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Chapter 1

Neuroscience: Exploring the Brain - Chapter 1.pdf

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Human Biology
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John Yeomans

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  Chapter  1  -­‐  Neuroscience:  Past,  Present,  and  Future   The  Origins  of  Neuroscience   -­‐ evidence  of  our  ancestors  boring  holes  into  each  others  skulls   à  trepanation   -­‐ but  aim  was  not  to  kill,  but  instead  to  cure   à  skull  showed  signs  of  healing  after  operation   à  indicated  that  the  procedure  was  carried  out  on  live  subjects  and  was  not  a  ritual  after  death   -­‐ in  ancient  Egypt,  it  was  indicated  that  they  were  aware  of  many  symptoms  of  brain  damage   à  but  to  them,  the  heart  was  the  seat  of  the  soul  and  memories   à  the  rest  of  the  body  was  preserved  for  the  afterlife,  but  the  brain  was  thrown  away   à  this  theory  of  the  heart  being  most  important  was  not  challenged  until  Hippocrates     Views  of  the  Brain  in  Ancient  Greece   -­‐ clear  correlation  between  structure  and  function   -­‐ differences  in  appearance  predict  differences  in  function   -­‐ head  is  specialized  for  sensing  the  environment   -­‐ in  the  head  are  your  eyes,  nose,  ears,  and  tongue   -­‐ Hippocrates  was  the  father  of  Western  Medicine   –  stated  his  belied  that  the  brain  was  not  only  involved  in   sensation,  but  was  also  the  seat  of  intelligence   -­‐ Aristotle:  clung  to  belief  that  the  heart  was  the  center  of  intellect   à  proposed  the  brain  was  a  radiator  for  t he  cooling  of  blood  that  was  overheated  by  the  seething  heart   à  explained  that  the  rational  temperament  of  humans  was  explained  by  the  large  cooling  capacity  of  the  brain     Views  of  the  Brain  During  the  Roman  Empire   -­‐ Galen:  embraced  the  Hippocratic  view  of   the  brain  function  (brain  was  the  seat  of   intelligence)   o physician  of  gladiators   o found  2  major  parts:  cerebrum  (in  the  front)  and  the  cerebellum  (in  the  back)   o cerebellum  was  hard,  and  the  cerebrum  was  soft   à  deducing  function  from  structure,   Galen  proposed  the  cerebrum  was  the  recipient  for  sensations,  and  cerebellum   commands  the  muscles   o he  proposed  this  because  he  recognized  that  to  form  memories,  sensations  must  be   imprinted  to  the  brain  à  must  occur  by  cerebrum   o later  on,  it  was  found  the  cerebrum  is  connected  to  sensation  and  perception  (and  also   the  repository  of  memory),  and  that  cerebellum  was  the  movement  control  center   (even  thought  Galen    reached  the  right  conclusions  from  the  wrong  reasons)   o Galen  cut  open  the  brain  and  found  that  it  was  hollow   o In  these  hollow  spaces  are  ventricles  (similar  to  the  chambers  in  the  heart)   à  in  these   ventricles  are  fluids   o To  Galen,  his  discovery  fits  perfectly  with  the  theory  that  the  body  functioned   according  to  balance  of  4  vital  fluids  (humours)   o Sensations  were  registered  and  movements  initiated  by  the  movement  of  humours  to  or  from  the  brain   ventricles  via  nerves  (nerves  were  believed  to  be  hollow  tubes,  like  blood  vessels)     Views  of  the  Brain  from  Renaissance  to  the  Nineteenth  Century   -­‐ more  details  of  the  brain  was  added  by   Vesalius  (  1514-­‐ 1564)  during  the  Renaissance     -­‐ 1700s:  French  investors  began  developing  hydraulically   controlled  mechanical  devices  à  these  devices  supported   the  notion  that  the  brain  could  be  machinelike  (that  fluid   forced  out  of  the  ventricles  through  the  nerves  might   literally  “pump  you  up”  and  cause  the  movement  of  the   limbs)   -­‐ Descartes  (1596-­‐1960)  from  France:  advocate  of  this   fluid-­‐mechanical  theory  of  brain  function   o Proposed  that  brain  mechanisms  control  human   behaviour  only  to  the  extent  this  behaviour   resembles  that  of  the  beasts   o Human  mental  capacities  exist  outside  the  brain  in  the  “mind”       o Descartes  believed  that  the  mind  is  a  spiritual  entity  that  receives  sensations,  and  commands  movement   by  communicating  with  the  machinery  of  the  brain  via  the  pineal  gland   o Believed  there  is  a  “mind -­‐brain”  problem  à  somehow  the  human  mind  is  distinct   from  the  brain   o However,  modern  neuroscience  research  supports:  The  mind  has  a  physical  basis,   which  is  the  brain   -­‐ scientists  in  the  1700-­‐1800s  broke  away  from  Galen’s  tradition  of  focusing  on  the  ventricles   and  began  to  give  the  substance  of  the  brain  a  closer  look   o observed  that  the  brain  tissue  was  divided  into  2  parts:   grey  matter  and  white   matter   o white  matter:  continuous  with  th e  nerves  of  the  body,  was  correctly  believed  to   contain  fibers  that  bring  information  to  and  from  the  grey  matter   -­‐ end  of  1800s:  nervous  system  has  been  completely  dissected     -­‐ scientists  recognized  that  the  nervous  system  has  :   1. a  central  division,  consisting  of  the  brain  and  spinal  cord   2. a  peripheral  division,  consisting  of  the  network  of  nerves  that  course  though  the  body   -­‐ important  discovery:  observation  that  the  same  general  pattern  of  bumps  (called   gyri)  and   grooves  (called  sulci  and  fissures)  can  be  identified  on  the  surface  of  the  brain  in  every   individual   Ø this  pattern  enables  the  parceling  of  the  cerebrum  into   lobes  à  this  patter  was  the  basis       for  speculation  that  different  functions  might  be  localized  to  the  different  bumps  on  the  brain     Nineteenth  Century  Views  of  the  Brain     Understanding  of  the  nervous  system  at  the  end  of  the  1800s)   -­‐ injury  to  the  brain  can  disrupt  sensations,  movement,  and  thought,  and  can  cause  death   -­‐ the  brain  communicates  with  body  via  the  nerves   -­‐ the  brain  has  different  identifiable  parts,  which  probably  perform  different  functions   -­‐ the  brain  operates  like  a  machine  and  follows  the  laws  of  nature     Nerves  as  Wires   -­‐ Benjamin  Franklin  (1751):  published  a  pamphlet  titled   Experiments  and  Observations  on  Electricity  –  heralded  a   new  understanding  of  electrical  phenomena   -­‐ (Italian  scientist)  Luigi  Galvani  and  (German  biologist)  Emil  du  Bois-­‐Reymond:  shown  that  muscles  can  be   caused  to  twitch  when  nerves  are  stimulated  electrically,  and  the  brain  itself  can  generate  electricity   à  got  rid  of  the  theory  that  nerves  communicate  with  the  brain  by  the  movement  of  fluid   à  new  concept:  nerves  are  “wires”  that  conduct  electrical  signals  to  and  from  the  brain   -­‐ what  was  unresolved  then?  Whether  the  signals  to  the  muscles  causing  movement  use  the  same  wires  as  those  that   register  sensations  from  the  skin   o bidirectional  communication  along  the  same  wires   à  this  was  suggested  by  the  observation  that  when  a   nerve  in  the  body  was  cut,  there  is  a  loss  of  both  sensation  and  movement  in  the  affected  regio n   o however,  it  was  also  known  that  within  each  nerve  of  the  body,  there  are  many  thin  filaments  ( nerve  fibers)  –   each  one  could  serve  as  an  individual  wire  carrying  information  in  a  different  direction   -­‐ 1810  –  (Scottish  Physicians)  Charles  Bell  and  (French  physiologist)  Francois  Magendie:     • know  that  before  the  nerves  attach  to  the  spinal  cord,  the  fibers  divide  into  2  branches/roots   • dorsal  root:  enters  toward  the  back  of  the  spinal  cord   • ventral  root:  enters  toward  the  front     • Bell  tested  the  possibility  that  these  2  spinal  roots  carry  information  in  different  directions   à  by  cutting   each  root  separately  and  observing  the  consequences  in  experimental  animals   à  found  that  cutting  ventral  roots:  caused  muscle  paralysis   • Magendie  later  showed  that  the  dorsal  roots  carry  sensory  information  to  the  spinal  cord   • Bell  and  Magendie  concluded  that  within  each  nerve  there  is  a  mixture  of  many  wires,  some  of  which   bring  information  into  the  brain  and  spinal  cord,  others  send  information  out  to  th e  muscles   • in  each  sensory  and  motor  nerve  fiber,  transmission  is  strictly  one  way   à  the  2  fibers  are  bundled   together  for  most  of  their  length,  but  are  anatomically  segregated  when  they  enter/exit  the  spinal  cord     Localization  of  Specific  Functions  to   Different  Parts  of  the  Brain   -­‐ 1811:  Bell  proposed  that  the  origin  of  the  motor  fibers  is  the  cerebellum,  and  the  destination  of  the  sensory  fibers   is  the  cerebrum   • how  would  you  test  this?       • You  could  do  what  Bell  and  Magendie  did,  destroy  these  parts  of  the  b rain  and  test  for  sensory  and  motor   deficits   • This  approach,  in  which  parts  of  the  brain  was  systematically  destroyed  to  determine  the  function,  is  called   experimental  ablation  method   -­‐ 1823:  (French  physiologist)  Marie-­‐Jean-­‐Pierre  Flourens  used  this  method  (experimental  ablation  method)  in  a   variety  of  birds  to  show  that  the  cerebellum  does  play  a  role  in  the  coordination  of  movement   • also  concluded  that  the  cerebrum  is  involved  in  sensation  and  perception  (as   Bell  and  Galen  had   suggested  before)   • only  difference  between  Flourens  and  Bell  &  Galen?  Flourens  was  able  to  provide  solid  experimental   support  for  this  conclusions   -­‐ Franz  Joseph  Gall  (Austrian  medical  student):  believed  that  the  bumps  on  the  brain’s  surface  performed  different   functions  à  seemed  to  have  the  right  idea   • Believed  that  the  bumps  on  the  surface  of  the  skull  reflect  the  bumps  on  the  surface  of  the  brain   • Gall  proposed  in  1809  that  the  propensity  for  certain  personality  traits  (ex.  generosity,  secretiveness,   destructiveness)  could  be  related  to  the  dimensions  of  the  head
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