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Chapter 2

Neuroscience - Exploring the Brain - Chapter 2.pdf
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Department
Human Biology
Course
HMB200H1
Professor
John Yeomans
Semester
Winter

Description
  Chapter  2:  Neurons  and  Glia   Introduction   -­‐ neurons  sense  changes  in  the  environment,  communicate  these  changes  to  other  neurons,  and  command  the   body’s  responses  to  these  sensations   -­‐ glia  (glial  cells)  are  thought  to  contribute  to  brain  function  mainly  by  insulating,  supporting,  and  nourishing   neighboring  neurons     The  Neuron  Doctrine   -­‐ progress  in  cellular  neuroscience  was  not  possible  before  the  development  of  the  co mpound  microscope  because   of  the  brain  cells’  small  size   -­‐ but  the  brain  was  too  “jello”   à  in  order  to  study  the  anatomy  of  brain  cells,  had  to  wait  for  a  development  of  a   method  to  harden  the  tissue  without  disturbing  its  structure  and  instrument  that  could  produce  very  thin  slices   -­‐ in  early  1900s,  scientists  discovered  formaldehyde  can  be  used  to  harden  tissues  by  immersing  them  into  it   à   developed  the  microtome  to  make  thin  slices   -­‐ these  technical  advances  spawned  the  field  of   histology:  the  microscopic  study  of  the  structure  of  tissues   -­‐ next  problem?  Freshly  prepared  brain  has  a  uniform,  cream -­‐coloured  appearance  under  the  microscope   à  tissue   had  no  differences  in  pigmentation  to  enable  histologists  to  resolve  individual  cells   -­‐ next  breakthrough:  introduction   of  stains  that  could  selectively  colour  some  (not  all)  parts  of  the  cells  in  the  brain   tissue   -­‐ Franz  Nissl  (German  neurologists)  1900s:   introduced  a  stain  used  today  still.  Showed  that  a   class  of  basic  dyes  would  stain  the  nuclei  of  all  cells   and  also  stain  clumps  of  material  surrounding  the   nuclei  of  neurons  à  these  clumps  are  called  Nissl   bodies  à  the  stain  was  called  Nissl  stain   à  the  Nissl  stain  was  powerful  for  2  reasons:   1) distinguishes  neurons  and  glial  from  one   another   2) enables  histologists  to  study  the  arrangement,  or   cytoarchitecture,  of  neurons  in  different  parts  of  the  brain   -­‐ the  study  of  cytoarchitecture  led  to  the  realization  that  the  brain  consists  of  many  specialized  regions   à  we  now   know  each  region  performs  diffe rent  functions     The  Golgi  Stain   -­‐ Problem  with  the  Nissl  stain?  he  Nissl  stain  does  not  tell  the  whole  story   à  Nissl-­‐stained-­‐neuron  looks  like  a  lump   of  protoplasm  containing  a  nucleus   -­‐ 1873  Camillo  Golgi  (Italian  histologist):  discovered  that   soaking  brain  tissue  in  a  silver  chromatic  solution  ( Golgi   stain),  a  small  percentage  of  neurons  became  darkly   coloured  in  their  entirety     § revealed  that  in  the  neuronal  cell  body  (the  region  of   the  neuron  around  the  nucleus  that  is  shown  with   the  Nissl  stain)  is  actually  only  a  small  fraction  of  the   total  structure  of  the  neuron   § the  Golgi  stain  shows  that  neurons  have  at  least  2   distinguishable  parts:   1) central  region  that  contains  the  cell  nucleus   2) numerous  tubes  that  radiate  away  from  the   central  region   -­‐ the  swollen  region  containing  the  cell  nucleus  has  several   names:  cell  body,  some,  perikaryon   -­‐ the  thin  tubes  that  radiate  away  from  the  soma  are  called   neuritis  à  2  types  of  neuritis:  axons  and  dendrites   -­‐ the  cell  body  gives  rise  to  a  SINGLE  axon     -­‐ axon  is  of  uniform  diameter  throughout  its  length,  and  if  it   braches,  the  branches  generally  extend  at  right  angles   -­‐ axons  can  travel  great  distances  in  the  body  =  recognized  by  histologists  that  axons  must  act  like  “wires”  that  carry   output  of  the  neurons   -­‐ dendrites  on  the  other  hand ,  rarely  extend  more  than  2mm  à  many  extend  from  the  cell  body         -­‐ since  dendrites  come  into  contact  with  many  axons,  they  must  act  as  the  antennae  of  the  neuron  to  receive   incoming  signals     Cajal’s  Contribution   -­‐ Santiago  Ramon  y  Cajal  (histologist)  1888:  learned  about   Golgi’s  method   -­‐ In  a  series  of  publications  over  the  next  25  years  after  1888,  Cajal   used  the  Golgi  stain  to  work  out  the  circuitry  of  main  regions  of   the  brain   -­‐ Golgi  and  Cajal  drew  completely  opposite  conclusions  about   neurons   -­‐ Golgi  was  correct  about  how  neuritis  of  different  cells  are  fused   together  to  form  a  continuous  reticulum  or  network  (like  the   veins  and  arteries  system)   à  According  to  the  reticular  theory:  the  brain  is  an  exception  to   the  cell  theory  (states  that  the  individual  cell  is  the  elementary   functional  unit  of  all  animal  tissues)   -­‐ Cajal  argued  that  the  neuritis  of  different  neurons  are  not   continuous  with  one  another  and  must  communicate  by  contact,   not  continuity     -­‐ The  idea  that  the  neuron  adhered  to  the  cell  theory  =  Neuron   Doctrine   -­‐ 1950s:  electron  microscope  à  with  increased  resolving  power  of  microscopes,  it  was  discovered  that  neuritis  of   different  neurons  are  not  continuous  with  one  another   -­‐ Cajal:  discovered  dendritic  spines   -­‐ Willliam  Greenough :  discovered  that  spine  number  is  also  very  sensitive  to  the  quality  of  environment   experienced  during  early  development  and  in  adulthood         The  Prototypical  Neuron         -­‐ difference  between  proteins  synthesized  on  the  rough  ER  and  those  on  the  free  ribosomes?   -­‐ If  it  is  designed  to  reside  within  the  cytosol  of  the  neuron   à  the  proteins  mRNA  transcript  goes  towards  the  free   ribosomes   -­‐ If  protein  is  destined  to  be  inserted  into  the  m embrane  of  a  cell/organelle  à  synthesized  in  the  rough  ER   -­‐ Neurons  are  so  endowed  with  rough  ER  because  special  membrane  proteins  are  what  gives  them  their  information -­‐ processing  abilities   -­‐ When  a  mitochondria  inhales,  it  pulls  in  pyruvic  acid
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