The Vestibular System

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The Vestibular System Powered By Docstoc
					         The Vestibular System
• generates compensatory responses to head
  motion
  – postural responses
  – ocular-motor responses
  – visceral responses
• To achieve this the vestibular system measures
  – Head rotation
  – Head acceleration
     • Einstein’s equivalency theorem states that an accelerometer
       cannot distinguish between translational accelerations and
       tilts
Vestibularly driven eye
  movements, or the
  VESTIBULO-
  OCULAR REFLEX (or
  VOR) is a very basic
  and evolutionarily old
  neural response
  designed to maintain
  clear and stable
  vision in the presence
  of head movements.

                     From Leigh and Zee, The Neurology of Eye Movement
Life is hard for those who don’t have a VOR
   During a walk I found too much motion in my visual
   picture of the surroundings to permit recognition of fine
   detail. I learned that I must stand still in order to read
   the lettering on a sign
                      --J.C, 1952
                      M.D. with no
                      vestibular system
His vision was disturbed by head movements as small as
those induced by the beat of his heart while at rest.
 Why do we need a vestibular system to hold gaze stable
during head movement when we have a variety of ocular
following mechanisms that cause the eyes to track a target
that moves with respect to the head?


•Visual latencies are on the order of ~70 msec, while VOR
responses occur an order of magnitude faster.
•If images on our retina slipped across photodetectors by
more than about 4°/sec, visual acuity goes down. We
cannot achieve this level of gaze stability by using long-
latency visually mediated responses.
•Because of this, the VOR must operate in the absence of
feedback, and is thus open loop!!
                      Why Study the VOR?


Answer: While having the same basic goals of other motor
reflexes, the VOR is comparatively simple:
• Relatively easy to control head motion
•There are no changes in load in the effector organ (i.e., the eye)
•The eye moves with relatively few degrees of freedom
•Head and eye movements are easily measured
•Every cell involved in the VOR resides within the cranial vault,
and are thus relatively easy to record from in animal preparations
The Magnetic Scleral Search Coil
Eye movements can be
measured with very
high precision using
only slightly invasive
procedures




                         From Leigh and Zee
Vestibular End Organs




         From Leigh and Zee
Semicircular canals lie orthogonal to each
other. Further, canals are located on both
sides of the head, forming push-pull pairs
with their contralateral counterparts.
                         IV
                         abducens
       III
oculomotor




             From Leigh and Zee
From Kandel and Schwartz
   Mechanical Model of Semicircular Canal

• Circular lumen of overall radius
  R and lumen radius r. The
  lumen is filled with fluid with
  moment of inertia I. The
  moment of viscous friction per
  relative angular velocity of the
  fluid is B.
• The lumen is rotated through
  angle q. A piston lags behind
  the lumen (due to inertia and
  friction, and thus only rotates
  through angle p, and the
  relative displacement of the
  piston is Q (Note that q=Q+p).
• Further, the piston has a
  spring-like restorative force, K.
                       (s)     T1T2 s
                            
                       q(s) T1s  1T2 s  1
                       



•Canal is a high-
pass transducer of
rotational velocity.
•10°/sec rotation
causes roughly
0.03° of cupula
deflection!!
 1 Hz


          Low freq stimulus
          results in diminished
          gain and a phase
          lead.


0.01 Hz
Input/Output Relationship of
    Semicircular Canal
            Vestibular Nystagmus

If the vestibular system
responded to large head
movements with a smooth eye
movement, the eye position
would rapidly exceed the
physiological range. For this
reason, the vestibular system
produces a sawtooth-like
waveform known as vestibular
nystagmus, in which quick
phases rapidly bring the eye
back into the physiological
range, while the slow phases are
compensatory for head
movement.
Velocity Storage
(positive feedback)
effectively raises the
time constant of the
VOR to above that of
canals (by about a
factor of three,
improving low-
frequency performance.
Visual information
combines with vestibular
information to further
supplement low-
frequency behavior. The
resulting eye movements
are known as optokinetic
nystagmus (OKN) and
optokinetic
afternystagmus
(OKAN). This
information apparently
enters ocular motor
pathways through the
velocity storage loop.
From Leigh and Zee
                   Otolith Organs




Contribute to:
   •Image stabilization during head translation and tilt
   •Perception of motion
   •Orientational mechanisms
   •Postural Stability
           Translational VOR
• The translational
  VOR is more
  complex than the
  angular VOR
• Size of response
  is highly
  dependent on
  the distance of
  the target
LVOR During Interaural Translation (Monkey)
    Naso-Occipital Translational Motion

• The kinematics
  to naso-occipital
  motion are even                                      d
  more complex        Y

• Response                                                              
                                                       X
  magnitude is
  highly              Given

  dependent on                      
                        tan 1 Y X
                                                                    Motion

                      Then
  both target         
                         
                                        Y
                      X            Y 2 
  distance and               X 2 1  2  
                                    X 

  gaze angle              
                              Y
                              
                            X  Y2
                              2
                                         
                               sin 
                                            ( in rads / unit d )
                                 d
                       Horizontal NO-LVOR Response
                 10
Head Velocity
 (cm/sec)

                  0



                 -10



                1.25     A                       B
Eye Velocity
 (deg/sec)




                0.00


           -1.25


           -2.50

                 10
Eye Position




                  5
  (deg)




                  0

                  -5
                                                     0.5 sec
                 -10
               Otolith Ambiguity
               Translation                  Tilt



    Acceleration


                   Both Angles are equal.
•The otolith organs transduce acceleration

•Thus, translational accelerations and tilts of the head
cause similar activity on otolith afferents.

•Otolith information is therefore ambiguous
(Einstein’s equivalency principle) and must be
resolved in order to provide proper stabilizing and
orientation responses.
                                      EYE MOVEMENT Trials
                 100                                                                    100



Sled Velocity




                                                                                                Eye Velocity
                  50                                                                    50

  (cm/sec)




                                                                                                 (deg/sec)
                   0                                                                    0


                  -50                                                                   -50


                 -100                                                                   -100

                        0        10         20                30         40        50


                 100                                                                    50
Sled Velocity




                                                                                               Eye Velocity
                  50                                                                    25
  (cm/sec)




                                                                                                (deg/sec)
                   0                                                                    0


                  -50                                                                   -25


                 -100                                                                   -50

                        0        10         20                30         40        50

                                                                                        20


                  50
 Sled Velocity




                                                                                        10




                                                                                               Eye Velocity
   (cm/sec)




                                                                                                (cm/sec)
                   0                                                                    0



                                                                                        -10
                  -50


                                                                                        -20
                        0   10         20         30           40   50        60

                                                 Time (sec)
                          Sled Velocity                             Sled Velocity                     Sled Velocity
                            (cm/sec)                                  (cm/sec)                          (cm/sec)




                                              50
                                                             -100
                                                                               50
                                                                                    100
                                                                                               -100
                                                                                                                 50
                                                                                                                      100




                                     0
                                                                           0
                                                                                                             0




                         -50
                                                                     -50
                                                                                                       -50




             0
                                                        0
                                                                                          0




             10
                                                        10
                                                                                          10




             20
                                                        20
                                                                                          20




Time (sec)
             30
                                                        30
                                                                                          30




             40
                                                        40
                                                                                          40
                                                                                                                            Translation PERCEPTION




             50
                                                        50
                                                                                          50




                                     0
                                                                           0
                                                                                                             0




                                         20
                                                   40
                                                                               50
                                                                                                                 50




                  -40
                               -20
                                                                     -50
                                                                                                       -50




                                                             -100
                                                                                    100
                                                                                               -100
                                                                                                                      100




                        Percept Velocity                     Percept Velocity                  Percept Velocity
                           (cm/sec)                             (cm/sec)                          (cm/sec)
                                                 Translational-LVOR

                               1
Sensitivity (°/cm)


                              0.1


                             0.01
                                                                                             Translational VOR Response is
                                                                          UP-IA Horizontal   HIGH PASS, even though otolith
                                                                         ND-IA Horizontal
                      0.001                                               RD-DV vertical     organs are ALL PASS
                                                                          LD-DV vertical
                                                                         Average
                     0.0001




                             270
                             225
               Phase (deg)




                             180
                             135
                              90
                              45
                                0
                              -45
                                          0.01                     0.1      1
                                                              Frequency (Hz)
                                    Squirrel Monkey: Avg. Values
                      Tilt PERCEPTION

                  3
                  0          0
                            0 z
                            .5
                            0 H
                  5
                  1
                  0
              1
              -5
              3
              -0

                  3
                  0          1
                            0 z
                            .0
                            0 H
                  5
                  1
                  0
              1
              -5
              3
              -0
Positon(degres)

                  3
                  0          2
                            0 z
                            .5
                            0 H
                  5
                  1
                  0
              1
              -5
              3
              -0
                      0c
                      0s
                      1e
   .
   4
   1                                 .
                                     0
                                     00



   .
   2
   1                                 11
                                      .
                                     -25




                                      PHASE(Degrees)
   .
   0
   1                                 22
                                      .
                                     -50
GAIN
   .
   8
   0                                 33
                                      .
                                     -75



   .
   6
   0                                 45
                                      .
                                     -00
     .5
     0
     00         0
                .
                01              .5
                                0
                                02

                re
                ec
                qu
                Fny



       Tilt responses are LOW PASS
                          Translational
                          VOR
                          Responses
              High
              frequency   Translational
                          Perception
                          Responses
Otolith
information



              Low
                          Tilt VOR
              frequency
                          Responses

                          Tilt Perception
                          Responses
Input/Output Relationship of the
              Eye
               Motoneuron Activity




                                                  From Robinson,Handbook of Physiology, 1981

•Ocular motoneurons vary firing rate by both eye position and eye velocity.
•Recall that muscles require mostly position commands, but require some velocity
information to overcome viscous forces.
•Since the vestibular endorgans generate velocity information, we clearly have a
communication problem between the vestibular system and the eye muscles
              Neural Integration
Problem: Eye muscles require a combination of position and
velocity information, while the vestibular system (and the saccadic
system) generate only velocity information.


Solution: Mathematically integrate velocity information, then sum
velocity and position information in correct proportion to satisfy
the needs of the ocular motor plant.




                         From Leigh and Zee
Easiest place to study neural integration is in the saccadic system,
which changes gaze during normal “looking around.
Neural integration of the vestibular signal entails many of the same
features and neural substrates.




         From Leigh and Zee
Neural integration is
essentially a summing
operation, and can be
achieved using a simple
positive feedback neural
circuit.
However, such a circuit
will be inherently
unstable, and will saturate
in the presence of tonic
nerve activity.
                     Lateral Inhibition
These drawbacks can be overcome by using a laterally-inhibitory
net. In lateral inhibition, each cell projects in an inhibitory fashion
to its neighbors. Thus, activity on a cell causes inhibition of
neighboring cells, thus disinhibiting itself.
From Robinson, 1981
                      Motor Learning
                    (Adaptive Plasticity)
Because the vestibular system must be
fast, the VOR functions in an open
loop manner. Despite this, the VOR
can correct long-term errors. The
cerebellum is particularly influential
in this process.
When climbing fibers (cf) fire in
response to retinal slip (poor VOR
gain) during head movement, the b of
the cerebellar loop changes. It is not
clear whether this change takes place       From Robinson, 1981
in the synapse between the parallel
fiber (gc) and the Purkinje cell (pc), or
between the Purkinje cell and the
vestibular nucleus.

				
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