A mathematical model of neural information processing at the cellular level

Computational modeling and experimental validation of odor detection behaviors of classically conditioned parasitic wasp, Microplitis croceipes

A prototype chemical sensor named Wasp hound® that utilizes five classically conditioned parasitoid wasps, Microplitis croceipes (Cresson) (Hymenoptera: Braconidae), to detect volatile odors was successfully implemented in a previous study. To improve the odor-detecting ability of Wasp Hound®, searching behaviors of an individual wasp in a confined area are studied and modeled through stochastic differential equations in this paper. The wasps are conditioned to 20 mg of coffee when associated with food and subsequently, tested to 5, 10, 20, and 40 mg of coffee. A stochastic model is developed and validated based on three positive behavioral responses (walking, rotation around odor source, and self-rotation) from conditioned wasps at four different test dosages. The model is capable to reproducing the behaviors of conditioned wasps, and can be used to improve the ability of Wasp Hound® to assess changes in odor concentration. The model simulation results show the behaviors of conditioned wasps are significantly different when tested at different coffee dosages. We conjecture that the searching behaviors of conditioned wasps are based on the temporal and spatial neuron activity of olfactory receptor neurons and glomeruli, which are strongly correlated to the training dosages. The overall results demonstrate the utility of mathematical models for interpreting experimental observations, gaining novel insights into the dynamic behavior of classically conditioned wasps, as well as broadening the practical uses of Wasp Hound.

Neurons evaluate both the amplitude and the meaning of signals

The neuronal threshold, which can be determined by the level of depolarization immediately prior to spike generation, is different for responses to conditioned and discriminated stimuli after conditioning. However, it is impossible to determine excitability within a response to stimuli that failed to generate a spike. In the present study we examined the role of the AP threshold of two related Helix defensive neurons in the initiation of an AP during elaboration of the neuronal analog of a classical conditional reflex. These neurons fire in a synchronous manner though spikes sometimes are not generated simultaneously. We compared the change in spike threshold within the responses with other parameters of intracellular activity that also affect the neuronal response, but which can be measured without an analysis of the spike waveform (spike latency, slope of excitatory postsynaptic potentials, etc.). We found that the change in slope is not the reason for the change in the threshold during pairing. The thresholds within conditioned and discriminated responses affected spike generation and their values correlated with the spike presence or absence in related neurons. The excitability changed transiently within the responses, since these alterations were selective for the conditioned and discriminated stimuli and applied primarily to the first spike of the response. The firing threshold seems to be a dynamic property of excitable membrane. Neurons appear to evaluate the significance of the input signal, transiently change their own excitability and only after that compare the magnitude of this signal and threshold.