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PSYC 370
Janet L Menard

PSYC  370   Chapter  9   Development  of  the  Nervous  System     Phases  of  Neurodevelopment     1. Induction  of  the  neural  plate   2. Neural  proliferation   3. Migration  and  aggregation   4. Axon  growth  and  synapse  development   5. Neuron  death  and  synapse  rearrangement     Each  of  these  will  be  discussed  in  greater  detail.       Ovum  +  Sperm  à  Zygote.     In  order  for  the  zygote  to  become  a  mature  organism,  there  must  be  cell  multiplication  and...   i. Differentiation   ii. Travelling  to  the  correct  location   iii. Functional  relationships  with  other  cells     Layers  of  embryonic  cells:   i. Ectoderm  (outermost)   ii. Mesoderm   iii. Endoderm     (1)  Induction  of  the  Neural  Plate.       - Development  of  the  neural  plate  begins  with   chemical  signals  from  the  organizer  area.     - Background  information  on  this  listed   below   - The  lips  of  the  neural  plate  fold  to  form  the   neural  groove.   - The  neural  tube  is  formed  once  the  lips  of  the   neural  groove  close.     - Eventually  becomes  the  cerebral   ventricles  and  spinal  canal.       Neural  plate:    small  patch  of  ectodermal  tissue  on  the   dorsal  surface  of  the  developing  embryo.       The  development  of  the  neural  plate  is  the  first  major   step  of  neurodevelopment.       Development  is  induced  by  chemical  signals  from  the   organizer  area  (area  under  the  mesoderm  layer).   (9  :  1)   PSYC  370   The  earliest  cells  in  the  embryo  are  totipotent  –  they  can  differentiate  into  any  type  of  cell.  Once   the  embryo  develops  the  future  function  of  the  cell  becomes  more  specified.       Once  the  neural  plate  develops,  its  cells  lose  some  of  their  potential.  They  become  multipotent  –   able  to  differentiate  into  many  different  types  of  mature  cells  but  not  all  (to  end  up  normal).       Stem  cells:  the  cells  of  the  neural  plate;  meeting  two  criteria:   i. Unlimited  self-­‐renewal  when  contained  in  appropriate  cell  culture   ii. Ability  to  develop  into  different  kinds  of  mature  cells     This  ability  is  lost  once  the  neural  plate  is  formed  into  the  neural  tube.  Continued  on  next  page.   As  the  neural  tube  develops,  cells  on  the  neural  plate  will  either  become  glial  cells  or  neurons.   They  are  then  called  glial  stem  cells  or  neural  stem  cells,  respectively.       40  days  after  conception,  three  swellings  are  visible  at  the  anterior  of  the  neural  tube.  These  will   become  the  forebrain,  midbrain,  and  hindbrain.       (2)  Neural  Proliferation.     Once  the  neural  tube  is  created,  the  cells  of  the  tube  begin  to  proliferate  (increase  rapidly  in   number).       Proliferation  is  not  equal  in  all  areas  of  the  tube.   - Most  cell  division  occurring  in  the  ventricular  zone   - Proliferation  occurring  at  different  times  creates  the  sequence  that  gives  rise  to  the  pattern   of  swelling  and  folding  that  gives  the  brain  its  characteristic  shape   - Different  in  each  species     Patterns  of  proliferation  are  controlled  by  two  organizer  areas  in  the  neural  tube:   i. Floor  plate     ii. Roof  plate     (3)  Migration  and  Aggregation.     Migration.     Migration:  movement  of  cells  from  their  site  of  creation  (in  the  ventricular  zone  of  the  neural   tube)  to  their  ultimate  location  in  the  mature  nervous  system.       Once  cells  have  been  created  through  cell  division  in  the  ventricular  zone,  they  migrate  to  the   appropriate  location.       Major  factors  governing  migration:  time  and  location.     - Neuron  subtypes  arise  at  the  same  time  and  travel  to  their  destination  together       (9  :  2)   PSYC  370   Types  of  migration   i. Radial  migration:  proceeds  in  a  straight  line  from  the   outer  wall  of  the  neural  tube.   ii. Tangential  migration:  proceeds  at  a  right  angle  to   radial  migration  (i.e.,  parallel  to  the  tube’s  walls).       Methods  of  migration   i. Somal  translocation:  the  cell   body  moves  along  an   extension  that  scans  the   environment  ahead.   First. Radial  or  tangential     ii. Glia-­‐mediated  migration:   filaments  called  radial  glial   cells  develop  in  the  neural   tube;  cells  move  along  these   filaments  to  their   destinations.   - Radial             Inside-­‐out  pattern:  migration  of  cells  to  deeper  layers  occurs  first,  with  superficial  layer  cells   migrating  last;  this  means  that  a  cell  migrates  through  already  formed  layers  before  reaching  its   destination.       Neural  crest:  located  dorsal  to  the  neural  tube;  its  cells  eventually  form  the  peripheral  nervous   system.       Aggregation.     Aggregation:  alignment  of  cells  within  different  areas  of  the  embryo  to  form  various  structures.     Cell-­‐adhesion  molecules  (CAMs):  located  on  the  surfaces  of  neurons  and  glial  cells;  recognize   molecules  on  other  cells  and  adhere  to  them.     - Important  for  aggregation   - Also  mediates  migration     Gap  junctions  (narrow  gaps  between  neurons  that  are  connected)  are  also  implicated  in   aggregation.           (9  :  3)   PSYC  370   (4)  Axon  Growth  and  Synapse  Development.     Axon  growth.       Axons  and  dendrites  begin  to  grow  once  neurons  have  i)  migrated  to  the  appropriate  location  ii)   aggregated  into  neural  structures.     Growth  cone:  structure  at  the  end  of  a  dendrite  that   searches  the  external  environment  and  proceeds  to  the   appropriate  location.   - It  is  important  for  dendrites  to  project  to   the  appropriate  locations     Filopodia:  projections  at  the  end  of  growth  cones  that   search  the  environment.       Chemoaffinity  hypothesis  of  axonal  development:  idea   that  each  postsynaptic  surface  releases  a  specific  chemical   label  that  growing  axons  can  be  attracted  to.  This  applies   during  development  and  regeneration.     The  chemoaffinity  hypothesis  fails  to  account  for  the  fact  that  axons  do  not  take  a  direct  route  to   their  destination.  Therefore  the  hypothesis  was  revised:     Growth   cones   are   attracted   to   a   series   of   chemical   signals   along   a   route   to   the   axon’s   final   destination.       Chemical  messengers  of  the  chemoaffinity  hypothesis   - Guidance  molecules:  attract  or  repel  the  growth  cone,  depending  on  where  the  axon  is   intended  to  go   - Pioneer  growth  cones:  first  growth  cones  to  travel  along  a  specific  route  in  a   developing  nervous  system  (see  fasciculation).     - Signals  sent  out  by  other  neurons     Fasciculation:  refers  to  the  fact  that  developing  axons  tend  to  follow  the  path  established  by  a   preceding  axon.     Axons  tend  to  project  in  topographic  arrays  (e.g.,  the  retina).       Topographic  gradient  hypothesis:  idea  that  axonal  growth  is  guided  by  the  relative  position  of   the  cell  bodies  on  intersecting  gradients,  rather  than  point-­‐to-­‐point  coding  of  neural  connections.     - Guided  by  intersecting  signal  gradients  (anterior-­‐posterior  and  medial-­‐lateral)       In  other  words,  axons  growing  from  one  topographic  surface  to  another  are  guided  to     specific  targets  that  are  arranged  on  the  terminal  surface  in  the  same  way  that  the  axon’s     cell  bodies  are  arranged  on  the  originating  surface.   (9  :  4)   PSYC  370     - What  to  take  away  from  this:  the  idea  of  signal  gradients,  how  they  work,  and  that   ephrin  is  an  important  family  of  chemicals  in  this  process     Ephrins:  guidance  molecules  implicated  in  signal  gradients.       Synapse  formation.     Once  axons  have  reached  their  intended  sites,  they  need  to  establish  an  appropriate  pattern  of   synapses.     Synaptogenesis:  creation  of  new  synapses.   - Depends  on  the  presence  of  glial  cells  (esp.  astrocytes)     (5)  Neuron  Death  and  Synapse  Rearrangement.     Neuron  death.     About  50%  more  neurons  than  are  needed  are  produced.  Neuron  death  occurs  in  various  parts  of   the  brain  at  different  times  in  development.       Necrosis:  passive  cell  death  in  which  the  cell  breaks  open  and  spills  its  contents;  this  can  cause   inflammation.     Apoptosis:  active  cell  death  in  which  DNA  triggers  the  death  and  the  contents  of  the  cell  are   packaged  before  the  death  occurs.   (9  :  5)   PSYC  370   Apoptosis  can  have  consequences,  though...   - Genetic  inhibition  of  apoptosis  is  cancer   - Inappropriate  apoptosis  is  neurodegenerative  diseases     Triggers  of  apoptosis   i. Some  neurons  programmed  for  early  cell  death   ii. Some  neurons  die  because  they  do  not  receive  life-­‐preserving  chemicals  from  their  targets   (see  next)     Neurotrophins:  promote  growth  and  survival  of  neurons,  function  as  axon  guidance  molecules,   and  stimulate  synaptogenesis.     - Nerve  growth  factor  (NGF)     Synapse  rearrangement.     Cell  death  results  in  mass  synapse  rearrangement   - When  a  neuron  dies,  synapses  of  the  remaining  neurons  fill  the  empty  space     Postnatal  Cerebral  Development  in  Human  Infants     The  human  brain  does  not  reach  full  maturity  until  late  adolescen
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