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Lecture 4

BIO271 2014 Lecture 4.pdf

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University of Toronto St. George
Christopher Garside

  Lecture  4:  Sensory  Physiology   Mechanoreceptors   -­‐ can  be  simple  undifferentiated  nerve  endings  –  nerve  ends  in  the   process  (nothing  else  attached  to  it) ,  or   -­‐ can  be  complex  accessory  structures,  like  the  Pacinian  Corpuscle   -­‐ Mechanoreceptors  also  include  stretc h  receptors  in  the  skin,   auditory,  and  vestibular  structures  of  the  vertebrate  ear   -­‐ Mechanoreceptors  are  activated  by  a  stretch  or  distortion  of  their   plasma  membrane   -­‐ Transform  mechanical  stimuli  into  electrical  signals   -­‐ All  organisms  (and  most  cells)  sense  and  respond  to  mechanical   stimuli   -­‐ Two  main  types  of  mechanoreceptor  proteins:   o ENaC   § Epithelial  sodium  channels   o TRP  channels   § Transient  receptor  potential  channels   -­‐ Structure  for  the  2  mechanoreceptors  are  essentially  the  same   o Ion  channel  sitting  on  a  membrane   o Ion  channel  is  gated  by  distortion  of  the  membrane   o Pressure  against  the  membrane  causes  the  channel  to   open   -­‐ Channels  are  linked  to  extracellular  matrix   o Links  are  bound  to  something  in  the  extracellular  matrix   o Also  have  an  intracellular  link  on  the  inside   à  bound  to  cytoskeleton   o When  you  pull  on  them,  you  will  have  opposite  forces  action  on  each   other  =  pulls  the  channel  open   o Mechanical  stimuli  alter  channel  permeability     Touch  and  Pressure   Three  classes  of  receptors   § Baroreceptors   § Interoceptors  detect  pressure  changes   § Tactile  receptors   § Exteroceptors  detect  touch,  pressure,  and  vibration   § Senses  stimuli  from  the  external  environment   § Specialized  based  on  the  accessory  structures  that  they  have   § Proprioceptors   § Monitor  the  position  of  the  body     Vertebrate  Tactile  Receptors   o Many  different  types,  but  all  co -­‐exist  in  similar  regions  of  the  skin   o Widely  dispersed  in  skin   o Receptor  structure   § Free  nerves  endings   • Nerve  branches  and  it  ends   • On  the  endings:  mechanoreceptors   • When  you  apply  pressure,  the  channel  opens  and  you  generate  a  potential   à  transmitted  to  action  potentials   § Nerve  endings  enclosed  in  access ory  structures     • Accessory  structures  allow  modeling  the  response  of  the  nerve  to  be  more  tonic  or   more  phasic   • Can  sense  whether  it’s  something  brushing  on  the  skin,  crawling  on  the  skin,  etc.   • Specialized  to  tell  us  more  information  about  the  external  stim uli   • (e.g.,  Pacinian  corpuscle)       Free  Nerve  Endings  in  Merkel’s  Disks   § Merkel’s  Disks  are  free  nerve  endings  associated  with  an  enlarged  epidermal  cell   § Small  receptive  fields   § Region  a  receptor  cell  can  respond  to       § Slowly  adapting  tonic  receptors  most  sensiti ve  to  indentation  of  the  skin   § Over  time,  the  frequency  of  the  action  potential  is  going  to  decay   § What  do  you  think  they  are  used  for?  (read  text)     Root  Hair  Plexus   -­‐ Nerve  ending  comes  in  and  wraps  itself  all  around  the  base  of  this  hair   -­‐ Root  hair  plexus  are  wrapped  around  the  base  of  the  hair  follicles  and  monitor   movements  across  the  body  surface   -­‐ When  hair  moves/shifts  with  position  of  the  base  of  the  hair   –  this  afferent  can  pick   it  up   -­‐ Monitor  movement  across  the  body  surface   -­‐ Rapidly  adapting  phasic  rec eptors;  only  sense  when  it  passes  over,  not  how  long  its   there  for   -­‐ Sensitive  to  changes  in  movement   -­‐ Ex.  Insect  crawling  on  skin   –  movement  can  be  detected     Pacinian  Corpuscle     • Free  nerve  ending  wrapped  in  layers  of  connective  tissue   • Rapid  adaptation  by  the  Pacinian  Corpuscle  is  a  good  example  of  mechanical  filtering  by  accessory  structures   • accessory  structures  (rapidly  adapting)   à  accessory  structure  allows  for  mechanical  filtering   • The  Pacinian  Corpuscle  is  a  pressure  and  vibration  receptor  found  in  the   skin,  muscle,  tendons  and  joints       B)  accessory  structure  is  intact:  stimulus  is  maintained     o potential  change  only  at  the  on -­‐set,  and  off-­‐set  =  phasic  receptor   o for  the  duration  of  the  application,  there  is  no  maintained  response     C)  removed  the  accessory  structure  to  see  why  there  was  no  maintained  response   o get  on-­‐set  of  response  and  it  adapts   o therefore,  it’s  the  accessory  structure  itself  that’s  responsible  for  mediating  the  phasic  response  of  the  nerve  ending   • these  layers  are  actually  filled  with  fluid   • when  they  push  on  the  outside  of  the  accessory  structure,  it  creates  a  wave   à  the  wave  travels  à  initial  wave   travelling  contacts  free  nerve  ending  =  rapid  response   • but  very  quickly,  everything  returns  to  normal   à  diffuses  away   • the  layers  actually  absorb  the  pressure  that  you  are  apply,  and  come  to  a  steady  state  when  no  other  pressure  is   applied   • change/wave  that  creates  the  pressure  =  activation  of  nerve  ending   • response  of  these  nerves  are  different  because  of  the  accessory  structure       • accessory  structure  causes  mechanical  filtering  –  allows  pressure  in  the  fluid  (in  these  layers  of  connective   tissue)  to  diffuse  away  quickly   • the  accessory  structure  can  be  associated  with  another  cell  type  or  with  layers  of  tissue  that  can  modify  their   response   • With  the  corpusc le  intact  the  neuron  depolarized  transiently  at  the  onset  and  offset  of  the  stimulus   • With  the  accessory  layers  removed  the  neuron  remained  depolarized  during  most  of  the  stimulus   • How  do  the  accessory  layers  produce   sensory  adaptation?     What  is  the  disadvan tage  for  a  sensory  cell  with  a  wide  dynamic  range  compared  with  a  narrow  dynamic  range?   A. Higher  threshold  of  detection  (do  not  know  anything  about  this)   B. Lower  sensitivity  for  low  intensity  stimulus  (don’t  know  shape  of  curve  =  no  information  about  intensity)   C. Higher  sensitivity  for  low  intensity  stimulus  (don’t  know  shape  of  curve  =  no  information  about  intensity)   D. Lower  sensitivity  for  all  stimuli   -­‐ wide  dynamic  range:  sensitive  over  many  inputs  and  in tensities   o can  respond  to  a  lot,  but  not  very  well  to  a  small  change   o trade  off  ability  to  recognize  small  change   -­‐ narrow  dynamic  change:  high  sensitivity,  but  respond  to  only  a  small  change   o steep  curve     Touch  and  Pressure   Three  classes  of  receptors   § Baroreceptors   § Interoceptors  detect  pressure  changes   § Tactile  receptors   § Exteroceptors  detect  touch,  pressure,  and  vibration   § Proprioceptors   § Monitor  the  position  of  the  body     Vertebrate  Proprioceptors   -­‐ Monitor  the  position  of  the  body   -­‐ Free  nerve  ending  monitors   tensions  in  structures   -­‐ Three  major  groups   o Muscle  spindles   § Free  nerve  ending   § Innervates  muscle  itself  to  monitor  muscle  length   § Located  in  skeletal  muscles   § Monitor  muscle  length   o Golgi  tendon  organs   § Free  nerve  ending  comes  in  and  innervates  the  tendon   § Located  in  tendons   § Monitor  tendon  tension   o joint  capsule  receptors   § Located  in  capsules  that  enclose  joints   § Monitor  pressure,  tension,  and  movement  in  the  joint         Insect  Tactile  Receptors   Two  common  types  of  sensilla   § Trichoid   § Hair  like  projection  of  cuticle     § Sensory  neuron  does  not  extend  directly  into  it   § Have  another  accessory  structure   § When  hair  like  projection  is  moved,  it  will  put  pressure  on  the   accessory  structure   § When  you  push  onto  it,  it  transmits  a  pressure  onto  the  dendrite   of  sensory  neuron   § At  the  tip  of  the  dendrite  is  the  sensory  receptors       § Pushing  on  it  =  activaton  of  mechanoreceptors  (ion  channels  gated   –  generate  a  potential  change  =  action   potential  to  the  nervous  system)   § Bipolar  sensory  neuron   § Different  neuronal  structure   § Cell  body  is  along  the  line  of  the  axon   § TRP  channel     § Campaniform   § Dome-­‐shaped  bulge  of  cuticle  (instead  of  hair  like  projection)   § Pressing  on  it  will  put  pressure  onto  accessory  structure  onto  the  bipolar  sensory  neuron   § Bipolar  sensory  neuron     Cross  Section  of  a  Mechanoreceptor   from  the  Drosophila   v Sensilia  coming  out  from  the  top   -­‐ Neuron  is  on  the  base   -­‐ Can  record  the  response  by  cutting  and  gaining  access  to  the  sensory  neuron   -­‐ Bend  it  in  the  preferred  direction/reverse  direction  =  get  2  different  responses   -­‐ Preferred  direction:  larger  response  than  opposite  direction   v Longitudinal  section  through  a  bristle  mechanoreceptor  of   Drosophila   v Cutting  the  shaft  allows  electrical  access  to  the  sensory  neuron   v Mechanical  distortion   (in  preferred  or  opposite  direction)   activates   ion  channels  which  produced  the  receptor  potential   v The  dendrite  is  bathed  in  endolymph,  which  has  a  high   concentration  of  K   + v Second  exception:  for  sensory  neurons   -­‐  solution  bathing  the   neuron  (endolymph)  has  a  high  concentration  of  K+   o First  exception:  high  chloride  in   the  cell  of  the  olfactory   neuron   v When  the  bending  of  the  shaft  opens  stretch -­‐activated  cation  selective  channels,  there  is  an  inward  driving  force   for  K   o Influx  of  this  positive  ion  =  depolarization  of  membrane     Equilibrium  and  Hearing   § Utilize  mechanoreceptors  as  well   § Equilibrium  (“balance”)   § Detect  position  of  the  body  relative  to  gravity   § Impossible  to  stand  up  when  imbalanced   § Hearing   § Detect  and  interpret  sound  waves     § Vertebrates   § Ear  is  responsible  for  equilibrium  and  hearing   § Invertebrates  
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