FoundationalConceptsNeuroscience

of three orthogonal (spatially perpendicular, at 90-degree angles to
one another) semicircular canals, together with two bulbous cavities
called the utricle and the saccule. These are the sensory structures of
the vestibular system, detecting our orientation relative to gravity
and our acceleration as we move, walk, and turn. This allows us to
maintain balance and execute smooth and coordinated movements;
without a vestibular system we would be bumbling around pretty
badly.
Unlike hearing, seeing, tasting, and smelling, the functioning of
our vestibular system is pretty much out of our conscious awareness,
except when something disrupts it—like spinning around and getting
dizzy, or vertigo. Vertigo is a condition in which one feels dizzy or in
motion even while sitting or standing still. It is generally associated
with something unusual happening in the vestibular system, for ex-
ample, infection or inflammation.
Within the utricle and saccule are receptor cells that detect the
movement of fluid in the attached semicircular canals. Because there
are three orthogonal canals, complete information can be gathered as
to how the head is oriented and accelerating in the three dimensions
of space. Changes in orientation and movement cause the fluid to
move around differently in the canals. The receptor cells are hair cells
of the same type as along the basilar membrane in the cochlea. The
moving fluid bends the hairs, and neural signals are generated and
passed into cranial nerve 8, which carries vestibular as well as audi-
tory information to the brain.
An additional feature is present in the vestibular system that
is not found in the spiral part of the cochlea. Associated with the
vestibular hair cells are tiny microscopic stones, crystals of calcium
carbonate, like little crystals of limestone. They are called otoliths: ear
stones (Greek otos = ear, lithos = stones). The otoliths are suspended