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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 & spinal cord • Peripheral nervous system (PNS) • = cranial & spinal nerves 7-4

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

  6. Neurons 7-6

  7. Neurons • Gather & transmit information by: • Responding to stimuli • Sending electrochemical impulses • Releasing chemical messages 7-7

  8. Neurons continued • Have a cell body, dendrites, & axon • Cell body contains nucleus Fig 7.1 7-8

  9. Neurons continued • Cell body makes macromolecules • Groups of cell bodies in CNS are called nuclei; in PNS are called ganglia Fig 7.1 7-9

  10. Neurons continued • Dendrites receive information, convey it to cell body • Axons conduct impulses away from cell body Fig 7.1 7-10

  11. Neurons continued • Axon length necessitates special transport systems: • Axoplasmic flow moves soluble compounds toward nerve endings • Via rhythmic contractions of axon • Axonal transport moves large & insoluble compounds bidirectionally • Along microtubules; very fast • Viruses & toxins enter CNS this way 7-11

  12. 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 Fig 7.3 7-12

  13. Structural Classification of Neurons • Pseudounipolar: • Cell body sits along side of single process • e.g. sensory neurons • Bipolar: • Dendrite & axon arise from opposite ends of cell body • e.g. retinal neurons • Multipolar: • Have many dendrites & one axon • e.g. motor neurons Fig 7.4 7-13

  14. Supporting/Glial Cells 7-14

  15. Supporting/Glial Cells • PNS has Schwann&satellite cells • Schwann cells myelinate PNS axons Fig 7.2 7-15

  16. Supporting/Glial Cells continued • CNS has oligodendrocytes, microglia, astrocytes, & ependymal cells Fig 7.5 7-16

  17. Supporting/Glial Cells continued • Each oligodendrocyte myelinates several CNS axons • Ependymal cells are neural stem cells • Other glial cells are involved in NS maintenance Fig 7.8 7-17

  18. Myelination • In PNS each Schwann cell myelinates 1mm of 1 axon by wrapping round & round axon • Electrically insulates axon Fig 7.6 7-18

  19. Myelination continued • Uninsulated gap between adjacent Schwann cells is called node of Ranvier Fig 7.2 7-19

  20. Nerve Regeneration • Occurs much more readily in PNS than CNS • Oligodendrocytes produce proteins that inhibit regrowth Neurotrophins • Promote fetal nerve growth • Required for survival of many adult neurons • Important in regeneration 7-20

  21. Nerve Regeneration continued Fig 7.9 • 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 7-21

  22. Astrocytes • Most common glial cell • Involved in: • Inducing capillaries to form blood-brain barrier • Buffering K+ levels • Recycling neurotransmitters • Regulating adult neurogenesis • Maintain interstitial fluid Fig 7.10 7-23

  23. Blood-Brain Barrier • Allows only certain compounds to enter brain • Formed by capillary specializations in brain • Capillaries are not as leaky as those in body • Do not have gaps between adjacent cells • Closed by tight junctions 7-24

  24. Membrane Potential 7-25

  25. Resting Membrane Potential (RMP) • At rest, all cells have a negative internal charge & unequal distribution of ions: • Results from: • Large anions being trapped inside cell • Na+/K+ pump & limited permeability keep Na+ high outside cell • K+ is very permeable & is high inside cell • Attracted by negative charges inside 7-26

  26. Excitability • Excitable cells can discharge their RMP quickly • By rapid changes in permeability to ions • Neurons & muscles do this to generate & conduct impulses 7-27

  27. Membrane Potential (MP) Changes • Measured by placing 1 electrode inside cell & 1 outside • Depolarization occurs when MP becomes more positive • Hyperpolarization: MP becomes more negative than RMP • Repolarization: MP returns to RMP Fig 7.11 7-28

  28. Membrane Ion Channels • MP changes occur by ion flow through membrane channels • Some channels are normally open; some closed • Closed channels have molecular gates that can be opened • Voltage-gated (VG) channels are opened by depolarization • 1 type of K+ channel is always open; other type is VG & is closed in resting cell • Na+ channels are VG; closed in resting cells 7-29

  29. Action Potential 7-31

  30. The Action Potential (AP) Fig 7.13 • 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 • Depolarization causes more channels to open (positive feedback loop 7-32

  31. Mechanism of Action Potential continued • Depolarization & repolarization occur via diffusion • Do not require active transport • After an AP, Na+/K+ pump extrudes Na+, recovers K+ • APs Are All-or-None • When MP reaches threshold, an AP is irreversibly fired • Because positive feedback opens more & more Na+ channels • Shortly after opening, Na+ channels close • & become inactivated until repolarization 7-35

  32. Refractory Periods Fig 7.16 • 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 7-38

  33. Axonal Conduction 7-39

  34. Cable Properties • Refers to ability of axon to conduct current • Axon cable properties are poor because: • Cytoplasm has high resistance • Though resistance decreases as axon diameter increases • Current leaks out through ion channels 7-40

  35. Conduction in an Unmyelinated Axon Fig 7.18 • After axon hillock reaches threshold & 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 7-41

  36. Conduction in Myelinated Axon • Ions can't flow across myelinated membrane • Thus no APs occur under myelin • & no current leaks • Increases current spread Fig 7.19 7-42

  37. Conduction in Myelinated Axon continued • Gaps in myelin are called Nodes of Ranvier • APs occur 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 Fig 7.19 7-43

  38. Synaptic Transmission 7-44

  39. Synapse • Is a functional connection between a neuron (presynaptic) & another cell (postsynaptic) • There are chemical & electrical synapses • Synaptic transmission in chemicals is via neurotransmitters (NT) • Electrical synapses are rare in NS 7-45

  40. Electrical Synapse • Depolarization flows from presynaptic into postsynaptic cell through channels called gap junctions • Formed by connexin proteins • Found in smooth & cardiac muscles, brain, and glial cells Fig 7.20 7-46

  41. Chemical Synapse Fig 7.22 • 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 7-47

  42. Synaptic Transmission • APs travel down axon to depolarize bouton • Open VG Ca2+ channels in bouton • Ca2+ driven in by electrochemical gradient • Triggers exocytosis of vesicles; release of NTs 7-48

  43. Neurotransmitter Release • Is rapid because vesicles are already docked at release sites on bouton before APs arrive • Docked vesicles are part of fusion complex • Ca2+ triggers exocytosis of vesicles 7-49

  44. Synaptic Transmission continued • NT (ligand) diffuses across cleft • Binds to receptor proteins on postsynaptic membrane • Chemically-regulated ion channels open • Depolarizing channels cause EPSPs (excitatory postsynaptic potentials) • Hyperpolarizing channels cause IPSPs (inhibitory postsynaptic potentials) • These affect VG channels in postsynaptic cell 7-50

  45. Synaptic Transmission continued • EPSPs & IPSPs summate • If MP in postsynaptic cell reaches threshold, a new AP is generated Fig 7.23 7-51

  46. Acetylcholine (ACh) • Most widely used NT • NT at all neuromuscular junctions • Used in brain • Used in ANS • Where can be excitatory or inhibitory • Depending on receptor subtype • Nicotinic or muscarinic 7-52

  47. Ligand-Operated Channels • Ion channel runs through receptor • Opens when ligand (NT) binds 7-53

  48. Nicotinic ACh Channel • Formed by 5 polypeptide subunits • 2 subunits contain ACh binding sites • Opens when 2 AChs bind • Permits diffusion of Na+ into and K+ out of postsynaptic cell • Inward flow of Na+ dominates • Produces EPSPs Fig 7.24 7-54

  49. G Protein-Operated Channels • Receptor is not part of the ion channel • Is a 1 subunit membrane polypeptide • Activates channel indirectly through G-proteins 7-55

  50. Muscarinic ACh Channel • Binding of 1 ACh activates G-protein cascade • Opens some K+ channels, causing hyperpolarization • Closes others, causing depolarization Fig 7.25 7-56

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