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Chapter 7

Chapter 7. The Nervous System: Neurons and Synapses. 7-1. Chapter 7 Outline Structure of NS Neurons Supporting/Glial Cells Membrane Potential Action Potential Axonal Conduction Synaptic Transmission Neurotransmitters Synaptic Integration. 7-2. Structure of Nervous System. 7-3.

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Chapter 7

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  1. Chapter 7 The Nervous System: Neurons and Synapses 7-1

  2. Chapter 7 Outline • Structure of NS • Neurons • Supporting/Glial Cells • Membrane Potential • Action Potential • Axonal Conduction • Synaptic Transmission • Neurotransmitters • Synaptic Integration 7-2

  3. Structure of Nervous System 7-3

  4. Nervous System (NS) • Is divided into: • Central nervous system (CNS) • = brain and spinal cord • Peripheral nervous system (PNS) • = cranial and spinal nerves 7-4

  5. Nervous System (NS) continued • Consists of 2 kinds of cells: • Neurons and supporting cells (= glial cells) • Neurons are functional units of NS • Glial cells maintain homeostasis • Are 5X more common than neurons 7-5

  6. Neurons 7-6

  7. Neurons • Gather and transmit information by: • Responding to stimuli • Producing and sending electrochemical impulses • Releasing chemical messages 7-7

  8. Neurons continued • Have a cell body, dendrites and axon • Cell body contains the nucleus 7-8

  9. Neurons continued • Cell body is the nutritional center and makes macromolecules • Groups of cell bodies in CNS are called nuclei; in PNS are called ganglia 7-9

  10. Neurons continued • Dendrites receive information, convey it to cell body • Axons conduct impulses away from cell body 7-10

  11. Neurons continued • Long axon length necessitates special transport systems: • Axoplasmic flow moves soluble compounds toward nerve endings • Via rhythmic contractions of axon 7-11

  12. Neurons continued • Axonal transport moves large and insoluble compounds bidirectionally along microtubules;very fast • Anterograde transport moves materials away from cell body • Uses the molecular motor kinesin • Retrograde transport moves materials toward cell body • Uses the molecular motor dynein • Viruses and toxins can enter CNS this way 7-12

  13. Functional Classification of Neurons • Sensory/Afferent neurons conduct impulses into CNS • Motor/Efferent neurons carry impulses out of CNS • Association/ Interneurons integrate NS activity • Located entirely inside CNS 7-13

  14. Structural Classification of Neurons • Pseudounipolar: • Cell body sits along side of single process • e.g. sensory neurons • Bipolar: • Dendrite and axon arise from opposite ends of cell body • e.g. retinal neurons • Multipolar: • Have many dendrites and one axon • e.g. motor neurons 7-14

  15. Supporting/Glial Cells 7-15

  16. Supporting/Glial Cells • PNS has Schwann and satellite cells • Schwann cells myelinate PNS axons 7-16

  17. Supporting/Glial Cells continued • CNS has oligodendrocytes, microglia, astrocytes and ependymal cells 7-17

  18. Supporting/Glial Cells continued • Each oligodendrocyte myelinates several CNS axons • Ependymal cells appear to be neural stem cells • Other glial cells are involved in NS maintenance 7-18

  19. Myelination • In PNS each Schwann cell myelinates 1mm of 1 axon by wrapping round and round axon • Electrically insulates axon 7-19

  20. Myelination continued • Uninsulated gap between adjacent Schwann cells is called the node of Ranvier 7-20

  21. Axon Regeneration • Occurs much more readily in PNS than CNS • Oligodendrocytes produce proteins that inhibit regrowth • And form glial scar tissue that blocks regrowth 7-21

  22. Nerve Regeneration continued • When axon in PNS is severed: • Distal part of axon degenerates • Schwann cells survive; form regeneration tube • Tube releases chemicals that attract growing axon • Tube guides regrowing axon to synaptic site 7-22

  23. Neurotrophins • Promote fetal nerve growth • Required for survival of many adult neurons • Important in regeneration 7-23

  24. Astrocytes • Most common glial cell • Involved in: • Buffering K+ levels • Recycling neurotransmitters • Regulating adult neurogenesis • Releasing transmitters that regulate neuronal activity 7-24

  25. Blood-Brain Barrier • Allows only certain compounds to enter brain • Formed by capillary specializations in brain • That appear to be induced by astrocytes • Capillaries are not as leaky as those in body • Gaps between adjacent cells are closed by tight junctions 7-25

  26. Membrane Potential 7-26

  27. Resting Membrane Potential (RMP) • At rest, all cells have a negative internal charge and unequal distribution of ions: • Results from: • Large cations being trapped inside cell • Na+/K+ pump and limited permeability keep Na+ high outside cell • K+ is very permeable and is high inside cell • Attracted by negative charges inside 7-27

  28. Excitability • Excitable cells can discharge their RMP quickly • By rapid changes in permeability to ions • Neurons and muscles do this to generate and conduct impulses 7-28

  29. Membrane Potential (MP) Changes • Measured by placing 1 electrode inside cell and 1 outside • Depolarization occurs when MP becomes more positive • Hyperpolarization: MP becomes more negative than RMP • Repolarization: MP returns to RMP 7-29

  30. Membrane Ion Channels • MP changes occur by ion flow through membrane channels • Some channels are normally open; some closed • K+ leakage channels are always open • Closed channels have molecular gates that can be opened • Voltage-gated (VG) channels are opened by depolarization • VG K+ channels are closed in resting cells • Na+ channels are VG; closed in resting cells 7-30

  31. Model of a Voltage-gated Ion Channel) 7-31

  32. Action Potential 7-32

  33. The Action Potential (AP) • Is a wave of MP change that sweeps along the axon from soma to synapse • Wave is formed by rapid depolarization of the membrane by Na+ influx; followed by rapid repolarization by K+ efflux 7-33

  34. Mechanism of Action Potential • Depolarization: • At threshold, VG Na+ channels open • Na+ driven inward by its electrochemical gradient • This adds to depolarization, opens more channels • Termed a positive feedback loop • Causes a rapid change in MP from –70 to +30 mV 7-34

  35. Mechanism of Action Potential continued • Repolarization: • VG Na+ channels close; VG K+ channels open • Electrochemical gradient drives K+ outward • Repolarizes axon back to RMP 7-35

  36. Mechanism of Action Potential continued • Depolarization and repolarization occur via diffusion • Do not require active transport • After an AP, Na+/K+ pump extrudes Na+, recovers K+ 7-36

  37. Mechanism of Action Potential continued 7-37

  38. APs Are All-or-None • When MP reaches threshold an AP is irreversibly fired • Because positive feedback opens more and more Na+ channels • Shortly after opening, Na+ channels close • and become inactivated until repolarization 7-38

  39. How Stimulus Intensity is Coded • Increased stimulus intensity causes more APs to be fired • Size of APs remains constant 7-39

  40. Refractory Periods • Absolute refractory period: • Membrane cannot produce another AP because Na+ channels are inactivated • Relative refractory period occurs when VG K+ channels are open, making it harder to depolarize to threshold 7-40

  41. Axonal Conduction 7-41

  42. Cable Properties • Refer to how axon’s properties affect its ability to conduct current • Includes high resistance of cytoplasm • Resistance decreases as axon diameter increases • Current leaks out through ion channels 7-42

  43. Conduction in an Unmyelinated Axon • After axon hillock reaches threshold and fires AP, its Na+ influx depolarizes adjacent regions to threshold • Generating a new AP • Process repeats all along axon • So AP amplitude is always same • Conduction is slow 7-43

  44. Conduction in Myelinated Axon • Ions can't flow across myelinated membrane • Thus no APs occur under myelin • and no current leaks • This increases current spread 7-44

  45. Conduction in Myelinated Axon continued • Gaps in myelin are called Nodes of Ranvier • APs occur only at nodes • VG Na+ channels are present only at nodes • Current from AP at 1 node can depolarize next node to threshold • Fast because APs skip from node to node • Called saltatory conduction 7-45

  46. Synaptic Transmission 7-46

  47. Synapse • Is a functional connection between a neuron (presynaptic) and another cell (postsynaptic) • There are chemical and electrical synapses • Synaptic transmission at chemical synapses is via neurotransmitters (NT) • Electrical synapses are rare in NS 7-47

  48. Electrical Synapse • Depolarization flows from presynaptic into postsynaptic cell through channels called gap junctions • Formed by connexin proteins • Found in smooth and cardiac muscles, brain, and glial cells 7-48

  49. Chemical Synapse • Synaptic cleft separates terminal bouton of presynaptic from postsynaptic cell • NTs are in synaptic vesicles • Vesicles fuse with bouton membrane; release NT by exocytosis • Amount of NT released depends upon frequency of APs 7-49

  50. Synaptic Transmission • APs travel down axon to depolarize bouton • Open VG Ca2+ channels in bouton • Ca2+ is driven in by electrochemical gradient • Triggers exocytosis of vesicles; release of NTs 7-50

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