1. Why We Don’t Fall Down
By: Bryan. D
Have you ever strolled along a beach just as the sun was setting across the water,
enjoying the feeling of your bare feet sinking into the sand as you walk? Perhaps you have
attended a party that required you to navigate across a crowded room, dodging dancing party
goers and weaving around bustling servers to get to your table. I would guess that most people
have experienced situations like this many times in their lives. Moving around our surroundings
is something most humans gracefully do every day. We do this without a second thought to the
complex inner mechanisms that keep us from falling down on those uneven sandy beaches or
getting dizzy amongst the dancing party goers. The amazing body system that allows humans to
accomplish these effortless feats of balanced mobility is the vestibular system.
To be able to maintain equilibrium and balance, as well as proper gait and posture, the
vestibular system depends on inputs from three main areas: the vestibular apparatus of the inner
ear, visual inputs from the eyes, and inputs from pressure and stretch proprioceptive sensors
located throughout the body (Fig. A). These parts of the vestibular system provide the cerebral
cortex, brainstem and cerebellum with vital information about changes in head and body
movement with respect to gravity [1]. These inputs are analogues to the legs of a three-legged
stool, working together to evenly balance and support the weight of its load.
Fig. A: Balancing systems.

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The first leg of the stool is the vestibular apparatus inside the inner ear consisting of three
fluid filled semicircular canals and two maculae (the Utricle & Saccule). The semicircular canals
are filled with a fluid called endolymph and are positioned at right angles to one another in three
planes. Also inside these canals are hair cells that sense movement of the endolymph as the head
is rotated [3]. The semicircular canals help us to perceive rotatory acceleration [2]. The two
maculae are essential for perceiving linear acceleration/deceleration such as starting or stopping
a car, riding in an elevator or when jumping or falling. They do this by nerve signals sent to the
brain by special hair cells in the maculae that are displaced by a gelatinous membrane during
translatory movements [2, 3].
Fig. B: Semicircular Canals and Maculae. The cochlea (which functions in hearing) is also shown.
The second leg of the stool is the visual input provided by the eyes. Not only does vision
allow us to perceive where we physically are in space, it helps us to keep our body coordinated
to upcoming obstacles. It also allows us to see an object and prepare the muscles and limbs to
respond accordingly in a balanced fashion. For example, you go to pick up a large box that you
visually perceive to be extremely heavy. Because the visual information being sent to the cortex
and cerebellum are indicating the box is heavy, motor commands are priming the body to
balance for a heavy lift. If the box turns out to be extremely light, it is most likely you will be
thrown off balance as you lift it.

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The eyes are also directly tied into the vestibular organs of the inner ear and play a role in
the vestibule-ocular reflex (VOR). As Dr. Mason discussed during the neurobiology course, the
VOR moves the eyes in opposition to the head in order to maintain a steady gaze while the head
is moving. If the visual inputs are not coordinating with information being sent by the inner ear
balance organs, it may lead to dizziness. This explains why some people get car sick while
reading in a moving automobile.
The last leg of the stool is the proprioceptive sensors. These sensors are located in
muscles, tendons, joints and the inner ear. They provide vital information about body position,
muscle length and tension, as well as the position and movement of joints [3]. Rita Carter, in her
book Mapping the Mind, describes proprioception as the sixth sense. She states that
“proprioception is the sense of body awareness telling us the position of our limbs, our posture
and equilibrium. It involves the integration of several sensory inputs: touch and pressure,
sensations from skin, muscles and tendons, visual and motor information from the brain, and
data about our balance from the inner ear.” She adds that some people who suffer brain injuries
resulting in disturbance to proprioceptive inputs, have sensations of disembodiment [4].
The cerebellum (Fig C) receives input from the proprioceptors that reveals what is
actually happening in the joints and muscles in real time. Nerve impulses from the equilibrium-
sensing apparatus in the inner ear and from the eyes also enter the cerebellum. If there is a
discrepancy between intended and actual movements, the cerebellum sends feedback to upper
motor neurons. As movements occur, the cerebellum continuously provides corrections to upper
motor neurons to decrease errors and smooth the motions of the muscle movements being
executed [3].
Fig. C: Lateral image of human brain showing location of Cerebellum.

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There is an especially high density and complex array of proprioceptive sensors in the
cervical spine responsible for position sense. These important sensors provide a vital ability to
plan and execute effective and purposeful movements. Sensory information from the neck
receptors is processed in tandem with information from the vestibular system of the inner ear. B.
Armstrong, with the Health and Rehabilitation Research Centre at Auckland University of
Technology, found that if neck proprioceptive information is inaccurate or fails to be
appropriately integrated with vestibular inputs, then errors in head position may occur, resulting
in an inaccurate reference for head and neck position senses. He found evidence that
impairments in position sense are observed in individuals who have experienced whiplash-type
injuries and individuals with chronic head and neck pain of non-traumatic origins. It is also
documented to be prevalent among athletes exposed to recurrent impact trauma or repetitive
forces resulting in premature degenerative changes [5].
While each leg of the vestibular stool is capable of compensating if one of the other legs
is compromised (such as decreased vision in low light, a neck injury, or impaired balance due to
an inner ear infection), it is when all three legs of the system are in coordination with each other
that we are able to maintain posture, navigate in our surroundings, coordinate smooth motion of
body parts, modulate fine motor control and take carefree walks on the beach in loose sand at sun
set [1].
While the vestibular system was not directly covered in Dr. Mason’s neurobiology
course, her in-depth analysis of how the brain processes sensory information and sends out motor
commands, as well as her detailed instruction on neural anatomy has allowed me to research and
understand the amazing system that allows us to experience equilibrium and not fall down.

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References
1. Stack, B., and A. Sims. "The Relationship between Posture and Equilibrium and the Auriculotemporal Nerve in
Patients with Disturbed Gait and Balance."Cranio 27.4 (2009): 248-60. Web.
2. Zenner, H. P., and A. W. Gummer. "The Vestibular System." (n.d.): 697-707. Print.
3. Tortora, Gerard J., and Bryan Derrickson. "Chapter 16 Sensation." Principles of Anatomy & Physiology. 13th ed.
Hoboken, NJ: Wiley, 2012. 609+. Print.
4. Carter, Rita, and Christopher D. Frith. "A World of One's Own." Mapping the Mind. Berkeley: U of California,
1999. 115. Print.
5. Armstrong, Bridget, Peter Mcnair, and Denise Taylor. "Head and Neck Position Sense." Sports Medicine 38.2
(2008): 101-17. Web.