A nonrandom dynamic component in the synaptic noise of a central neuron.

Continuous segments of synaptic noise were recorded in vivo from teleost Mauthner cells and were studied with the methods of nonlinear analysis. As in many central neurons, this ongoing activity is dominated by consecutive inhibitory postsynaptic potentials. Recurrence plots and first or third order Poincaré maps combined with surrogate shuffling revealed nonrandom patterns consistent with the notion that synaptic noise is a continuously varying mixture of periodic and chaotic phases. Chaos was further demonstrated by the occurrence of unstable periodic orbits. The nonrandom component of the noise is reproducibly and persistently reduced when the level of background sound, a natural stimulus for networks afferent to the Mauthner cell, is briefly elevated. These data are consistent with a model involving a reciprocally connected inhibitory network, presynaptic to the Mauthner cell and its intrinsic properties. The presence of chaos in the inhibitory synaptic noise that regulates the excitability of the Mauthner cell and its sensitivity to external stimuli suggests that it modulates this neuron's function, namely to trigger a fast escape motor reaction following unexpected sensory information.