1 / 163

Chapter 48

Chapter 48. Nervous Systems. Overview: Command and Control Center. The human brain contains about 100 billion nerve cells, or neurons Each neuron may communicate with thousands of other neurons.

flavia-hancock
Download Presentation

Chapter 48

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Chapter 48 Nervous Systems

  2. Overview: Command and Control Center • The human brain contains about 100 billion nerve cells, or neurons • Each neuron may communicate with thousands of other neurons

  3. Functional magnetic resonance imaging is a technology that can reconstruct a three-dimensional map of brain activity • Brain imaging and other methods reveal that groups of neurons function in specialized circuits dedicated to different tasks

  4. Concept 48.1: Nervous systems consist of circuits of neurons and supporting cells • All animals except sponges have a nervous system • What distinguishes nervous systems of different animal groups is how neurons are organized into circuits

  5. Organization of Nervous Systems • The simplest animals with nervous systems, the cnidarians, have neurons arranged in nerve nets

  6. LE 48-2a Radial nerve Nerve ring Nerve net Hydra (cnidarian) Sea star (echinoderm)

  7. Sea stars have a nerve net in each arm connected by radial nerves to a central nerve ring

  8. LE 48-2b Eyespot Brain Brain Nerve cord Ventral nerve cord Transverse nerve Segmental ganglion Planarian (flatworm) Leech (annelid)

  9. Relatively simple cephalized animals, such as flatworms, have a central nervous system (CNS)

  10. LE 48-2c Ganglia Brain Anterior nerve ring Ventral nerve cord Longitudinal nerve cords Segmental ganglia Insect (arthropod) Chiton (mollusc)

  11. 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)

  12. LE 48-2d Brain Spinal cord (dorsal nerve cord) Brain Sensory ganglion Ganglia Squid (mollusc) Salamander (chordate)

  13. Nervous systems in molluscs correlate with lifestyles • Sessile molluscs have simple systems, whereas more complex molluscs have more sophisticated systems

  14. In vertebrates, the central nervous system consists of a brain and dorsal spinal cord • The PNS connects to the CNS

  15. Information Processing • Nervous systems process information in three stages: sensory input, integration, and motor output

  16. LE 48-3 Sensory input Sensor Integration Motor output Effector Central nervous system (CNS) Peripheral nervous system (PNS)

  17. 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 • The three stages of information processing are illustrated in the knee-jerk reflex

  18. LE 48-4 Gray matter Cell body of sensory neuron in dorsal root ganglion Quadriceps muscle White matter Hamstring muscle Spinal cord (cross section) Sensory neuron Motor neuron Interneuron

  19. Neuron Structure • Most of a neuron’s organelles are in the cell body • 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 • Many axons are covered with a myelin sheath

  20. LE 48-5 Dendrites Cell body Nucleus Synapse Signal direction Axon hillock Axon Presynaptic cell Synaptic terminals Myelin sheath Postsynaptic cell

  21. Neurons have a wide variety of shapes that reflect input and output interactions

  22. LE 48-6 Dendrites Axon Cell body Interneurons Sensory neuron Motor neuron

  23. Supporting Cells (Glia) • Glia are essential for structural integrity of the nervous system and for functioning of neurons • Types of glia: astrocytes, radial glia, oligodendrocytes, and Schwann cells

  24. In the CNS, astrocytes provide structural support for neurons and regulate extracellular concentrations of ions and neurotransmitters

  25. LE 48-7 50 µm

  26. Oligodendrocytes (in the CNS) and Schwann cells (in the PNS) form the myelin sheaths around axons of many vertebrate neurons

  27. LE 48-8 Nodes of Ranvier Layers of myelin Axon Schwann cell Schwann cell Nucleus of Schwann cell Nodes of Ranvier Axon Myelin sheath 0.1 µm

  28. 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 cell’s inside is negative relative to the outside • Membrane potential of a cell can be measured

  29. LE 48-9 Microelectrode –70 mV Voltage recorder Reference electrode

  30. The Resting Potential • Resting potential is the membrane potential of a neuron that is not transmitting signals • Resting potential depends on ionic gradients across the plasma membrane Animation: Resting Potential

  31. Concentration of Na+ is higher in the extracellular fluid than in the cytosol • The opposite is true for K+ • By modeling a neuron with an artificial membrane, we can better understand resting potential

  32. LE 48-10 CYTOSOL EXTRACELLULAR FLUID [Na+] 150 mM [Na+] 15 mM [K+] 150 mM [K+] 5 mM [Cl–] 120 mM [Cl–] 10 mM [A–] 100 mM Plasma membrane

  33. LE 48-11 –92 mV +62 mV Inner chamber Outer chamber Inner chamber Outer chamber 150 mM NaCl 150 mM KCl 15 mM NaCl 5 mM KCl Cl– K+ Na+ Cl– Sodium channel Potassium channel Artificial membrane Membrane selectively permeable to K+ Membrane selectively permeable to Na+

  34. A neuron that is not transmitting signals contains many open K+ channels and fewer open Na+ channels in its plasma membrane • Diffusion of K+ and Na+ leads to a separation of charges across the membrane, producing the resting potential

  35. Gated Ion Channels • Gated ion channels open or close in response to one of three stimuli: • Stretch-gated ion channels open when the membrane is mechanically deformed • Ligand-gated ion channels open or close when a specific chemical binds to the channel • Voltage-gated ion channels respond to a change in membrane potential

  36. 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 • Some stimuli trigger a hyperpolarization, an increase in magnitude of the membrane potential

  37. LE 48-12 Stimuli Stronger depolarizing stimulus Stimuli +50 +50 +50 Action potential 0 0 0 Membrane potential (mV) Membrane potential (mV) Membrane potential (mV) Threshold Threshold –50 –50 –50 Threshold Resting potential Resting potential Resting potential Depolarizations Hyperpolarizations –100 –100 –100 1 1 4 1 4 0 0 5 2 3 0 6 2 3 5 2 3 5 4 Time (msec) Time (msec) Time (msec) Graded potential hyperpolarizations Graded potential depolarizations Action potential

  38. Other stimuli trigger a depolarization, a reduction in the magnitude of the membrane potential

  39. Hyperpolarization and depolarization are called graded potentials • The magnitude of the change in membrane potential varies with the strength of the stimulus

  40. Production of Action Potentials • Depolarizations are usually graded only up to a certain membrane voltage, called the threshold • A stimulus strong enough to produce depolarization that reaches the threshold triggers a response called an action potential

  41. An action potential is a brief all-or-none depolarization of a neuron’s plasma membrane • It carries information along axons

  42. Voltage-gated Na+ and K+ channels are involved in producing an action potential • When a stimulus depolarizes the membrane, Na+ channels open, allowing Na+ to diffuse into the cell

  43. As the action potential subsides, K+ channels open, and K+ flows out of the cell • During the refractory period after an action potential, a second action potential cannot be initiated Animation: Action Potential

  44. LE 48-13_5 Na+ Na+ Na+ Na+ K+ K+ Rising phase of the action potential Falling phase of the action potential +50 Action potential Na+ Na+ 0 Membrane potential (mV) Threshold –50 K+ Resting potential –100 Time Depolarization Na+ Na+ Extracellular fluid Potassium channel Activation gates Na+ K+ Plasma membrane Undershoot Cytosol Sodium channel Inactivation gate K+ Resting state

  45. Conduction of Action Potentials • An action potential can travel long distances by regenerating itself along the axon • At the site where the action potential is generated, usually the axon hillock, an electrical current depolarizes the neighboring region of the axon membrane

  46. LE 48-14c Axon Action potential Na+ An action potential is generated as Na+ flows inward across the membrane at one location. Action potential K+ Na+ K+ 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. Action potential K+ Na+ K+ The depolarization-repolarization process is repeated 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.

  47. Conduction Speed • The speed of an action potential increases with the axon’s diameter • In vertebrates, axons are myelinated, also causing an action potential’s speed to increase • Action potentials in myelinated axons jump between the nodes of Ranvier in a process called saltatory conduction

  48. LE 48-15 Schwann cell Depolarized region (node of Ranvier) Cell body Myelin sheath Axon

  49. Concept 48.4: Neurons communicate with other cells at synapses • In an electrical synapse, current flows directly from one cell to another via a gap junction • The vast majority of synapses are chemical synapses • In a chemical synapse, a presynaptic neuron releases chemical neurotransmitters stored in the synaptic terminal

More Related