Afferent activity recorded during rotation from single fibres of the posterior nerve in the isolated frog labyrinth

Summary

EPSPs and spikes were recorded at rest and during rotation from single fibres of the posterior nerve in the isolated frog labyrinth. The spike discharge properties of 57 units were examined at rest and during repetitive acceleratory-velocity steps. Forty of these units were subjected to excitatory steps of 5–12 s duration and 45% displayed an evident discharge adaptation. In the non-adapting units, the excitatory response also deviated from that expected on the basis of the torsion-pendulum model and exhibited an exponential time-course in only 36% of the fibres examined. The time constant T₂ of the response rising phase was significantly longer than that of the decay (2.5 s versus 1.7 s). When all the 57 units were considered, a linear behaviour was found in 67%. The average gain in these linear units was 1.9 ± 1.4 spikes · s⁻¹/deg · s⁻². Adaptive fibres exhibited a lower resting firing rate and a higher gain (3.8 spikes/s and 2.3 spikes · s⁻¹/deg · s⁻², respectively) when compared with the non-adapting ones (7.1 spikes/s and 1.5 spikes · s⁻¹/deg · s⁻²). An undershoot was present in 57% of the units; it increased with acceleration and was not strictly related to adaptation. Fifteen of the 40 units tested with the 5–12 s duration excitatory steps survived repeated inhibitory accelerations of the same duration. In these units a marked response asymmetry was evident since their resting activity could be abolished by accelerations not larger than 10 deg/s². In 40% of the units inhibited by acceleration the mean response was proportional to the stimulus logarithm, while the others saturated for weak stimulations. A consistent overshoot of the discharge was evident in most of the units (60%). Analysis of the EPSP emission rates demonstrates that even a 10–20% increase in their frequency during excitation results in a two-three fold increase in the corresponding spike frequency. Similarly, a decrease of 15–35% in their numbers during inhibition is sufficient to completely block the spike firing. These findings reveal the high sensitivity of the afferent synapse, spike discharge being modulated by slight modifications in the release of the excitatory transmitter.