Laser interferometric measurements on the dynamic behaviour of the cupula in the fish lateral line.

The motion of the cupula in the supra-orbital lateral line canal of the ruff in response to sinusoidal water movements was measured in vivo by means of a novel laser interferometer microscope. The interferometer utilized the laser light scattered back from a small bead (phi approximately equal to 1 micron) which was positioned under visual control on a selected location inside the cupula. The displacement of the cupula was measured in the range of 0.3 to 300 nm. The stimulus-response relation is found to be linear in this range. The frequency response of the cupula measured at the top is identical to that measured at the base near the sensory hair bundles. From this result it is concluded that the cupula is sliding over the sensory epithelium during motion in the physiological displacement range. A hydrodynamical model of cupular motion which incorporates a frequency dependent boundary layer around the cupula is presented. The model accurately describes the experimental data.