The Sixth Sense - Sensation of Self Motion and Orientation

What are the basic senses? Sight, sound, taste, touch, smell. We use all these senses to explore and engage the world around us. But what about sensation and perception of self? While many may not view the body's perception of self as interesting, it is definitely necessary and important. How else would we know where are arms are? And how to tie an apron behind our back? And when we are walking? And when we are falling?

The body's perception of self motion and orientation is constantly yet subconsciously being processed by our brains. Chief among these sensations are proprioception and vestibular sensation. Proprioceptive input is provided by 'stretch' receptors located within muscle that provide the brain with a feedback signal of where our body parts are. This allows one to perform complex motions without visual input (such as tying a knot behind one's back). 

Vestibular sensation, on the other hand, provides inputs indicating head motion and head orientation with respect to gravity. The vestibular labyrinths, which contain the actual sensory structures, are located in each of the two inner ears. Each labyrinth consists of three semicircular canals (SCCs), which sense three-dimensional (3D) rotational velocity, and two otolith end organs, which sense 3D linear acceleration. Each vestibular end organ, be it SCC or otolith end organ, contains mechanoreceptive 'hair cells' (named so for their hair like protrusions), which detect the different types of motions. The SCCs are hula-hoop shaped structures filled with fluid with a membrane spanning the canal, in which the hair cells are embedded. Any time the head moves, the fluid pushes on that membrane and the hair cells deflect, resulting in signal that represents rotational velocity in the plane of that SCC. Three such canals, oriented orthogonally, provide 3D sensation of rotational motion. The otolith end organs, called the utricle and saccule, sense linear acceleration. Each otolith end organ contains small calcium crystals that sit on a bed of hair cells. Whenever the head moves, the crystals exert a different force on the hair cells, which is detected as a linear acceleration on that particular hair cell. Hair cells located in otolith end organs are not oriented in the same direction and, as such, do not report forces in the same direction. The brain is able to reconstruct the force due to gravitational acceleration and linear acceleration from all the information provided by the hair cells of the otolith end organs. Truly, the vestibular labyrinth is evidence of nature's ability to engineer a means to detect motion and sense orientation. 

Because we live our lives blissfully unaware of just how important sensation of head motion and orientation is, bilateral loss of vestibular function (inner ear balance sensation) due to ototoxic hair cell injury can be disabling, with patients suffering disequilibrium and inability to maintain stable vision during head movements typical of daily life. While most individuals with partial loss compensate through rehabilitative strategies enlisting other senses, those who fail to compensate for profound loss have no good therapeutic options. Because the vestibular nerve should be intact in many of these patients, electrical stimuli encoding head rotation should be able to drive the nerve and restore sensation of head movement, much like a cochlear implant restores auditory function.