PSY 873 Exam (Dr. Albery)

17 May 1995

Kaz Shimada, M.D.

Aerospace Medicine, WSU

#1            Semicircular canal sensory structure

            In man, the sensor for angular acceleration is located in the inner ear.  It is functionally an organ which senses inertial behavior of fluid.  There is an automobile application of this principle for two-dimensional acceleration sensor, which uses gas as fluid.

            There are two sets of semicircular canals in humans, one on each side of the skull.  One set is internally connected by lymphatic ducts.  Anterior and posterior semicircular ducts share a crus.

            The lumen of each canal has a mean diameter of 0.3 mm in humans.  The horizontal and posterior canals have a radius of curvature of 1.6 - 1.9 mm, the anterior canal has 2.2 mm.  There is an ampulla in each canals, which contains a gelatinous, elastic structure called cupula.

             The horizontal canal lies parallel to the horizon when a man pitches down head 30 deg.  In cats, horizontal canals are parallel tot the horizon when they are in a natural position.  The three canals in one side are spatially oriented generally orthogonally, but there are significant deviation from normal relationship.  The angle between the horizontal canal and the anterior canal is 111.76 deg; between the horizontal and the posterior 95.75 deg.

            Based on the neural firing pattern, there are three sets of Òcomplementary canals.Ó  For example, when a man is turned around body Z-axis, clockwise seen from the top, there will be an ampulopetal deflection of cupula in the right side, and primary afferent fiber will be stimulated.  However, again, there are not completely aligned in the same plane.  Left anterior-right posterior canals are off plane by 24 deg; left horizontal-right horizontal 20 deg.

            The semicircular canal system is embedded in the osseous labyrinth.  The functional part of the duct systems is the endolymphatic part.  Although cross-section of perilymph duct is ten times larger than that of endolymph duct in man, circulation of endolymph in the endolymphatic duct is the main contributor tot he canalÕs mechanical response to angular acceleration.  There are evidences that the expected maximum extent of cupular movement would remain on the order of one degree, even a man is submitted to 500 deg/sec stimulus.

            There are type I and II hair cells as sensory cells in the crista in the ampulla.  Even though the displacement of cupula is small, the shearing movement of the cilia, produced by the deflection of the cupula, will alter the resting potential and leads to either depolarization or hyperpolarization of the hair cells.

            The time constant of onset of deflection of cupula is 3-5 ms.  The time constant for its recovery is about 3.8 sec (monkey).  Human oculomotor response restore time is more than three times longer due to CNS response delay.  With repeated exposure, the recovery time becomes close to cupula response (about 7 sec).  In the stimulus frequency range of 0.1 - 5 Hz, gain and phase response is quite flat.

Response to 30 sec angular acceleration

            In addition to the short onset time (3-5 ms) of cupula, there is a longer time constant for low-frequency response.  Its value is about 10 sec for a man.  After 2-3 times of this length, i.e. 30-40 sec, the cupula would have deviated through an angle closely proportional to the imposed acceleration.  Thus, it is a stimulus that will cause a response for a steady, maintained cupula deviation.

            When a subject is decelerated, the inertia of the endolymph displaces the cupula in a direction opposite to that in which it was displaced during the acceleration.  During the first phase of deceleration (30 sec), the subject will continue to show nystagmus to the same direction; then the direction reverses (as does sense of rotation direction).  It can take up to several minutes before the behavioral effects are dissipated.