Comparisons between Active Properties of Distal Dendritic Branches and Spines: Implications for Neuronal Computations.

The specific contributions of distal dendrites to the computational properties of cortical neurons are little understood and are completely ignored in most network simulations of higher brain functions. Compartmental models, based on realistic estimates of morphology and physiology, provide a means for exploring these contributions. We have pursued analysis of a model of synaptic integration in a distal dendrite bearing four spines, using a new general-purpose simulation program called SABER. We have analyzed this model under the assumption that the dendrite contains sites of impulse-generating membrane, and we have compared its responses to synaptic activation with the case of impulse-generating membrane located instead in the spine heads, as previously reported. Both types of models generate basic logic operations, such as AND, OR, and AND-NOT gates. Active spine heads require lower excitatory synaptic conductances, but active branch segments lead to larger responses in the soma. The transients recorded near the soma give no evidence of their origin in either active branch or active spines, indicating that the interpretation of experimental recordings with regard to sites of distal active responses must be viewed with caution. The results suggest the hypothesis that a hierarchy of logic operations is virtually inherent in the branching structure of dendritic trees of cortical pyramidal neurons. Inclusion of these properties in representations of cortical neurons would greatly enhance the computational power of neural networks aimed at simulating higher brain functions.