| Abstract No.: | B-B2059 |
| Country: | Canada |
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| Title: | BACKPROPAGATION AND POST SYNAPTIC POTENTIAL IN THALAMOCORTICAL CELL ARE CONTROLLED BY BRANCHING POINTS |
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| Authors/Affiliations: | 1 Reza Zomorrodi Moghaddam*; 2 Helmut Kroger; 3 Igor Timofeev;
1 Physics Department & CRULRG, Laval University, Quebec, QC, Canada; 2 Physics Department, Laval University, Quebec, QC, Canada; 3 The Centre de recherche Université Laval Robert-Giffard (CRULRG), Laval University, Quebec, QC, Canada
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| Content: | Objectives: Modeling studies have indicated that the geometry of the dendritic tree plays a key role in controlling the extent of the action potential backpropagation (AP-BP). Recent electrophysiological data distinguish attenuation in proximal from distal dendrites in thalamocortical (TC) neurons. The multi-branched structure of thalamocortical (TC) neuron dendrites allows direct investigation of the role of dendritic branch points in the control of AP-BP. The failure of action potential invasion into parts of a multi-branched dendritic tree may have important functional consequence. For example, AP-BP may invade some areas of the dendritic tree resetting synaptic integration but fail to influence activity in other dendrites.
Materials and Methods: We investigated dendritic structure of seven TC cells from adult cat in VPL nucleus and performed modeling experiments. Results: We found that TC neurons do not obey Rall’s power law. In about 50% of dendritic bifurcations, the diameter of parent branch was equal or smaller than the diameter of the daughter branch. These have been mainly found in the proximal and middle region. In the bifurcation points that obey Rall's power law, the exponent (n) in Rall’s power law has a different value for proximal (n=1.79±0.07) versus distal (n=2.24±0.08) dendrites. To determine how this specific behavior at bifurcation points in TC neuron influence forward and back propagations, we developed a multi compartment model for an artificial cell with 22 bifurcation points obeying Rall’s power law with different exponents numbers; n=1.2, 1.5, 2. The model of the TC neuron contained fast Na+/K+ currents inserted in the soma, T-current and h-current distributed in the dendrites with higher density in proximal dendrites. Since, the input resistance of the cell was decreasing with increase in Rall’s power from 1.2 to 2, we changed the leak conductance to obtain the same input resistance for adequate comparison of results. Simulation results on the artificial cell show the following behavior: attenuation of AP-BP and somatic response to a distal synaptic input has inverse and direct relation to the exponential number in the Rall’s law, respectively. These results suggest that independent of channel type and density, different attenuation of AP-BP is related to the Rall’s law at bifurcation points. Our simulation for a reconstructed TC neuron shows: (a) There are different attenuation factors for proximal and distal dendrites in agreement with experimental data. (b) According to a bigger value of the exponent number in Rall’s law in distal dendrites than in proximal ones, a generated postsynaptic potential at distal dendrite can reach the soma with little attenuation in amplitude. We conclude that inputs arriving at distal dendrites of TC cell would have stronger influence than it was previously thought.
Supported by NSERC.
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