Abstract:

In the slugs and snails odor input signal, partly processed by the tentacle ganglion, propagates through the tentacle nerve (TN) to the cerebral ganglion, initially activating the meso-meta-region and finally the procerebral region (PC). The PC, equivalent to mammalian olfactory bulb, exerts slow spontaneous neuroelectrical oscillation, which changes its frequency and amplitude pattern responding to stimulus input. This has been related to a mechanism of signal processing for odor encoding. Three neuronal substructures, the cell mass (CM), the terminal mass (TM) and internal mass (IM) form the PC. Records from IM and CM have extensively been studied, but those from TM have scarcely been investigated. In the present study we aimed to clarify network dynamics among these cell ensembles with particular interest in the property of TM. Methods: We isolated the cerebral ganglia from the slug Incilaria together with TNs. We applied to TN electrical stimulation of weak to strong intensities (0.1 - 1.0 μA) and recorded activities at the three loci of PC by glass suction electrodes at a sampling rate of 200 Hz. The data were stored on hard drive and later off-line analysed by wavelet tools. Results: Wavelet analysis revealed that the major power of the spontaneous oscillations laid below 1.6 Hz. Namely, in the Incilaria PC, mainly the frequency components < 1.6 Hz take part in the dynamical signal processing. The frequency components, that are time-dependently, interacting with each other, contribute together to altering total entropy of a cell mass at a given time. Notably, the 0.1 - 0.2 Hz component contributing most strongly to total energy attributes most to dropping entropy ("ordering of neuronal state"). Response to the weakest stimulus is most sensitively elicited as "desynchronization" in TM-IM, but that to the stronger stimuli, as "synchronization or frequency ordering" in TM-CM, and finally "synchronization" in TM-IM-CM (the whole PC). The fact that the entropy of TM in general remains lower than IM and CM regardless with stimulation suggests that the neurons of TM are in more ordered state than the other masses playing some governing function in the procerebral network. © 2007 American Institute of Physics.