And make up a peripheral nervous system (PNS)

  10. Anteriornerve ring Ganglia Brain Longitudinalnerve cords Ganglia (g) Squid (mollusc) (f) Chiton (mollusc) Figure 48.2f, g • Nervous systems in molluscs • Correlate with the animals’ lifestyles • Sessile molluscs have simple systems • While more complex molluscs have more sophisticated systems

  11. Brain Sensoryganglion Spinalcord (dorsalnerve cord) Figure 48.2h (h) Salamander (chordate) • In vertebrates • The central nervous system consists of a brain and dorsal spinal cord • The PNS connects to the CNS

  12. Sensory input Integration Sensor Motor output Effector Central nervoussystem (CNS) Peripheral nervoussystem (PNS) Figure 48.3 Information Processing • Nervous systems process information in three stages • Sensory input, integration, and motor output

  13. Sensory neurons transmit information from sensors • That detect external stimuli and internal conditions • Sensory information is sent to the CNS • Where interneurons integrate the information • Motor output leaves the CNS via motor neurons • Which communicate with effector cells

  14. 2 6 5 4 1 3 Gray matter Sensory neurons convey the information to the spinal cord. Sensors detect a sudden stretch in the quadriceps. The sensory neurons communicate with motor neurons that supply the quadriceps. The motor neurons convey signals to the quadriceps, causing it to contract and jerking the lower leg forward. Cell body of sensory neuronin dorsal root ganglion Sensory neurons from the quadriceps also communicate with interneuronsin the spinal cord. Quadricepsmuscle Hamstringmuscle White matter The interneurons inhibit motor neurons that supply the hamstring (flexor) muscle. This inhibition prevents the hamstring from contracting, which would resist the action of the quadriceps. Spinal cord(cross section) Sensory neuron Motor neuron The reflex is initiated by tapping the tendon connected to the quadriceps (extensor) muscle. Interneuron Figure 48.4 • The three stages of information processing • Are illustrated in the knee-jerk reflex

  15. Dendrites Cell body Nucleus Synapse Signal direction Axon Axon hillock Presynaptic cell Postsynaptic cell Myelin sheath Synapticterminals Figure 48.5 Neuron Structure • Most of a neuron’s organelles • Are located in the cell body

  16. Most neurons have dendrites • Highly branched extensions that receive signals from other neurons • The axon is typically a much longer extension • That transmits signals to other cells at synapses • That may be covered with a myelin sheath

  17. Dendrites Axon Cell body (c) Motor neuron (b) Interneurons Figure 48.6a–c (a) Sensory neuron • Neurons have a wide variety of shapes • That reflect their input and output interactions

  18. Supporting Cells (Glia) • Glia are supporting cells • That are essential for the structural integrity of the nervous system and for the normal functioning of neurons

  19. 50 µm Figure 48.7 • In the CNS, astrocytes • Provide structural support for neurons and regulate the extracellular concentrations of ions and neurotransmitters

  20. Node of Ranvier Layers of myelin Axon Schwann cell Schwann cell Nodes of Ranvier Nucleus of Schwann cell Axon Myelin sheath 0.1 µm Figure 48.8 • Oligodendrocytes (in the CNS) and Schwann cells (in the PNS) • Are glia that form the myelin sheaths around the axons of many vertebrate neurons

  21. Concept 48.2: Ion pumps and ion channels maintain the resting potential of a neuron • Across its plasma membrane, every cell has a voltage • Called a membrane potential • The inside of a cell is negative • Relative to the outside

  22. TECHNIQUE APPLICATION Electrophysiologists use intracellular recording to measure the membrane potential of neurons and other cells. A microelectrode is made from a glass capillary tube filled with an electrically conductive salt solution. One end of the tube tapers to an extremely fine tip (diameter < 1 µm). While looking through a microscope, the experimenter uses a micropositioner to insert the tip of the microelectrode into a cell. A voltage recorder (usually an oscilloscope or a computer-based system) measures the voltage between the microelectrode tip inside the cell and a reference electrode placed in the solution outside the cell. Microelectrode –70 mV Voltage recorder Referenceelectrode Figure 48.9 • The membrane potential of a cell can be measured

  23. The Resting Potential • The resting potential • Is the membrane potential of a neuron that is not transmitting signals

  24. EXTRACELLULARFLUID CYTOSOL – + [Na+]15 mM [Na+]150 mM + – [K+]5 mM [K+]150 mM + – [Cl–]10 mM [Cl–]120 mM + – [A–]100 mM – + Plasmamembrane Figure 48.10 • In all neurons, the resting potential • Depends on the ionic gradients that exist across the plasma membrane

  25. The concentration of Na+ is higher in the extracellular fluid than in the cytosol • While the opposite is true for K+

  26. Inner chamber Outer chamber Inner chamber Outer chamber –92 mV +62 mV + – + – 150 mMNaCl 150 mMKCL 5 mMKCL 15 mMNaCl Cl– + – + – K+ Na+ Cl– + – Potassiumchannel Sodium channel + – Artificialmembrane (b) Membrane selectively permeable to Na+ Figure 48.11a, b (a) Membrane selectively permeable to K+ • By modeling a mammalian neuron with an artificial membrane • We can gain a better understanding of the resting potential of a neuron

  27. A neuron that is not transmitting signals • Contains many open K+ channels and fewer open Na+ channels in its plasma membrane • The diffusion of K+ and Na+ through these channels • Leads to a separation of charges across the membrane, producing the resting potential

  28. Gated Ion Channels • Gated ion channels open or close • In response to membrane stretch or the binding of a specific ligand • In response to a change in the membrane potential

  29. Concept 48.3: Action potentials are the signals conducted by axons • If a cell has gated ion channels • Its membrane potential may change in response to stimuli that open or close those channels

  30. Stimuli +50 0 Membrane potential (mV) Threshold –50 Restingpotential Hyperpolarizations –100 0 1 2 3 4 5 Time (msec) (a) Graded hyperpolarizations produced by two stimuli that increase membrane permeability to K+. The larger stimulus producesa larger hyperpolarization. Figure 48.12a • Some stimuli trigger a hyperpolarization • An increase in the magnitude of the membrane potential

  31. Stimuli +50 0 Membrane potential (mV) –50 Threshold Restingpotential Depolarizations –100 0 1 2 3 4 5 Time (msec) (b) Graded depolarizations produced by two stimuli that increase membrane permeability to Na+.The larger stimulus produces alarger depolarization. Figure 48.12b • Other stimuli trigger a depolarization • A reduction in the magnitude of the membrane potential

  32. Hyperpolarization and depolarization • Are both called graded potentials because the magnitude of the change in membrane potential varies with the strength of the stimulus

  33. Production of Action Potentials • In most neurons, depolarizations • Are graded only up to a certain membrane voltage, called the threshold

  34. Stronger depolarizing stimulus +50 Actionpotential 0 Membrane potential (mV) Threshold –50 Restingpotential –100 0123456 Time (msec) (c) Action potential triggered by a depolarization that reaches the threshold. Figure 48.12c • A stimulus strong enough to produce a depolarization that reaches the threshold • Triggers a different type of response, called an action potential

  35. An action potential • Is a brief all-or-none depolarization of a neuron’s plasma membrane • Is the type of signal that carries information along axons

  36. Both voltage-gated Na+ channels and voltage-gated K+ channels • Are involved in the production of an action potential • When a stimulus depolarizes the membrane • Na+ channels open, allowing Na+ to diffuse into the cell

  37. As the action potential subsides • K+ channels open, and K+ flows out of the cell • A refractory period follows the action potential • During which a second action potential cannot be initiated

  38. –  – –  – –  – –  – +  + +  + +  + +  + +  + +  + +  + +  + –  – –  – –  – –  – 3 4 Falling phase of the action potential 3 +  + +  + +  + +  + 2 4 –  – –  – –  – –  – 5 1 1 Depolarization 2 +  + +  + +  + +  + Activationgates Extracellular fluid Potassiumchannel –  – –  – –  – –  – +  + Plasma membrane –  – 5 Inactivationgate Resting state 1 • The generation of an action potential Na+ Na+ Na+ Na+ K+ K+ Rising phase of the action potential Depolarization opens the activation gates on most Na+ channels, while the K+ channels’ activation gates remain closed. Na+ influx makes the inside of the membrane positive with respect to the outside. The inactivation gates on most Na+ channels close, blocking Na+ influx. The activation gates on mostK+ channels open, permitting K+ effluxwhich again makesthe inside of the cell negative. +50 Actionpotential Na+ Na+ 0 Membrane potential (mV) Threshold Threshold –50 K+ Resting potential –100 Time A stimulus opens the activation gates on some Na+ channels. Na+ influx through those channels depolarizes the membrane. If the depolarization reaches the threshold, it triggers an action potential. Na+ Na+ Na+ +  +  +  +  +  +  +  + +  + +  + K+ –  –  –  –  –  –  –  – –  – –  – Undershoot Both gates of the Na+ channelsare closed, but the activation gates on some K+channels are still open. As these gates close onmost K+ channels, and the inactivation gates open on Na+ channels, the membrane returns toits resting state. Cytosol Sodiumchannel K+ The activation gates on the Na+ and K+ channelsare closed, and the membrane’s resting potential is maintained. Figure 48.13

  39. Conduction of Action Potentials • An action potential can travel long distances • By regenerating itself along the axon

  40. – + + + + + + – – + + + + + + Axon Actionpotential An action potential is generated as Na+ flows inward across the membrane at one location. 1 + + – – – – – – Na+ – – – – – – + + – – + + + + + + Actionpotential The depolarization of the action potential spreads to the neighboring region of the membrane, re-initiating the action potential there. To the left of this region, the membrane is repolarizing as K+ flows outward. 2 K+ – – + – – – + – Na+ – – – – – – + + – – + + + + + + K+ Actionpotential The depolarization-repolarization process isrepeated in the next region of the membrane. In this way, local currents of ions across the plasma membrane cause the action potential to be propagated along the length of the axon. 3 K+ – – – – + + + + – + + + + – – – Na+ – – – + + – + + Figure 48.14 – + + – – – + + K+ • At the site where the action potential is generated, usually the axon hillock • An electrical current depolarizes the neighboring region of the axon membrane

  41. Conduction Speed • The speed of an action potential • Increases with the diameter of an axon • In vertebrates, axons are myelinated • Also causing the speed of an action potential to increase

  42. Schwann cell Depolarized region(node of Ranvier) Myelin sheath – –– – – – ++ + Cell body ++ ++ + Axon – – – ++ + – – – Figure 48.15 • Action potentials in myelinated axons • Jump between the nodes of Ranvier in a process called saltatory conduction

  43. Concept 48.4: Neurons communicate with other cells at synapses • In an electrical synapse • Electrical current flows directly from one cell to another via a gap junction • The vast majority of synapses • Are chemical synapses

  44. Postsynapticneuron Synapticterminalof presynapticneurons 5 µm Figure 48.16 • In a chemical synapse, a presynaptic neuron • Releases chemical neurotransmitters, which are stored in the synaptic terminal

  45. Postsynaptic cell Presynapticcell Na+ Neuro-transmitter Synaptic vesiclescontainingneurotransmitter K+ Presynapticmembrane Postsynaptic membrane Ligand-gatedion channel Voltage-gatedCa2+ channel Ca2+ Postsynaptic membrane 3 Synaptic cleft Ligand-gatedion channels 5 4 6 1 2 Figure 48.17 • When an action potential reaches a terminal • The final result is the release of neurotransmitters into the synaptic cleft

  46. Direct Synaptic Transmission • The process of direct synaptic transmission • Involves the binding of neurotransmitters to ligand-gated ion channels

  47. Neurotransmitter binding • Causes the ion channels to open, generating a postsynaptic potential

  48. Postsynaptic potentials fall into two categories • Excitatory postsynaptic potentials (EPSPs) • Inhibitory postsynaptic potentials (IPSPs)

  49. After its release, the neurotransmitter • Diffuses out of the synaptic cleft • May be taken up by surrounding cells and degraded by enzymes

  50. Summation of Postsynaptic Potentials • Unlike action potentials • Postsynaptic potentials are graded and do not regenerate themselves