Vestibular organs

Photo courtesy NASA

How You Sense Position and Motion

Orientation and motion are sensed by using the vestibular system, which is located in the upper portion of the inner ear.

Here is how the vestibular system senses orientation with respect to gravity: It has otolithic organs that contain crystals of calcium carbonate (chalk). The crystals are attached to hair-like sensory nerve cells in different orientations (x-, y- and z-axes).

  1. When you bend your head in different directions (forward, backward, sideways), gravity pulls on the crystals that are oriented in the direction of the pull.
  2. The affected crystals stimulate the attached hair cells to send nerve impulses to the brain.
  3. The brain interprets these signals to find out which way the head is oriented in space.

Here is how the vestibular system senses motion: There are three semicircular canals for sensing motion, specifically acceleration. They are oriented at right angles to one another, and each is in one of the three directions (x-, y- or z-axis). They contain fluid called endolymph and hair-like sensory nerve cells.

  1. As your head accelerates in a given direction, the endolymph lags behind because of its initial resistance to change in motion (inertia).
  2. The lagging endolymph stimulates the appropriate hair cells to send nerve signals to the brain.
  3. The brain interprets them to find out which way the head has moved.