Essential Principles of Vestibular Physiology
Traditionally there have been five “senses,” namely vision, hearing, smell, touch, and taste. It is entirely reasonable to name balance as the true “sixth sense.” What sets balance apart from the other senses is that it is not used to actively “investigate” our surroundings in the same way the others are. Balance is automatic and subconscious until there is a disruption of the vestibular system and symptoms develop. These symptoms are often substantial.
The vestibular system has two broad functions—the maintenance of balance and the maintenance of stable gaze. The vestibular end organs comprise the otolith organs (the utricle and saccule) and the three semicircular canals (lateral, superior, and posterior). The semicircular canals (SCCs) are activated during rotational movements and the otolith organs during linear movements.
The SCCs are paired structures. While the lateral canals are paired with each other, the superior canal on the left is functionally paired with the posterior canal on the right and vice versa. Eye movements are produced in the plane of the canal being stimulated. Stimulation of the semicircular canal occurs when the cupula is deflected as a result of endolymph within the canal remaining relatively still, as a result of its inertia, as the head is moved.
The vestibulo-ocular reflex (VOR) serves to maintain the visual field in a stable fashion on an area of interest. The area of high visual acuity afforded by the fovea centralis is relatively small when compared to the entire visual field and must be kept accurately directed toward the area of interest even during head and body movements. Systems of smooth pursuit are not sufficiently fast to allow this to be undertaken voluntarily and thus the VOR is used to ensure that eye movements are produced that are equal and in an opposite direction to head movements. Defects in this reflex cause reduced dynamic visual acuity owing to the “retinal slip” caused by an image not being held consistently over the fovea.
Although there is substantial crossover between the function of these systems, the otolith organs play the greatest role in the maintenance of an upright posture through the detection of body or head tilt while the SCCs play the greatest role in the VOR.
Hair cells within the SCCs fire at a baseline rate when at rest with no head movement. When the head is moved in a rotational fashion, one of the pair of canals will increase its firing rate while the other will decrease. This differential will signal a head movement in the plane of that semicircular canal. In the case of the lateral canal, there will be an increased rate of firing of the hair cells ...