The Physiology of Equilibrium

Equilibrium (balance) is mainly based around the function of the vestibular system of the inner ear. Sight and proprioception also play a lesser role in balance.

NOTE: Any two of the 3 methods (vestibular, sight, and proprioception) can be intact for full balance, but when 2 out of 3 go, they're going down! 

The Vestibular System

The vestibular system includes the

• semicircular canals, which respond to rotational movements or angular acceleration; and the
• utricle and saccule, which respond to changes in the position of the head with respect to gravity or linear acceleration.

The utricle and saccule are called the otolithic organs, informing the brain about head location when not moving, and the semicircular canals update the brain during physical movement.

SEMICIRCULAR CANALS: The semicircular canals (SCC) respond to rotational movements and angular acceleration. There are three semicircular canals within each ear. One is oriented in each cardinal direction and is labeled:

1. superior,
2. inferior, and
3. horizontal.

Each contains ENDOLYMPH, a fluid similar to the cerebrospinal fluid that pushes and pulls against a collection of fluid and gelatinous material--the ampullary cupula; thus, it is a gelatin-like bulb that translates movement into neural impulses.

UTRICLE: responds to linear accelerations and head tilting in the horizontal plane (head to shoulder).

SACCULE: responds to linear accelerations and head tilting in the vertical plane (up and down).

These two organs use the pull of gravity on a gelatin-like substance to detect movement, as opposed to the semicircular canals that rely on the movement of fluid.

Changes in one direction vs. another will be present as an algebraic sum to the brain, which is transmitted to the ocular motor cortex so that the eyes move smoothly with head turns to keep an item of intention in stable view.