THE VESTIBULAR SYSTEM
The vestibular system monitors the position and movement of the head, gives us a sense of
balance and equilibrium, and helps coordinate movements of the head and eyes.
The Vestibular Labyrinth
The vestibular and auditory systems both use hair cells to transduce movements. In mammals,
all hair cells are contained within sets of interconnected chambers called the vestibular
labyrinth (See Fig. 11.28) which include two types of structures with different structures:
Otolith organs – detects force of gravity and tilts of the head
o Contain the saccule and the utricle
o Located near the center of the labyrinth
Semicircular canals – sensitive to head rotation
o Three arcing structures
o Lie in approximately orthogonal planes
Ultimately, these structures transmit mechanical energy, derived from head movement, to its
The saccule and utricle detect changes of head angle as well as linear acceleration of the head.
Each otolith organ contains a sensory epithelium called a macula.
Vertically oriented within the saccule and horizontally oriented within the utricle
The vestibular macula contains hair cells (see Fig.11.29). Movements are transduced when the
hair bundles are deflected.
The otolith organs contain tiny crystals of calcium carbonate called otoliths which
encrust the surface of the macula’s gelatinous cap. This is key to the tilt sensitivity of the
When the head accelerates, a force is exerted on the otoliths. This in turn exerts a force in the
same direction on the gelatinous cap, which moves slightly, and the cilia of the hair cells bend.
Each hair cell has one especially tall cilium called the kinocillium.
The bending of hairs toward the kinocillium results in depolarizing, excitatory potential
The bending of hairs in the other direction will lead to hyperpolarization and inhibition.
If the hairs are perpendicular to the preferred direction, there is barely any response
The response saturates when the hairs are bent less than 0.5 µm.
The responses of hair cells are, therefore, direction selective. A given head movement excites
hair cells on one side while inhibiting hair cells in the corresponding location on the other side.
Thus, any tilt or acceleration of the head will excite some hair cells, inhibit others, and have no
effect on the rest.
Semicircular organs detect turning movements of the head and also sense acceleration but of a
different nature. Angular acceleration is generated by sudden rotational movements (e.g.
spinning in circles). The hair cells of the semicircular canals are clustered within a sheet of cells
called the crista which are located within a bulge along the canal called the ampulla. Hair cells of the ampulla have their kinocillia oriented in the same direction. Therefore,
they are all excited or