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J Neurophysiol (April 4, 2007). doi:10.1152/jn.00084.2007
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Submitted on January 25, 2007
Accepted on April 2, 2007

Synaptic Amplification versus Bistablility in Motoneuron Dendritic Processing: A Top-down Modeling Approach

Nicholas Pabon Shapiro1 and Robert Lee2*

1 Biomedical Engineering, Georgia Institute of Technology, Altanta, Georgia, United States
2 Biomedical Engineering, Georgia Institute of Technology, Altanta, Georgia, United States; Dept Biomed Engineering, Emory Univ, Atlanta, Georgia, United States

* To whom correspondence should be addressed. E-mail: rlee2{at}emory.edu.

Motoneurons have been shown to exhibit both bistable firing and synaptic amplification. Both of these behaviors have generally been attributed to a single mechanism, namely, dendritic plateau potentials based on L-type Ca2+ conductances. However, our recent discovery of a fast amplification-mode (Jones and Lee 2006) calls this into question. Here we examine the possibility that two mechanisms underlie these behaviors; one being a slow-mode bistability mechanism (i.e. the L-type Ca2+ conductance-based dendritic plateaus), and the other being a theoretical fast-mode amplification mechanism. A "top-down" motoneuron model that encapsulated these and other hypotheses was developed in which these mechanisms could be explored. The resulting final model simultaneously exhibits synaptic amplification, plateau potential formation, bistable firing patterns, current-voltage (I-V) and frequency-current (F-I) hystereses. This model suggests that amplification and plateaus are mutually exclusive in the same dendrite/dendritic branch. Thus, we predict that plateau generation does not occur in all dendritic branches. This could be readily accomplished by a large degree of variation in the density of L-type Ca2+ channels believed to underlie plateau formation in these cells with the added benefit of spreading plateau onset over a wider voltage range, as is observed experimentally.




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