Anat 6.8 Auditory Pathways_Bayaoa

Anatomy  6.8    

February  21,  2012   Dra.  Bayaoa  

Auditory Pathways  

  Outline   I.  Auditory  and  Vestibular  system   II.  Auditory  System   A. External  Ear   B. Middle  Ear   C. Inner  Ear   III.  Central  Auditory  Pathway   IV.  Vestibular  System   A.  Inner  Ear  (Labyrinth)   B. Vestibular  Functions  of  the  ear   Macula   Objectives:   • Define  function  of  the  cochlear  system  and  receptors  for  hearing     • Tracing  the  pathway  of  sound  from  the  environment  to  the  sense  organ  for  hearing     • Describing  the  origin,  course,  and  termination  of  the  cochlear  nerve   • Tracing  the  central  auditory  pathway  from  the  organ  of  Corti  until  the  nerve  impulse   reaches  the  auditory  cortex     • Role  of  the  olivocochlear  bundle  of  Rasmussen  in  sound  perception     • Pathways  of  auditory  reflexes     • Differentiating  between  conductive  and  sensory  deafness     • Explaining  tinnitus,  Rinne’s  &  Weber’s  test     • Function  of  the  vestibular  system     • Parts  of  Static  labyrinth  vs  kinetic  labyrinth     • Location  of  the  receptors  for  vestibular  control     • Origin,  course,  and  termination  of  the  vestibular  nerve     • Describing  the  role  of:  MLF,  medial  vestibulospinal  tract,  lateral  vestibulospinal  tract   in  the  maintenance  of  equilibrium   • Connections  of  the  vestibular  apparatus  with  the  cerebellum   • Define:  vertigo  and  nystagmus     • Tests  for  vestibular  function     • Clinical  manifestations  of  disturbances  in  vestibular  apparatus    

 

I.  AUDITORY  AND  VESTIBULAR  SYSTEM   CN  VIII/  Vestibulocochlear  nerve   • Has  two  distinct  divisions   1. Vestibular  –  position  and  movement  of  head;  for  equilibrium     2. Cochlear  –  mediates  auditory  function     • Grossly;  it  is  divided  into  external  ear,  middle  ear  and  inner  ear   • Stimulus:  SOUND   o Sinusoidal  waves  of  air  molecules   o Frequency  of  waves  (in  hertz)  -­‐>  PITCH  of  sound   o Amplitude  of  each  wave  (in  decibels)  –  LOUNDESS  of  sound   o The   human   ear   can   detect   sound   frequencies   from   20   to   20,   000   Hz  and  1-­‐120  db   II.  AUDITORY  SYSTEM   A.  External  Ear   Consists  of     • Auricle/pinna/ear  lobe   • External  auditory  meatus  (  ear  canal)   • Tympanic   Membrane   (ear   drum;   separates   the   external   ear   from   the  middle  ear)   Sound  waves  enter  the  external  auditory  meatus  -­‐>  impinge  on  the   tympanic  membrane  -­‐>  tympanic  membrane  vibrates  and  transmits   sound  into  the  middle  ear   B.    Middle  Ear   • Lies  in  the  petrous  part  of  temporal  bone   • Transmits  the  vibrations  of  the  tympanic  membrane  to  the  inner   ear   RG,  SOPIE,  KEIFER,  TASIE,  FILLE,  ELIZ,  MAY,  JANNA  J  

 

 

• Air-­‐filled  cavity   • Two  parts:   o Tympanic  cavity  proper   o Attic/  epitympanic  recess

Figure  1.  Structures  of  the  middle  ear  (Coronal  Oblique  Section)    

• Consists  of  the  three  auditory  ossicles  (MIS)   1. Malleus   (hammer)   –   attaches   to   the   inner   aspect   of   the   tympanic  membrane     2. Incus   (anvil)   –   receives   vibration   from   the   malleus   via   diarthrodial  joint;  articulates  with  stapes   3. Stapes  (stirrup)  –  sits  on  the  membrane  of  the  foramen  ovale   (oval   window)   through   which   vibration   is   transmitted   to   the   fluid  perilymph  of  the  inner  ear.     • The  3  auditory  ossicles  serve  as  an  amplifier  and  as  an  impedance-­‐   matching  device  that  decreases  the  amount  of  energy  lost  by    by   the  sound  waves  in  going  from  the  air  to  the  fluid  in  the  inner  ear   • Eustachian  tube     o From  the  cavity  of  the  middle  ear  to  the  posterior  nasopharynx   o Equalizes  air  pressure  inside  and  outside  tympanic  membrane     • Contains  two  muscles:   1. Stapedius  -­‐  innervated  by  CN  VII  (facial)   • pulls  on  stapes  and  causes  tightening  of  the  membrane  of   the  oval  window   2. Tensor  tympani  -­‐  innervated  by  CN  V  (mandibular  branch  of   trigeminal  nerve)   § pulls  on  malleus  and  causes  the  tightening  of  the  tympanic   membrane     • The   two   muscles   contract   to   decrease   transmission   of   vibrational   energy   from   external   to   inner   ear   to   protect   the   inner   ear   from   loud  sounds  (attenuation  reflex)  

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• Hyperacousis   -­‐   sounds   perceived   as   uncomfortable   and   louder.   May  be  experienced  by  person  with  paralysis  of  these  2  muscles  or   of  the  stapedius  alone  (as  in  Bell’s  facial  palsy)   C.  Inner  Ear   • Two  components:   o Cochlea    -­‐  organ  for  hearing,  where  cochlear  nerve  attaches   o Vestibular  apparatus  (3  semicircular  canals,  utricle  and  saccule)   –  where  vestibular  nerve  attaches   • The   oval   window/foramen   ovale,   to   which   the   stapes   communicates,   opens   to   the   vestibule   portion   which   contains   perilymph     • Bony  Labyrinth  (contains  perilymph)  is  made  up  of  the:   o Cochlea   o Semicircular  canals     o Vestibule     • Membranous  labyrinth  (with  endolymph)  is  found  within  the  bony   labyrinth  and  is  made  up  of  the:   o cochlear  duct  (in  bony  cochlea)   o utricle  and  saccule  (in  vestibule)   o semicircular  ducts  (in  semicircular  canals)   1.  Cochlea  

Figure  2.  The  cochlea    

 

• Resembles  a  snail   • Contains   helicotrema   which   is   the   apical   connection   between   the   scala  vestibuli  and  scala  tympani.  It  is  also  the  base  of  cochlea   • Turns  at  its  conical  central  axis  called  modiolus.   • 2  and  ¾  turns   • Consists  of  three  parallel  filled  channels:     o Scala  vestibuli  (w/  perilymph)   o Scala  tympani  (w/  perilymph)   o Scala  media  or  cochlear  duct  (w/  endolymph)       *Perilymph   is   similar   to   CSF   while   endolymph   is   similar   to   intracellular  fluid.   • Vestibular   membrane   divides   scala   vestibuli   into     another   space   called  scala  media  or  cochlear  duct   RG,  SOPIE,  KEIFER,  TASIE,  FILLE,  ELIZ,  MAY,  JANNA  J  

 

   

• Contains   a   spiral  lamina   (ridge   of   bone   dividing   scala   vestibuli   and   scala  tympani)   • Scala   media/cochlear   duct   -­‐   a   membranous   labyrinth   within   the   cochlea,   completes   the   separation   of   scala   vestibuli   and   scala   tympani   • Basilar  membrane  -­‐  where  the  cochlea  sits.  It  is  narrower  at  base   than  at  the  apex.   2.  Organ  of  Corti   • Within  the  cochlear  duct/scala  media   • Generates   action   potentials   in   the   bipolar   neurons   of   the   spiral   ganglion   of   the   cochlear   division   of   the   vestibulocochlear   nerve   when  hair  cells  vibrate  due  to  sound.   • Functions  as  an  audiofrequency  analyzer/organizer   o Highest   tones   stimulate   hair   cells   near   the   basal   cochlea   which   contains  the  narrowest  segment  of  basilar  membrane   o Lowest   tones   stimulate   hair   cells   in   the   apical   portion   of   the   cochlea   which   contains   the   widest   segment   of   the   basilar   membrane   o Intermediate   tones   stimulate   hair   cells   on   the   intermediate   portion  of  the  basilar  membrane   • Consists  of  two  receptor  cells:   a.  Inner  hair  cells     • Auditory  receptor  cells   • Base   o attached  to  basilar  membrane   o synapses   with   dendrites   of   spiral   ganglion   cells   (10   spiral   ganglion   cells   innervate   1   hair   cell   and   its   axons   form   the   cochlear  division  of  CN  VIII)   o inner   hair   cells   +   spiral   ganglion   =   produce   frequency-­‐ dependent  responses  to  sound   • Apex   o contains   stereocilia   which   lie   just   below   the   tectorial   membrane   o Stereocilia  touch  the  tectorial  membrane  when  sound  waves   enter   the   cochlea   à   bending   of   the   stereocilia   à   open   ionic   channels   à   causes   changes   of   potential   in   hair   cell   membrane  

Figure  3.  Organ  of  Corti  

 

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 (N1)  spiral  ganglion  

b.Outer  Hair  cell   • Stereocilia  is  found  embedded  in  the  tectorial  membrane     • Possesses  contractile  properties   • Controls  sensory  response  properties  of  the  organ  of  Corti  by   regulating  apposition  of  tectorial  membrane  to  inner  hair  cells     III.  CENTRAL  AUDITORY  PATHWAY   1. Receptor  (Organ  of  Corti)   2. N1  (spiral  ganglion)   3. N2  (ventral  and  dorsal  cochlear  nuclei)     *fibers  form  3  acoustic  striae  (in  bold  letters)   a. Dorsal   cochlear   nucleus   à   dorsal   acoustic   stria   à   (most   of   them  cross  to  join)  contralateral  lateral  lemniscus   b.Ventral   cochlear   nucleus   à     intermediate   acoustic   stria   (course   similar   to   that   of   the   dorsal   acoustic   stria)   à   contralateral  lateral  lemniscus   • Dorsal   and   intermediate   acoustic   striae   constitute   the   central   monaural   auditory   pathway,   carrying   information   about  the  frequency  of  auditory  signals.   c. Ventral  cochlear  nucleus  à  ventral   acoustic   stria  à  terminate   in   ipsilateral   and   contralateral   nuclei   of   the   trapezoid   body   and   superior   olivary   nuclei   à   ipsilateral   and   contralateral   lemnisci   • Ventral   acoustric   stria   forms   a   binaural   pathway   which   analyzes   the   location   of   origin,   or   direction,   of   auditory   stimuli.   4. N3  (inferior  colliculus)   • Sends   axons   to   medial   geniculate   nucleus   through   the   brachium  of  the  inferior  colliculus   5. N4  (medial  geniculate  body)   • Final  sensory  relay  station  of  hearing  pathway     • Special  sensory  nuclei  of  the  thalamus   • Auditory   radiation   through   the   sublenticular     portion   of   the   internal   capsule   will   go   to   the   transverse   temporal   gyri   (of   Heschl)  and  to  planum  temporal   6. Primary  auditory  cortex   • Heschl’s  gyrus  or  anterior  transverse  temporal  gyrus  or   • Brodmann    areas  41  and  42     *When  impulses  reach  areas  41  and  42,  sound  is  heard.  It  is  at  area   22   (auditory   association   area)   where   interpretation   occurs.   Commissural   fibers   exist   between   the   following   nuclei   in   the   auditory  pathway:     • Superior  olivary  nuclei   • Nuclei  of  trapezoid  body   • Nuclei  of  lateral  lemniscus   • Inferior  colliculi                

RG,  SOPIE,  KEIFER,  TASIE,  FILLE,  ELIZ,  MAY,  JANNA  J  

 

   

    (N2)          

 dorsal  cochlear  nucleus     (ipsilateral)  

ventral  cochlear  nucleus     (ipsilateral)  

Dorsal  acoustic  stria                      Intermediate  acoustic  stria                            Ventral  acoustic  stria  

                                               trapezoid  nuclei                          superior  olivary  nuclei                          (contralateral  and  ipsilateral)       Lateral  Lemniscus                                                                          Lateral  Lemniscus      (contralateral)                                                                                    (ipsilateral)         (N3)   Inferior  colliculus                                                      Inferior  colliculus       (N4)   MGB                                                                                                                      MGB                                      Primary  auditory  complex  (BA  41  &  42)    

Tonotopic  Representation  in  the  Auditory  System       Low  Tones   High  Tones   Organ  of  Corti   Upper   Lower   Cochlear  Nuclei   Ventral   Dorsal   MGB   Lateral   Medial   Heschl’s  gyrus   Anterolateral   Posteromedial     A.  Efferent  Transmission   • Heschl’s  gyrus  à  medial  geniculate  body  à  inferior  colliculus  à   superior  olive  à  cochlear  nuclei  à  organ  of  Corti   • Efferent   cochlear   bundle/olivocochlear   bundle   =   terminates   at   organ   of   Corti   where   fibers   end   in   synaptic   relationship   to   hair   cells   • Feedback  mechanism  to  sharpen  tone  of  perception     • For   modulation,   suppression,   selection   of   impulses   arising   in   organ  of  Corti   B.  Auditory  Reflexes   • Involuntary   responses   to   sound   mediated   by   branches   of   main   auditory  pathway   1. Audiomotor  reflexes   o High-­‐intensity   sound   stimulus   à   stapedius  and  tensor   tympani  muscles   of   the  ossicles  contract   (stapedius   pulls   the  stapes  (stirrup)   of   the   middle   ear   away   from   the   oval   window  of  the  cochlea  and  the  tensor  tympani  muscle  pulls   the  malleus  (hammer)   away   from   ear   drum)   à   diminish   vibration  of  middle  ear  ossicles  à  decreased  transmission  of   vibrational  energy  to  the  cochlea   o Impulse  goes  to  cochlear  nuclei  à  R  and  L  superior  olive  à   reticular  formation  à  CN  V  (for  tensor  tympani  ms)  and  CN   VII  (for  stapedius  ms)  

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• For  the  succeeding  pathways,  follow:   o Inferior   colliculus   à   superior   colliculus   à   tectospinal   tract     à  lower  motor  neurons  in  the  brain  stem  and  cervical  spinal   cord,  and  supply  the  oculomotor  system  and  muscles  of  the   head  and  neck  that  respond  to  sound   2. Head  and  neck  turning  in  response  to  sound   3. General  acoustic  muscle  reflex   o Generalized   jerking   of   the   body   in   response   to   a   loud,   sudden  sound   4. Auditory-­‐oculogyric  reflex   o Deviation  of  the  eyes  in  the  direction  of  the  sound   5. Auditory  palpebral  reflex   o Blinking  of  the  eyelids  in  response  to  a  loud  sound   6. Cochleopupillary  reflex   o Dilation  of  the  pupils  in  response  to  a  loud  sound     C.  Hearing  Defects     1.Nerve  deafness   • Destruction  of  cochlear  portion  of  CN  VIII     • Lesion   of   lateral   lemniscus   =   partial   deafness   only   since   pathways  are  bilateral   2.Occlusion  deafness   • When   conduction   of   sound   is   impaired   before   receptor   is   reached   • Due  to  impacted  cerumen,  otitis  media  (infection  in  middle  ear)   or  otosclerosis  (arthritis  of  auditory  ossicles)   3.Tinnitus   • hissing,   roaring,   buzzing,   humming   sounds   due   to   acoustic   neuroma,  streptomycin,  aspirin   • may  lead  to  nerve  deafness   E.  Screening  for  hearing  loss   1.  Weber  test   • Reflects  conduction  loss  in  the  ipsilateral  ear     o Conduction   problem   of   the  middle   ear   (incus,   malleus,   stapes,   and   eustachian   tube)   masks   the   ambient   noise   of   the   room,  making  the  sound  appear  louder   o The   well-­‐functioning  inner   ear  (cochlea   with   its   basilar   membrane)   picks   the   sound   up   via   the   bones   of   the   skull   causing   it   to   be   perceived   as   a   louder   sound   than   in   the   affected/abnormal   ear   (ipsilateral   sensorineural   hearing   heard  as  louder)   • Patient   with   unilateral  conductive   hearing   loss  would   hear   the   tuning  fork  loudest  in  the  affected  ear   • Normal  Weber  =  sound  is  loudest  in  midline   2.  Rinne  Test   • Used  in  cases  of  unilateral  hearing  loss  and  establishes  which   ear  has  the  greater  bone  conduction  by  comparing  perception   of  sounds  transmitted   by  air  conduction   to   those   transmitted   by  bone  conduction  (mastoid  process)  .     o A   "positive"   result   indicates   the   healthy   state,   in   contrast   to   many   other   medical   tests;   so   avoid   using   the   term   'positive'   or   'negative',   and   simply   state   if   the   test   was   normal  or  abnormal,  to  avoid  confusion.   o Normal  /  positive  Rinne:  air  conduction  >  bone  conduction   • Conductive  deafness  in  Weber  and  Rinne:   o Sound  lateralizes/is  louder  in  affected  ear   o In   Weber:   abnormality   in   the   middle   ear   structures   leads   to  increased  sound  perception  in  abnormal  ear  

RG,  SOPIE,  KEIFER,  TASIE,  FILLE,  ELIZ,  MAY,  JANNA  J  

 

   

o In  Rinne:  perceptions  by  bone  conduction  (via  undamaged   inner   ear)   is   enhanced   compared   to   air   conduction   (because  of  damaged  middle  ear)     • Sensorineural  deafness  in  Weber  and  Rinne:   o Sound  is  louder  in  normal  ear  (Weber)   o Bone   conduction   is   ineffective   in   stimulating   damaged   nerve     o Both   bone   conduction   and   air   conduction   are   diminished   but  air  conduction  is  slightly  >  bone  conduction  (Rinne)       IV.  VESTIBULAR  SYSTEM   Functions:   • Maintenance  of  body  balance   • Coordination  of  eyes,  head  and  body  movement   • Permits  eyes  to  remain  fixed  on  a  point  in  space  as  the  head   moves     Consists  of:   • Receptors  located  in  the  inner  ear   • vestibular  hair  cells   • Peripheral  nerves  of  the  vestibular  division  of  CN  VIII   • Central  connections  that  analyze  information  about  the  position   and  movement  of  head  in  space     A.  Inner  Ear  (Labyrinth)   1.  Bony  Labyrinth   • Consists  of  the  cochlea,  vestibule,  semicircular  canals   • Series   of   interconnected   cavities   in   the   petrous   portion   of   temporal  bone   • Contains   perilymph   which   fills   the   space   between   the   bony   and   the  membranous  labyrinth  

  2.  Vestibular/Membranous  Labyrinth   • Consists   of   the   saccule,   utricle,   semicircular   ducts,   cochlear   duct   • Where   the   peripheral   receptors   of   the   vestibular   system,   the   vestibular  hair  cells,  are  located   • Contains  endolymph   • 2  swellings  within  the  vestibule   o Utricle   o Saccule   • 3  semicircular  canals  within  the  vestibule   o Anterior   o Lateral/Horizontal   o Posterior    

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Vestibular  Hair  Cells   • Peripheral  receptors  of  the  vestibular  system   • Reside   inside   specialized   receptor   areas   within   the   membranous  labyrinth   • Sense  both  dynamic  and  static  functions   Dynamic   Function:   detects   linear   (translational)   and   angular   (rotational)  motion  of  head  in  space   o Mediated  by  the  semicircular  ducts   Static  Function:  detection  of  position  (tilt)  of  the  head       o Mediated  by  the  utricle   • The   apical   portion   contains   numerous   long,   rigid,   unbranched   stereocilia,  and  on  one  side,  a  single  kinocilium   o arranged  in  rows  of  increasing  length     B.  Vestibular  Functions  of  the  Ear   • Movement  of  the  head  causes  movement  of  the  endolymph  and   likewise  movement  of  the  otolithic  membrane  (over  the  macula)  and   cupula   (over   the   crista   ampullaris).   This   causes   bending   of   the   stereocilia.     1.  If  Hair  bundle/  stereocilia  bends  toward  the  kinocilium   • Tip   links   (very   small   strand   of   protein   that   connect   the   + stereocilia)   are   pulled   à   cation   channels   open   à   influx   of   K   + ions   à   depolarization   of   hair   cells   à   opening   of   Ca   channels   near  base  of  the  cell  à  stimulates  release  of  neurotransmitter     2.  If  hair  bundle/  stereocilia  bends  away  from  the  kinocilium   • Causes  tip  links  to  be  slack  à  closure  of  apical  cation  channels   + à   hyperpolarization   à   closure   of   Ca   channels   à   reduce   neurotransmitter  release    

C.  Macula    

• Specialized  receptor  region  found  in  the  utricle  and  saccule   • The  hair  cells  within  it  respond  to  linear  acceleration,  gravity  and   tilt  of  the  head   • Contains   hair   cells   that   synapse   with   the   peripheral   nerves/   distal  branches  of  the  vestibular  ganglion  cells   • The  tips  of  the  stereocilia  and  kinocilium  are  embedded  in    thick,   gelatinous   layer   of   proteoglycans   called   the   otolithic   membrane,   the   outer   part   of   which   is   filled   with   calcified   structures   (calcium   carbonate)   called   otoliths   (or   otoconia)   which   increases   the   specific   gravity   of   the   otolithic   membrane   to   about   twice   that   of   the  endolymph.     • otolithic  membranes  move  when  subjected  to  acceleration.   • 2  types:  macula  sacculi  and  macula  utriculi   o Macula   sacculi   (lies   in   the   floor   of   the   saccule)   lies   in   a   plane   perpendicular   to   macula   utriculi   (occupies   the   lateral   wall   of   the  utricle),  but  both  are  similar  histologically.   Utricle:  located  on  the  floor  of  the  vestibule   Saccule:  medial  wall   • The  arrangement  of  hair  cells  in  the  macula  creates  a  response   to  acceleration  in  any  direction.     1.    Striola  

Figure  6.  Striola     Figure  4.Movement  of  stereocilia     RG,  SOPIE,  KEIFER,  TASIE,  FILLE,  ELIZ,  MAY,  JANNA  J  

 

   

 

 

Figure  5.  Macula  

 

• Specialized  strip  through  the  middle  of  the  macula   o Curved  equatorial  line  

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• Defines  the  site  of  change  in  orientation  of  the  hair  cells   • Zone  that  runs  the  length  of  macula  of  both  utricle  and  saccule   • Because  striola  curves  through  the  macula,  hair  cells  are  polarized   in  different  directions   o Utricular   and   saccular   hair   cells   are   directionally   sensitive   to   a   wide  variety  of  head  positions  and  linear  movements   2.  Macula  Utricli   • Responds   to   changes   in   head   position   with   respect   to   gravity/   tilt  and  to  earth-­‐horizontal  linear  acceleration   • Each  hair  cell  is  oriented  with  the  kinocilium  toward  the  striola   o hair   cells   of   either   sides   of   striola   are   organized   as   mirror   images     o horizontal  acceleration  depolarizes  one  sector  of  the  macula   regardless  of  the  direction  of  movement   • hair  cells  are  polarized  toward  the  stiola     3.  Macula  Sacculi   • Similar  in  structure  with  that  of  the  macula  utriculi   • Responds  to  gravitational  pull   • Oriented  vertically,  approximately  in  a  parasagittal  plane.   • The  kinocilia  are  oriented  away  from  striola   • Linear  acceleration  in  the  vertical  direction  stimulates  the  macula   o Occurs   in   response   to   gravity   or   against   gravity   (e.g.   acceleration  or  deceleration  in  an  elevator)   • Hair  cells  are  polarized  away  from  the  striola       D.  Semicircular  Canals   • Anterior  (superior)  -­‐  Kinocilia  faces  away  from  the  utricle   • Lateral  (horizontal)  -­‐  Kinocilia  faces  the  utricle   • Posterior  -­‐  Kinocilia  faces  away  from  the  utricle   • Each  semicircular  canal  has  an  enlarged  end,  called  the  ampulla   • Within  the  ampulla  is  the  ampullary  crest  or  crista  ampullaris,  a   ridge  that  bears  hair  cells  like  those  of  the  maculae  

  o React  to  rotational  acceleration  or  angular  movement  (kinetic   equilibrium)   o Covered  with  a  gelatinous  capsule,  the  cupula   § Extends  almost  to  the  roof  of  the  ampulla   § Has  the  same  specific  gravity  as  the  endolymph,  therefore,  it   cannot  sense  the  effect  of  gravity   RG,  SOPIE,  KEIFER,  TASIE,  FILLE,  ELIZ,  MAY,  JANNA  J  

 

   

When   the   head   undergoes   angular   acceleration,   the   viscous   endolymph  in  the  semicircular  ducts  pushes  in  the  cupula   o Distortion  of  the  cupula  evokes  a  receptor  potential  in  the  hair   cells   of   the   ampullary   crest,   à   alters   the   level   of   activity   in   the  peripheral  fibers  of  CN  VIII,  innervating  the  hair  cells   o The   vestibular   nerve   fibers   to   each   duct   respond   with   an   increase  in  impulse  frequency  to  rotation  in  one  direction  and   with   a   decrease   in   impulse   frequency   to   rotation   in   the   opposite  direction   • Lodged  within  the  canals  are  the  semicircular  ducts  :   1.  Superior  semicircular  canal     o Vertical   o Perpendicular  to  the  long  axis  of  the  petrous  bone   2.  Posterior  semicircular  canal     o Vertical     o Parallel  with  the  long  axis  of  the  petrous  bone     3.  Lateral  semicircular  canal     o Horizontal     o Lies  in  the  medial  wall  of  the  aditus  to  the  mastoid  antrum   above  the  facial  nerve  canal     •

Figure  8.  Movement  of  the  cristae  ampullaris    

 

E.  Vestibular  Pathways   • Afferent  fibers  of  the  vestibular  nerve  have  their  cell  bodies  in  the   vestibular  ganglion  (of  Scarpa)   • Axons  of  bipolar  cells  of  the  vestibular  ganglion  pass  through  the   internal   auditory   canal   and   reach   the   upper   medulla   in   company   with  the  cochlear  nerve   o Most   of   the   fibers   of   the   vestibular   nerve   bifurcate   into   ascending   and   descending   branches   and   terminate   in   the   vestibular  nuclei,  which  are  clustered  in  the  lateral  floor  of  the   fourth  ventricle   o Inferior  vestibular  nucleus  (descending  spinal)   § Receives   input   from   the   semicircular   ducts   and   from   the   utricle  and  the  saccule   § Nucleus   projects   into   the   ascending   medial   longitudinal   fasciculus  (MLF)   o  Superior  vestibular  nucleus  (of  Bechterew)   § Receives  input  chiefly  from  the  cristae  of  semicircular  canals   § Neurons  project  into  the   ascending   part   of   the   MLF,  where   they  participate  in  vestibuloocular  reflexes   o Medial  vestibular  nucleus  (of  Schwalbe)   § Receives  input  chiefly  from  the  cristae  of  semicircular  canals   § Neurons  project  into  the   ascending   part   of   the   MLF,  where   they  participate  in  vestibuloocular  reflexes   o Lateral  vestibular  nucleus  (of  Deiter)   § Receives  input  chiefly  from  the  macula  of  the  utricle   § Neurons  project  to  the  ascending  portion  of  the  MLF  and   to   the  spinal  cord  through  the  lateral  vestibulospinal  tract  

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• Neurons   of   the   lateral   vestibular   nucleus   are   inhibited   monosynaptically  by  the  Purkinje  cells  in  the  cerebellum.   • From  the  vestibular  nuclei,  secondary  axons  distribute  this   information   to   4   sites:   spinal   cord   (muscle   control),   reticular   formation   (vomiting   center),   extraocular   muscles,   and  cortex  (conscious  perception).     • Some   primary   fibers   of   the   vestibular   nerve   pass   directly   to   the   cerebellum,  ending  in  the  cortex  of  the  flocculonodular  lobe     1.  Vestibulospinal  Tracts   • Both  tracts  have  a  strong  facilitating  effects  on  motor  neurons   innervating  antigravity  muscles     • Assists  the  local  myotactic  reflexes     • Reinforce  the  tonus  of  the  extensor  muscles  of  the  trunk  and   limbs     • Produces  enough  strength  to  support  the  body  against  gravity   and  maintain  an  upright  posture     • Both  terminate  along  their  course  almost  exclusively  upon   interneurons  in  laminae  VII  and  VIII  synapse  on  the  alpha  and   gamma  lower  motorneurons  in  Lamina  IX     • Two   major   projections   into   the   spinal   cord   arise   from   the   vestibular  nuclei:  lateral  (maintains  upright  posture)  and  medial   (coordinated  movements  of  neck  &  eye)  vestibulospinal  tract     • Lateral   vestibular   nucleus   →   uncrossed   lateral   vestibulospinal   tract  →  cervical  to  the  lumbosacral  spinal  cord  →  antigravity  ms,   neck  ms  that  control  gaze   • Medial   vestibular   nucleus   →   crossed   and   uncrossed   medial   vestibulospinal   tract   →   descending   portion   of   MLF   (medial   longitudinal  fasiculus)→  cervical  spinal  cord  only  →  antigravity  ms  

Figure  9.    Vestibulospinal  tract   RG,  SOPIE,  KEIFER,  TASIE,  FILLE,  ELIZ,  MAY,  JANNA  J  

 

   

 

V.  CENTRAL  CONNECTIONS  &  VESTIBULAR  NUCLEI  

  1. Conjugate  Eye  Movement:  in  response  to  head  movement  &   position  of  head  in  space     Vestibular  nuclei  via  MLF  (crossed  and  uncrossed)   ↓   Nuclei  of  CNs  III,  IV,  VI:  EOMs  (conjugate  eye  movement)   and   Nucleus  of  CN  XI  and  anterior  horn  cells  of  cervical  spinal  cord   (medial  vestibulospinal  tract:  maintains  the  position  of  the  head)     2. Maintain  upright  posture   Lateral  vestibular  nucleus   ↓   Lateral  vestibulospinal  tract  of  spinal  cord  to  sacral  level:  extensor   muscles  of  the  trunk  and  limits     3.  Fastigial  nuclei  of  the  cerebellum   0 0 -­‐ Vestibulocerebellar   fibers   (1   and   2 )   enter   cerebellum   via   juxtarestiform   body   (a   portion   of   the   inferior   cerebellar   peduncle).   Uncrossed   efferent   fibers   from   the   fastigial   nucleus   of   the   cerebellum   project   to   the   brainstem   also   through  the  juxtarestiform  body.     Secondary  vestibulocerebellar  fibers   ↓   Fastigial  nuclei  &  cortex  of  cerebellar  vermis,   sends  efferent  fibers  to   ↓   Lateral  vestibular  nucleus:  facilitating  influences  on  extensor  muscle   tone  via  vestibulospinal  tract     4. Reticular  formation  of  brainstem   o Vomiting  center  (group  of  reticular  neurons  in  medulla  near   dorsal  motor  nucleus  of  vagus)   § Parasympathetic   motor   impulses   to   thoracic   &   abdominal   viscera   § Connections  from  vestibular  nuclei  to  the  vomiting  center   probably  account  for  the  vomiting  associated  with  motion   sickness   o RAS  (Reticular  Activating  System)  or  Reticular  centers   § Vestibuloreticular   connections   to   the   RAS   may   alert   individual  to  sleep  (rocking  chair  for  babies)     5. MGB  (Medial  geniculate  body)     o Vestibular   influences   project   rostrally   via   the   MGB   to   a   vestibular  cortical  area  near  the  auditory  cortex   o Related   to   objective   sensations,   e.g.   dizziness   associated   with  the  vestibular  system                

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A.  Test  of  Vestibular  Functions    

1.

2.

 

Rotation  test   0 • Subject   will   sit   on   a   rotating   chair   with   head   tilted   30   forward  which  will  then  be  stopped  after  10-­‐12  rotations   • Induces   nystagmus,   which   lasts   about   30   seconds   in   neurologically  normal  persons     Caloric/Thermal  Test   • Subject  will  be  tested  on  both  sides   0 • Lying  down  with  head  tilted  forward  about  30  or  sitting  with   0 head   tilted   backward   60   to   bring   the   horizontal   SCC   into   a   vertical  plane   • Ear  will  be  irrigated  with  warm/cold  water  which  will  cause  a   convection   current   on   endolymph   which   will   stimulate   the   hair  cells  and  cause  nystagmus   • COWS  –  direction  of  nystagmus   o Cold  on  Opposite,  warm  on  Same  Side  

Reference:   2014A  trans   Lansang  Notes   Snell   Moore   Berne  &  Levy      “Everybody  is  a  genius.  But  if  you  judge  a  fish  by  its  ability  to  climb  a  tree,   it  will  live  its  whole  life  believing  that  it  is  stupid.”     -­‐-­‐  Albert  Einstein     "If  you  don't  go  after  what  you  want,  you'll  never  have  it.  If  you  don't  ask,   the   answer   is   always   no.   If   you   don't   step   forward,   you're   always   in   the   same  place."     J  

CLINICAL  CORRELATIONS   A.  Nystagmus  

  Persistent  stimulation  of  hair  cells  in  cristae  ampullaris  will  draw   eyes  slowly  to  one  side  until  a  limit  is  reached  then  jerk  quickly   to   opposite   side.   (involuntary   back   and   forth,   up   and   down   rotational  movement  of  the  eyeball)   • its  direction  is  designated  according  to  the  direction  of  the  fast   component  which  is  the  basis  for  tests  in  vestibular  function     • this  results  from  a  lesion  of  the  vestibular  system,  its  peripheral   and   central   connection   and   also   from   lesions   in   brainstem   and   cerebellum   • this   can   also   be   caused   by   chronic   visual   impairment   or   toxic   substances   • if  unbalanced:   o Unilateral  damage  to  the  vestibular  nuclei  or  their  connections   →   tonic   deviation   of   the   eyes   to   one   side   +   vertigo   +   nystagmus   o Destruction  of  the  vestibular  receptors  or  section  of  the  nerve:   (-­‐)  tonic  deviation     • Fast  case:  nystagmus  diagnosis   o Ex.  Lesion  on  R   o Caloric/thermal  test:   § Cold  water-­‐direction  of  nystagmus  is  opposite  the  lesion   § Warm  water-­‐direction  of  nystagmus  is  to  the  same  side   B.  Vertigo   •

  • Sensation   of   whirling,   dizziness   due   to   disturbance   of   equilibrium   • Stimulation  or  damage  to  the  vestibular  end  organ  (prolonged  or   excessive  stimulation  of  vestibular  apparatus  -­‐  motion  sickness)                         RG,  SOPIE,  KEIFER,  TASIE,  FILLE,  ELIZ,  MAY,  JANNA  J  

 

   

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