Computer model of passive signal integration based on whole-cell in vitro studies of rat lateral geniculate nucleus

Computer models were used to investigate passive properties of lateral geniculate nucleus thalamocortical cells and thalamic interneurons based on in vitro whole-cell study. Two neurons of each type were characterized physiologically and morphologically. Thalamocortical cells transmitted 37% of steady-state signal orthodromically (distal dendrite to soma) and 93% antidromically (soma to distal dendrite); interneurons transmitted 18% orthodromically and 53% antidromically. Lowering membrane resistance caused a dramatic drop in steady-state signal transmission. Simulation of brief signals such as orthodromically transmitted postsynaptic potentials and antidromically transmitted action potentials showed relatively poor transmission due to the low-pass filtering property of dendrites. This attenuation was particularly pronounced in interneurons. By contrast, bursts of postsynaptic potentials or action potentials were relatively well transmitted as the temporal summation of these recurring signals gave prolonged depolarizations comparable to prolonged current injection. While synaptic clustering, active channels and reduction of membrane resistance by ongoing synaptic activity will have additional profound effects in vivo, the present in vitro modelling suggests that passive signal transmission in neurons will depend on type of signal conveyed, on directionality and on membrane state. This will be particularly important for thalamic interneurons, whose presynaptic dendrites may either work independently or function in concert with each other and with the soma. Our findings suggest that bursts may be particularly well transmitted along dendrites, allowing firing format to alter the functional anatomy of the cell.