Resonance of spike discharge modulation in neurons of the guinea pig medial vestibular nucleus

The modulation of action potential discharge rates is an important aspect o f neuronal information processing. In these experiments, we have attempted to determine how effectively spike discharge modulation reflects changes in the membrane potential in central vestibular neurons. We have measured how their spike discharge rate was modulated by various current inputs to obta in neuronal transfer functions. Differences in the modulation of spiking ra tes were observed between neurons with a single, prominent after hyperpolar ization (AHP, type A neurons) and cells with more complex AHPs (type B neur ons). The spike discharge modulation amplitudes increased with the frequenc y of the current stimulus, which was quantitatively described by a neuronal model that showed a resonance peak > 10 Hz. Modeling of the resonance peak required two putative potassium conductances whose properties had to be ma rkedly dependent on the level of the membrane potential. At low frequencies (less than or equal to0.4 Hz), the gain or magnitude functions of type A a nd B discharge rates were similar relative to the current input. However, r esting input resistances obtained from the ratio of the membrane potential and current were lower in type B compared with type A cells, presumably due to a higher level of active potassium conductances at rest. The lower inpu t resistance of type B neurons was compensated by a twofold greater sensiti vity of their firing rate to changes in membrane potential, which suggests that synaptic inputs on their dendritic processes would be more efficacious . This increased sensitivity is also reflected in a greater ability of type B neurons to synchronize with low-amplitude sinusoidal current inputs, and in addition, their responses to steep slope ramp stimulation are enhanced over the more linear behavior of type A neurons. This behavior suggests tha t the type B MVNn are moderately tuned active filters that promote high-fre quency responses and that type A neurons are like low-pass filters that are well suited for the resting tonic activity of the vestibular system. Howev er, the more sensitive and phasic type B neurons contribute to both low- an d high-frequency control as well as signal detection and would amplify the contribution of both irregular and regular primary afferents at high freque ncies.