Physiological responses of frog vestibular fibers to horizontal angular rotation
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Single neuronal discharges in frog's vestibular nerve were recorded in unanesthetized preparations with glass microelectrodes. The nerve fibers supplying the horizontal semicircular canal are divided into two types according to the characteristics of their frequency responses to natural stimulation of the horizontal canal. The afferent fibers increase their firing rate only on ipsilateral rotation and cease to fire on contralateral acceleration. The efferent fibers usually increase their frequencies on rotation in either direction or show an increase in firing on contralateral rotation only. The thresholds of efferent fibers are generally higher as compared to afferent fibers. In addition, most of them show multisensory convergence.
Of the afferent fibers 65% showed frequency adaptation in response to prolonged acceleration steps whereas 35% did not show any sign of frequency decrease on prolonged stimulation.
Thirty out of 49 afferent units showed a non-linear relation between frequency increase and angular acceleration; in 19 units an approximately linear relationship was noted. In both types of responses the thresholds for frequency increase were in the range between 0.3–2.5°/sec2.
The time constants of the majority of fibers measured in the linear range were about 3 sec with a range between 1 and 10 sec. It is suggested that fibers having short time constants to acceleration and velocity step input represent acceleration-sensitive units whereas those having long time constants monitor angular velocity. Apparent ‘time constants’ were adopted for the non-linear range of non-linear units. These values decreased as the acceleration rate increased.
An approach to vestibular modeling, based on the present experimental results, is described in the Appendix.
Key WordsVestibular afferents Vestibular efferents Response to rotation Frog vestibular modeling
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