The semicircular canals are small membranous tubes with an inner diameter
of 0.4 mm and an outer diameter of 6.5 mm. The horizontal canal has a
backward tilt of 19.9° when the head is in an upright position with the nasaloccipital
axis, the superior and posterior semicircular canal make an angle of
94° (Della Santina et al. 2005).
The semicircular canals register angular accelerations during head
movements in 3D (Breuer 1874; Egmond et al. 1949). The capability of the
semicircular canals to detect angular accelerations is due to the small
diameter of the canals and the viscosity of the endolymph fluid within the
canals.
The hair cells in the sensory epithelium of the canals are positioned on a
crest-like septum, the crista, which is positioned perpendicular to the
longitudinal axis of the canal in an enlarged part of the canal, the ampulla.
The hair cells consist of multiple stereocilia and one kinocilium protruding into
General introduction
11
a gelatinous mass, the cupula, which reaches onto the roof of the canal
(Ramprashad et al. 1984; Mc Laren and Hillman 1979). When there is a
sudden head displacement the motion of endolymph will lag behind. This
leads to a deflection of stereocilia and the kinocilium located on the apical
side of each hair cell (Breuer 1874; Rasmussen and Windle 1960) (Figure 3).
The stereocilia are very sensitive: a deflection of one Angstrom (1 Å = 10-10
m) can be detected. Deflection of the stereocilia leads to a cascade of cellular
events causing receptor potentials at the base of the cell. The larger the
deflection of the stereocilia, the larger the change in membrane potential.