This research will investigate the mechanisms of synaptic integration in the lateral giant (LG) command neurons that trigger the escape tailflip of both developing and adult crayfish. Two questions will be addressed. The first asks how the LG's responses to mechanosensory inputs depend on the responses of tonic and phasic mechanosensory interneurons (MSIs) to tactile stimuli, the pattern of synaptic contact of individual MSIs on LG, the morphology of LG, and the passive and active membrane properties of LG. This question will be addressed by recording the responses of two identified MSIs, one phasic and one tonic, to mechanosensory stimuli that excite LG, by identifying possible MSI contact sites on LG and by recording LG's responses to them individually and in various patterns. The structural, biophysical and synaptic response data will be used to reconstruct the integrative properties of LG in a quantitative model that will acconut for the cell's responses to its normal patterns of input. The second question asks how the integrative properties of LG change during the animal's growth from post-natal fry to adult, and whether those changes and accompanying changes in synaptic inputs to LG can account for changes in tailflip behavior. We will analyse LG integrative properties in 2 cm, 6 cm and 12 cm animals in the manner described above, and develop quantitative models of them to determine how growth - related changes in cell shape, membrane properties and synaptic inputs should affect LG. We will also determine what changes occur in the number and pattern of inputs LG receives from identified MSIs and from the set of mechanosensory afferents. This study should identify the compensatory mechanisms that allow the labile disynaptic pathway to LG to remain viable during growth while the non- labile monosynaptic pathway fails. Since neuronal growth occurs in all nervous systems, both the changes in integrative properties during growth that we observe and the mechanisms that compensate for those changes may prove to be common features of developing nervous systems.
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