Majority  of  illustra3ons  in  this  presenta3on  are  from  Biological  Psychology
4 th  edi3on  (©  Sinuer  Publica3ons)
Atoms  are  stable  par3cles,  but  ions  are  not.
Depending  on  their  chemical  structure,  they  can   either  become  posi3ve  or  nega3ve.  In  the  neuron   these  play  an  dominant  role  in  propaga3on  of   neural  signals.
K +   Na +   Ca 2+   Cl -­‐   Protein -­‐
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Two  kind  of  forces  that  move   chemicals  (ions)  from  a  more   concentrated  loca3on  to  less   concentrated  region  are   concentra3on  gradient ,  and   forces  that  move  chemicals   with  charges  on  them   through  aPrac3on  and   repulsion  are   electrosta3c   forces.
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Diffusion  of  ions  through   a  semi-­‐permeable   membrane  produces  the   two  kinds  of  forces   men3oned  above.  A  cell   has  a  semi-­‐permeable   membrane  and  creates  a   electrical  poten3al   difference  because  of   these  forces.
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1.  Concentra3on  of  K +   outside  the  cell  is  more   than  inside.
2.  So  K +  starts  moving  in   the  cell  due  to   concentra3on  gradient  and   more  nega3ve  (protein)   charges  in  the  cell.
3.  An   equilibrium  poten3al   of  -­‐70  mV  is  generated.
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2

+
1.  Exchange  of  K +  ions   across  the  membrane  is   regulated  by  passive  K + channels.
2.  K+  passes  through
this  channel  back  and   forth,  un3l  a   res3ng   poten3al   is  reached.
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1.  Res3ng  poten3al  is   generated  by  Na + ,  K + ,  Ca ++ ,
Cl -­‐  and  protein  molecules -­‐ .
2.  Their  concentra3on   inside  and  outside  the   cells  is  different.
3.  Different  channels  in   the  neuronal  membrane   create  this  difference.
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1.  Axons  of  the  neurons   can  be  impaled  with   electrodes  to  measure   the  poten3al  difference
(voltage)  between  the   inside  and  outside  of  the   neuron.
2.  A  difference  of  -­‐60  to
-­‐70  mV  is  observed.
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Depolarizing  (posi3ve)  and  hyperpolarizing
(nega3ve)  currents  cause   graded  and   ac3on   poten3als .
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1.  In  phase  1,  Na channels  are  open.    The  neuron  is  at  res3ng   poten3al.
+  channels  are  closed  and  K +
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2.  In  phase  2,  when  a  brief  current  enters  the   neuron,  Na +  channels  open  le\ng  Na +  ions  to  enter   the  cell.  K +  channels  close.  The  neuron  starts   becoming  more  posi3ve.
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3.  In  phase  3,  If  the  Na mV  ( threshold ),  all  Na of  Na +
+
+  ions  raise  the  charge  to  -­‐40
channels  open  and  barrage
makes  the  inside  of  the  cell  very  posi3ve
(+40  mV).
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4.  In  phase  4,  Na +  channels  get  inac3vated.  K +   channels  open  and  K +  ions  start  moving  outside  the   cell.  The  neuron  poten3al  starts  rever3ng  back  to   equilibrium  poten3al.
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5.  In  phase  5,  Na +  and  K +  channels  close.  Na+  and  K+   pump  extrudes  Na+  ion  out  and  K+  ions  in  the  cell.
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5

During  the  5th  phase  the   Na +  -­‐  K undershoot.
+  pump   brings  back   the  electrical  poten3al  to  res3ng  state  a^er  the
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1.  A^er  a  neuron  has  fired  an  ac3on  poten3al,  the   neuron  goes  through  a   refractory  phase,   in  which  it   does  not  fire  another  ac3on  poten3al.  This   cons3tutes   absolute  refractory  phase .
2.  A  neuron  can  be  made  to  fire  an  ac3on  poten3al   in  the   rela3ve  refractory  phase   by  increasing   s3mulus  intensity.
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1.  The   intensity   of  an  ac3on  poten3al  remains   constant  across  its  propaga3on  extent.
2.  Ac3on  poten3als   fire  as   all-­‐or-­‐none   responses,  i.e.,  they   will  fire  if  above   threshold  and  not  if   they  are  below  it.
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In  myelinated  neurons  speed  of  conduc3on  is  fast,   due  to  Na +  ions  entering  at  nodes  of  Ranvier,  which   boost  current  and  speeding  the  signal  propaga3on.
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On  the  other  hand,  in  unmyelinated  neurons,  speed   of  conduc3on  is  slow  due  to  degrada3on  of   electrical  current  based  on   cable  proper3es .
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Presynap3c  neurons  can  either  inject  posi3ve  or   nega3ve  currents  into  a  postsynap3c  neuron.  When   the  current  is  posi3ve  it  is  called   Excitatory
Postsynap3c  Poten3al  (EPSP)  and  when  nega3ve  it   is  called   Inhibitory  Postsynap3c  Poten3al  (IPSP) .
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EPSP  and  IPSP  integrate  at  the  axon  hillock  and   make  the  neuron  fire  an  ac3on  poten3al  or  not.
EPSPs  increase  the  probability  of  an  ac3on   poten3al  and  IPSPs  decrease  that  likelihood.
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Weak  EPSPs  from  different  presynap3c  neurons   add-­‐up  in  “space”  to  cause  an  ac3on  poten3al
( spa3al  summa3on ).  Or  a  singular  presynap3c   neuron  could  “over  3me”  inject  enough  charge   inside  the  neuron  for  it  to  fire  an  ac3on  poten3al
( temporal  summa3on ).
Spa3al  Summa3on   Temporal  Summa3on
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Some  synapses  have  channels  that  directly  connect   one  neuron  to  the  other.  Ions  pass  between   neurons  freely  without  releasing  neurotransmiPers.
Therefore  conduct  messages  at  higher  speeds  than   regular  synap3c  neurons.
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1.  Unlike  ac3on  poten3als  that  are  digital  (all-­‐or-­‐   none)  graded  poten3als  are   analog  in  nature.  So
EPSPs  and  IPSPs  are  like  graded  poten3als.
2.  Intensity  of  graded  poten3als  is  not  constant  and   increase  and  decrease  with  the  intensity  of   s3mula3on.  In  ac3on  poten3als  it  is  registered  by   the  number  of  responses  fired.
3.  Many  sensory  receptors  (skin  etc.)  respond  with   graded  poten3als.
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1.  Ac3on  poten3al   reaches  presynap3c   terminal  buPon.
2.  Ca ++  enters  the   terminal  and  synap3c   vesicles  fuse  with  the   membrane.
3.  NeurotransmiPers  are   released  from  the   vesicles  in  the  cle^.
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4.  NTs  bind  to   postsynap3c  receptors,   opening  them  and  ions   flow  through.
5.  EPSPs  or  IPSP  spread   over  the  postsynap3c   cell  body  towards  the   axon  hillock  to  integrate   and  fire  an  ac3on   poten3al  or  not.
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6.  Enzymes  in  the   synap3c  cle^  break   excess  NTs.
7.  Reuptake  removes  the
NT  from  synapse  slows   down  the  process  and   recycles  the  NT.
8.  NT  binds  to   autoreceptors  to  signal   reduc3on  in  NT  in  the   synapse.
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Following  is  a  summary  of  ac3on  poten3als  and   synap3c  poten3als.
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Following  is  a  summary  of  ac3on  poten3als  and   synap3c  poten3als.
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We  can  use  various   techniques  like   patch   clamps  and   voltage   clamps  to  study  the   physiological  responses   of  receptors  and  based   on  these  studies  to   understand  their   structures.
Receptor
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In  patch  clamp  and   voltage  clamp   experiments,  the   receptors  can  be   manipulated  with   neurotransmiPers,  and/ or  voltages  to  study  their   behaviors.
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1.  Stoichiometry  aids  us   in  unraveling  the   structure  of  receptors.
2.  We  can  iden3fy   different  subunits.
3.  And  specific  loca3ons   for  exogenous  and   endogenous   neurotransmiPers.
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1.  NeurotransmiPers  that  bind  directly  to  the   receptor  and  open  the  channel  to  pass  ions  are   called   ionotropic  receptors.
2.  NeurotransmiPers  bind  to  receptors  using  G-­‐ protein  complexes  to  open-­‐up  channels  are  called   metabotropic  receptors.
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Simple  reflexes  like   the  knee-­‐jerk  involve   two  neurons  thus  take   shorter  3mes  (40  ms)   to  respond  compared   to  some  other  reflexes   that  take  longer  (220   ms;  Laming,  1968)   because  of  mul3ple   neurons  in  the  circuit.
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1.
Complexity  of  response   in  reflex  circuits  was   known  long  3me  ago   e.g.,  changes  in  s3mulus   intensity  was  faithfully   registered  in  the   response.
2.  Also  reflexes  could  be   modified  in   condi3oning   or   imprin3ng .
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More  complicated  circuits  have  more  neurons  and   show  proper3es  like   convergence  and   divergence ,   e.g.,  as  seen  in  the  visual  pathway.
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Feedback  circuits  connect  neurons  in  loops.
Nega3ve  feedback  circuits   usually  decrease  the   ac3vity  of  a  neuron,  and   posi3ve  circuits   increase  it.
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S3ll  others  make  oscillatory  or  rhythmic  circuits   used  in  many  bodily  func3ons,  like  heart,  breathing,   walking,  sleeping  etc.
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We  can  record  (EEG)  ac3vity  from  popula3on  of   neurons  using  scalp  electrodes.  Two  kinds  of   ac3vity  are  classified  as  spontaneous  and  event-­‐ related  evoked  poten3als.
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Spontaneous  brain  signals  do  not  require  any   specific  s3mula3on.  So  brain  ac3vity  during  sleep   and  dreaming  is  spontaneous.  Also  such  ac3vity  is   important  to  assess  and  diagnose  epilep3c  ac3vity.
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Brain  (EEG)  signals  that  require  specific  s3mula3on   are  called  event-­‐related  poten3als.  Such  signals   provide  informa3on  how  brain  responds  to  say,   auditory  signals  in  the  brain  stem.
Task-­‐related
Poten3als   43
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