Response dynamics of first order neurons in the isolated frog labyrinth

The sensory discharge properties of the posterior semicircular canal were examined at rest and during rotation in the isolated frog labyrinth by recording intracellularly the EPSPs and spikes from single nerve fibres. The preparation was mounted on a turn-table and the canal was stimulated by acceleratory steps (0.2 - 64 deg/sec2) of opposite polarity and increasing duration (1.25 - 12 sec), or by velocity steps produced by a sudden arrest of the table after periods (10 - 30 sec) of clockwise and counterclockwise rotation at constant velocity (1.2 - 130 deg/sec). 45 units were subjected to repetitive excitatory stimuli and the following parameters were evaluated: 1) Threshold for frequency increase (0.2 - 2 deg/sec2); 2) Latency of the response (1-2 msec - 1 sec); 3) Relationship between frequency increase and acceleration (32 linear; 13 log); 4) Gain of the linear uníts {1.97 +/- 0.79 spikes.sec-1/deg.sec-2 (M +/- SD)}; 5) Adaptation (apparent in 20 units); 6) Undershoot (present in 18 units); 7) Time constant for frequency increase during single acceleratory steps (which was found to be exponential, as predicted by the torsion pendulum model, only in 8 out of the 25 non-adapting units) {2.51 +/- 0.75 sec (M +/- SD ); 8) Time constant for frequency decrease at the end of both acceleratory and velocity steps {1.88 +/- 0.89 sec (M +/- SD), calculated in 32 units}. As regards inhibition, the relationship between frequency decrease and acceleration was investigated in 15 out of the 45 units. In 9 of these units the resting discharge was cancelled even by acceleratory steps of slight intensity, so that saturation of the effect was rapidly achieved. 7 of these uníts were linear and 2 were non-linear, when the excitatory response was considered. In the remaining 6 units, the response was related to the logarithm of acceleration, and, as concerns excitation, 3 were linear and 3 were non-linear. 12 out of these 15 units exhibited a clear-cut overshoot of the discharge at the end of the ínhibítory stimulation. In conclusìon, it is evident that the afferent discharge dìsplayed striking non-linearities with respect to the predictions of the torsion pendulum model: 1) Asimmetry between excitatory and ínhibitory response; 2) Non-linearity of the response for stimuli of increasing intensity; 3) Adaptation during long-lasting acceleratory stimulí; 4) Presence of undershoot and overshoot; 5) Deviation of the response time-course from the exponential. The observed non-linearities are most likely related to both the mechano-electrical transductíon mechanism at the receptor level and the properties of the afferent synapse. It is still unknown whether the efferent system may play a role in these effects.