Balance, the ability to sense and adjust to gravity and perceive any kind of acceleration is a fundamental sense. But if this sense is fundamental, why is it often ignored?
Known as the vestibular system, this sense functions below the level of our cerebral cortex and we often take it for granted. The vestibular nerve is the first fiber tract in the brain to begin myelination (a process by which fatty layer accumulates around nerve cells—enabling transmission of faster information for complex brain processes). By only 5 months after gestation, the vestibular apparatus is ready and the vestibular pathways to eyes and spinal cord have begun the process of myelination so the entire system works effectively.
How does this system work? The auditory system’s cochlea is part of a maze which sits in the vestibulum in the inner ear. Given that movement consists of rotations and transitions, the vestibular system consists of both the semicircular canal system (sensing angular acceleration in three planes) and the otoliths (sensing linear acceleration—gravity and translational movements). The receptor cells of the otoliths and the semicircular canals send signals through the vestibular nerve fibers to the neural structures that control eye movement, posture, and balance. What is projected from this anatomical basis of the vestibulo-ocular reflex is essential for an accurate vision. These projections to the muscles that control posture are also necessary to maintain an upright position. The brain obtains this information from the vestibular system and from our ability to sense the position, location, orientation, and movement of our bodies to understand our motion and dynamics.
A mature vestibular system allows a fetus to be able to sense his orientation with respect to gravity and therefore turn into proper position. There are various reflexes that are apparent at birth. The asymmetrical neck response occurs when limbs adjust in order to maintain balance, such as turning a baby’s head to the right as he automatically extends his right arm and leg and flexes limbs on his left side. The traction response occurs when a baby attempts to hold his head up while being pulled from lying to sitting. The vestibular system is aware of the forward movement of the head and contracts neck flexor muscles attempting to raise it to a vertical position. The doll’s eye reflex is the clearest vestibular function, apparent in the eye movements. If a baby moves his head to the right, his eyes will remain forward.
Babies crave vestibular stimulation through repetitive motion like swinging and bouncing. They even do it themselves by bouncing and shaking their heads at 6-8 months of age. Studies suggest that the stimulation of the vestibular system in babies results in more advanced development of reflexes and motor skills. This stimulation is also beneficial with very young infants; they cry less when they are being rocked or changed in position. Vestibular stimulation has a profound impact on a baby’s overall behavioral state, resulting in greater overall alertness. This alertness is ideal for learning and absorbing information effectively. Next time you go out for a run, you will appreciate this system. How would your perception change if jogging made you see the world bobbing up and down?
Eliot, L. (1999). What’s going on in there. How the brain and mind develop in the first five years of life, 237-239.
