Role of medullary networks and postsynaptic membrane properties in regulating Mauthner cell responsiveness to sensory excitation.

A benefit of studying well-defined networks at a cellular level is that it might be possible both to place these details in the context of the specific function of the network and to extract general principles applicable to more complex systems. The Mauthner cell system in teleosts is one such vertebrate network where a single impulse can trigger a vital escape reaction, the C start, in response to auditory or visual stimuli. We review here experiments concerned with the organization, at the cellular level, of the afferent circuits impinging on the Mauthner cell and with certain intrinsic membrane properties of the Mauthner cell that contribute to shaping the threshold and expression of the C start. One concept that emerges is related to the interaction between excitatory and inhibitory drives to the Mauthner cell. It seems that every major afferent drive to this neuron also excites a feedforward inhibitory network which, in turn, exerts a major role in establishing and regulating the threshold of the escape response. This design feature is complemented by the Mauthner cell's membrane properties which contribute to the behavioral threshold but exhibit nonlinearities, as excitation begins to overcome inhibition. Finally, we have compared in detail the frequency-dependent characteristics of inhibition and excitation, as revealed by studies of individual identified synaptic connections. This comparison emphasizes the notion that although inhibition is maximized for weak transient stimuli, it becomes depressed at auditory stimulus frequencies that facilitate excitatory transmission and evoke the escape response.