THE EAR: THE SEAT OF HEARING AND BALANCE
A Sixth Sense
Vestibular disorders affect about 45% of people above 40 years of age. Vertigo is a debilitating symptom resulting from such disorders. Still, until the function of the vestibular system is compromised, we are not conscious about it. Because of that, it is sometimes referred to as the sixth sense. Vestibular reflexes result in stabilization of eye in space and body posture. For example, during walking, the head moves with each step that you take. If you try to read a sign while walking, you need to have your eyes stabilized on the sign rather than moving with your head. This happens due to the vestibulo-ocular reflex (VOR), which moves the eyes in the opposite direction of head movements and with the same amount. In fact, if the VOR is compromised, the image on the retina will be similar to what you see with a movie camera attached to your head: blurry images jumping around!
The vestibular system is the main source for providing information regarding head movement and position. However, balance is the result of convergence of inputs from multiple sources. This multisensory integration is necessary for proper sense of equilibrium. Proprioception, vision, and somatosensory provide important extra-vestibular signals required for maintaining proper balance.
The nervous system has a great capacity for adapting to changes. This plasticity is presented very well in vesibular pathways. For example, the VOR changes with changes in the visual input, such as when wearing glasses. Another example is the adaptation of vestibulo-spinal reflexes (affecting the limbs) to changes in gravity (e.g., in astronauts).