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Lecture 5

BIO271 2014 Lecture 5.pdf

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Christopher Garside

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  Lecture  5:  Sensory  Physiology  –  Photoreceptor   Clicker:   Which  is  false?   a) Repolarization  occurs  when  the  membrane  potential  returns  to  the   resting  value   b) The  Goldman  equation  can  be  used  to  calculate  the  membrane   potential  if  you  know  the  relative  permeabilities  and  concentration   of  ions   c) A  positive  feedback  loop  underlies   the  depolarizing  phase  of  the   action  potential   d) Generator  potentials  occur  when  the  sensory  receptor  is   separate  from  the  afferent  neurons   e) Acetylcholinesterase  breaks  down  ACh  into  choline  and  acetate   -­‐ depolarize:  away  from  resting  value   -­‐ repolarize:  go  back  to  resting  value     Photoreception  and  the  Electromagnetic  Spectrum   § Ability  to  detect  visible  light   § 300nm-­‐350nm  of  the  electromagnetic  spectrum   § A  small  proportion  of  the  electromagnetic  spectrum  from   ultraviolet  to  near  infrared   § Ability  to  detect  this  range  of   wavelengths  supports  idea  that   animals  evolved  in  water   § Visible  light  travels  well  in  water;  other   wavelengths  do  not     Evolution  of  Photoreceptors   § receptors  that  absorb  photons  of  light  and  transmit  it  into  an   electrical  signal   § Two  major  types  of  photoreceptor  cells:   o Ciliary  photoreceptors  (orange):  Have  a  single,  highly   folded  cilium;  Folds  form  disks  that  contain   photopigments     § Humans  in  these   o Rhabdomeric  photoreceptors  (blue):  Apical  surface   covered  with  multiple  outfoldings  called  microvillar   projections;  Microvillar  projections  contain   photopigments   § Found  in  arthropods     § Many  microvilli  projections   § Photopigments:  Molecules  that  absorb  energy  from  photons   § Both  photoreceptor  cells  have  photopigments   à  differ  in  where  the  photopigment  is  located     Rods  and  Cones     § Vertebrates  have  ciliary  photoreceptors   § Both  have  the  same  general  shape   § At  the  back  of  the  Retina  (located  at  the  back  of  the  eye)   § These  photoreceptors  are  at  the  back  of  the  eye       § Rods   § Cones   § Difference:  the  properties  of  the  photopigments   § Both  have  inner  and  outer  segments   § Inner  and  outer  segments  connected  by  a  cilium   § Outer  segment  contains  photopigments   § Inner  segment  forms  synapses  with  other  cells     Feature   Rods   Cones   Class  of  Receptor   Ciliary   Ciliary   Shape   Outer  segment  rod  shaped   Outer  segment  cone   shaped   Sensitivity   Sensitive  to  very  dim  light   Sensitive  to  brighter  light   Type  of  Photopigment   One  type  (dim  light)   Up  to  3  types  in  mammals   -­‐  trying  to  find  washroom  at   (bright  light)   night?  Using  rods   -­‐  detect  colour   -­‐  gives  good  visual  acuity   (ability  to  discriminate   detail)       Diversity  in  Rod  and  Cone  Shape   § Diverse  shapes  of  rods  and  cones  among  vertebrates   § Shape  does  not  determine  properties  of  photoreceptor   § Properties  of  photoreceptor  depend  on  its  photopigment     Photopigments   § Photopigments  have  two  covalently  bonded  parts   § Chromophore   § Initially  the  chromophore  is  in  the  11-­‐cis  retinal  formation   § Once  a  photon  of  light  hits  this  retinal,  its  going  to  convert  this  retinal  to  all -­‐ trans  retinal   § Absorption  of  light  converts  bond  from   cis  to  trans   § This  is  the  first  step  in  the  transduction  pathway     § Opsin   § Once  the  conversion  of  chromophore  happens,  opsin  is  activated   § G-­‐protein-­‐coupled  receptor  protein   § Once  a  photon  of  light  has  been  absorbed  by  the  retinal,  and  converts  it  to  the  all -­‐trans  state  à  GPCR  pathway   is  activated   § Opsin  structure  determines  photopigment  characteristics;  For  example,  wavelength  of  light  absorbed       Phototransduction   § Photopigments  are  packed  into  the  highly  folded  cilium   § Packed  in  here  because  they’re  trying  to  maximize  the  possibility  of  absorbing  the  photon  of  light  when  its  shone  at   the  retina   § Take  the  photon  of  light,  and  convert  this  sensory  input  into     § Converting  a  photon  of  light  à  change  in  conformation  of  the  retinal   à  allows  activation  of  GPCR  pathway   § Steps  in  photoreception:   § Chromophore  absorbs  energy  from  photon       § Chromophore  changes  shape   § Double  bond  isomerizes  from   cis  to  trans   § Activated  chromophore  dissociates  from  opsi n   § Dissociation  is  called   “Bleaching”   § Opsin  activates  G-­‐protein   § Formation  of  second  messenger   § Ion  channels  open  or  close   § Change  in  membrane  potential   § Phototransduction  in  a  Rhabdomeric  and  Vertebrate  Photoreceptors  are  essemtially  the  same   à  just  different  GPCR   pathways     Phototransduction  in  a  Rhabdomeric   Photoreceptor  (arthropods)     -­‐ the  chromophore  +  opsin  =  rhodopsin   1. when  light  hits,  it  isomerizes  it  and   changes  the  comformation  of  the   chromophore   2. 11-­‐cis  retinal  absorbs  light  and   isomerizes  into  all-­‐trans  retinal   3. all-­‐trans  retinal  dissociates  from  the   opsin  =  bleaching   4. opsin  is  now  activated   5. once  opsin  is  activated,  it  actives  the  G   protein  coupled  pathway   6. G-­‐protein  coupled  pathway  activates  PLC   7. Converts  PIP2  to  DAG  and  IP3   8. As  a  result  of  this  conversion,  DAG  activates  a  TRP  channel   9. TRP  channels  open  and  now  cations  can  flow  in  and  depolarize  the  membrane     Phototransduction  of  Vertebrate  Photoreceptors   1. when  light  hits,  it  isomerizes  it  and  changes  the  comformation  of  the  chromophore   2. 11-­‐cis  retinal  absorbs  light  and  isomerizes  into  all -­‐trans  retinal   3. chromophore  dissociates  from  the  opsin  =  bleaching   4. opsin  is  now  activated   5. once  opsin  is  activated,  it  actives  the  G  protein  called  transducin   6. transducin  activates  PDE   7. PDE  converts  cGMP  to  GMP   8. The  decrease  of  cGMP  from  the   conversion  closes  Na+  channels   9. The  decrease  in  Na+  entry   hyperpolarizes  the  cell   -­‐ under  the  resting  conditions  (in  the   dark),  cGMP  levels  are  high   -­‐ high  cGMP  levels  keep  this  ion  channel   opened  at  all  times   -­‐ when  rhodopsin  transduce s  and   receives  a  proton  of  light  =  decrease  in   cGMP  levels   -­‐ when  cGMP  decrease,  they  can  no   longer  gate  this  cation  channel  –  cation  channel  closes  in  return   -­‐ less  Na+  and  Ca2+  ions  coming  in  =  hyperpolarization     -­‐ normally  in  the  dark,  the  membrane  potential   is  depolarize   -­‐ the  channel  is  normally  kept  opened  by  high  concentrations  of  cGMP   -­‐ because  this  photoreceptor  is  constantly  depolarized  =  constant  release  of  neurotransmitter   -­‐ when  we  absorb  light  =  decrease  cGMP  concentrations  =  ion  channel  closed  =  no  more   influx  of  Na+  and  Ca2+  =   membrane  potential  hyperpolarizes  =  decrease  in  transmitters     The  Eye   § Eyespots       § Cells  or  regions  of  a  cell  that  contain  photosensitive  pigment   § For  example,  protist  Euglena   § Eyes  are  complex  organs   § Detect  direction  of  light   § Light-­‐dark  contrast  (lateral  inhibition)   § Some  can  form  an  image  (the  human  eye)     Types  of  Eyes   Flat  sheet  eyes   § Photoreceptors  sitting  on  a  layer  of  pigment  layer  connected  directly  to  the   primary  afferent  neurons   § Some  sense  of  light  direction  and  intensity   § Often  in  larval  forms  or  as  accessory  eyes  in  adults     § In  the  human  eye,  the  retinal  layer  is  5  cell  layers  deep   Cup-­‐shaped  eyes  (e.g.,  Nautilus)   § flat  sheet  eye  curved  =  cup  shaped  eye   § advanced  à  aperture  at  the  top   § aperture  is  important  to  bear  the  amount  of   light  coming  in   § Retinal  sheet  is  folded  to  form  a  narrow  aperture   § Discrimination  of  light  direction  and  intensity   § Light-­‐dark  contrast   § Image  formation   § Poor  resolution   Vesicular  Eyes  (present  in  most  vertebrates)   § Lens  in  the  aperture  improves  clarity  and  i ntensity   § Lens  refracts  light  and  focuses  it  onto  a  single  point  on  the  retina   § Bend  the  direction  of  the  light   § Can  focus  that  light  into  a  particular  area  of  the  retina   § Need  to  focus  it  to  have  a  clear  image   § Lens  is  critical  for  clarity  and  acuity   § Image  formation   § Good  resolution     Compound  Eyes  of  Arthropods   § Use  the  Rhabdomeric  receptor   § Not  related  to  the  flat  sheet,  or  cut,  or  vesticular  eye   –  instead  its  called  a   compound  eye   § Composed  of  ommatidia  (photoreceptor)   § Each  ommatidium  has  its  own  lens   § Images  formed  in  two  ways   § Apposition  compound  eyes   § Ommatidia  operate  independently   § Each  one  detects  only  part  of  the  image   § Afferent  neurons  interconnect  to  form  an  image   § Pooled  together  to  make  an  image   § Superposition  compound  eyes   § Ommatidia  work  together  to  form   image   § Resolving  power  (the  more  detailed  processed)  is  increased  by  reducing  size   and  increasing  the  number  of  ommatidia     § The  more  ommatidia,  the  smaller  the  region  each  is  responsible  for  =  put  them   all  together,  you  get  a  better  image     § The  smaller  the  receptive  field  &  more  receptive  field  you  have  =  the  better  your  visual  view     Structure  of  the  Vertebrate  Eye   -­‐ The  cornea,  iris,  pupil,  and  lens  allow  us  to  focus  the  light  on  a  particular  part  of  the  retina  at  the  back   -­‐ Light  goes  through  the  cornea,  and   its  going  to  come  into  the  pupil  (changes  size  as  a  result  of  the  iris)   -­‐ Lens  can  change  shape  because  its  being  held  in  place  by  ciliary  bodies  (smooth  muscles)       -­‐ When  the  ciliary  body  smooth  muscles  contract  or  relax,  they  change  the  height  and  position  of  the  lens   -­‐ The  lens  reflects/refracts  light  onto  the  back  of  the  retina  allowing  you  to  focus   § Sclera   § “White”  of  the  eye   § Cornea   § Transparent  layer  on  anterior   § Retina   § Layer  of  photoreceptor  cells   § Choroid   § Pigmented  layer  behind  retina   § Tapetum   § Layer  in  the  choroid  of  nocturnal  animals  that  reflects  light   § Iris   § Two  layers  of  pigmented  smooth  muscle   § Pupil   § Opening  in  iris  allows  light  into  eye   § Lens   § Focuses  image  on  retina   § Ciliary  body   § Muscles  that  change  lens  shape     § Aqueous  humor   § Fluid  in  the  anterior  chamber   § Vitreous  humor   § Gelatinous  mass  in  the  posterior  chamber   -­‐ Photoreceptors:  absorbs  photons  of  light   -­‐ Light  goes  into  the  lens  à  Vitreous  humor  à  back  of  th
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