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Article: Sensory Physiology - on the Vestibular System

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  • Robert Karl Stonjek
    SENSORY PHYSIOLOGY: ON THE VESTIBULAR SYSTEM The following points are made by B.L. Day and R.C. Fitzpatrick (Current Biology 2005 15:583): 1) Small,
    Message 1 of 1 , Sep 10, 2005

      The following points are made by B.L. Day and R.C. Fitzpatrick (Current
      Biology 2005 15:583):

      1) Small, intricately formed and locked in the skull, the vestibular organs
      continuously bombard the brain with messages. The messages are quite unlike
      any others. They tell of accelerations, how the head is rotating and
      translating and its orientation in space. The messages never stop and cannot
      be turned off. Even when we are completely motionless, they signal the
      relentless pull of gravity. Perhaps because of their constant monologue, the
      vestibular sensation is different to the other senses. There is no overt,
      readily recognizable, localizable, conscious sensation from these organs.
      They provide a silent sense.

      2) Known as balance organs of the inner ear, the vestibular organs serve
      this complex motor function at a largely subconscious level, but their role
      does not stop with balance. They contribute to a surprising range of brain
      functions, from the highest levels of consciousness to the most automatic
      reflexes. The value of the vestibular sensory system to brain functions such
      as perception of self and non-self motion, spatial orientation, navigation,
      voluntary movement, oculomotor control, and autonomic control, comes from
      their unique and complete description of head motion and orientation in
      three dimensions.

      3) Locked in the bony structure of the inner ear in close association with
      the auditory organ, the cochlea, the vestibular organs form two functional
      units. The two otolith organs sense linear acceleration and its
      gravitational equivalent, and the three semicircular canals sense rotational
      movement in space. The hair cells of the utricle and saccule form a
      two-dimensional array with their cilia embedded in a membrane of dense
      calcium crystals known as otoliths ("ear stones"). Movement of the membrane
      by gravitational or inertial forces maximally activates those hair cells
      that are aligned with the movement. With the two organs oriented at right
      angles to each other, the direction of linear acceleration is spatially
      encoded in three dimensions and the magnitude of the acceleration is encoded
      by the firing rate. As the head rotates, the inertial force of the fluid in
      the semicircular canals deflects the cilia of hair cells aligned with the
      canals, modulating the firing of the afferent nerves. With the three
      semicircular canals aligned at right angles to each other, rotation in any
      direction can be resolved.

      Full Text at ScienceWeek

      In youth, we stimulate the vestibular system to the point of confusion on
      fairground rides. Later in life the same sort of thing happens, but without
      the need for the fairground ride (please remain seated whilst applauding my
      insight :)

      Posted by
      Robert Karl Stonjek
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