Study of signal propagation in a motoneuron based on a three dimensional detailed reconstruction

SERGIO DAICZ Y L. SZCZUPAK

Washington

Congreso; Annual Meeting of the Society for Neuroscience; 2006

The morphology of a neuron and the spatial distribution of its active conductances and synaptic inputs are fundamental components in determining how its input signals will be integrated to produce an output, and which of these interactions can be detected in somatic recordings. An example of this are the motor neurons of the leech Hirudo Medicinalis. Somatic recordings of their action potentials are only a few millivolts in amplitude, suggesting that they propagate passively from a spike initiation zone electrotonically distant from the soma. In our lab we are studying a circuit that combines electrical and chemical synapses that may serve to modulate the coordination of motor activity. In order to better interpret our observations and understand these interactions, it will be useful to have precise information about the morphology of motor neurons and hence of their electrotonic structure. To obtain a detailed reconstruction of AE (annulus erector) motor neurons we loaded them by iontophoresis with Tetramethylrhodamine-dextran and imaged the preparations at high resolution (0.23 x 0.23 x 0.24 µm) using a confocal laser scanning microscope with a 60X, 1.4 NA objective. Due to the reduced field of view at this magnification, each neuron had to be imaged in several separate volumes. The relative position of the volumes was later determined using the VIAS software. To obtain a detailed morphological model we used the semiautomatic skeletonization tool described in Evers et al. (J Neurophysiol 93: 2331-2342, 2005); the diameter of the neurites was sampled every 0.5 µm, resulting in about 17,000 compartments. This model was then reduced to one with 300 compartments, more suitable for computational simulations. Our initial simulations, carried out using the GENESIS neural simulation system, show that a purely passive propagation can account for most, but not all, of the observed attenuation of the action potentials. We are now analyzing how addition of different conductances contributes to the experimentally observed attenuation.Hirudo Medicinalis. Somatic recordings of their action potentials are only a few millivolts in amplitude, suggesting that they propagate passively from a spike initiation zone electrotonically distant from the soma. In our lab we are studying a circuit that combines electrical and chemical synapses that may serve to modulate the coordination of motor activity. In order to better interpret our observations and understand these interactions, it will be useful to have precise information about the morphology of motor neurons and hence of their electrotonic structure. To obtain a detailed reconstruction of AE (annulus erector) motor neurons we loaded them by iontophoresis with Tetramethylrhodamine-dextran and imaged the preparations at high resolution (0.23 x 0.23 x 0.24 µm) using a confocal laser scanning microscope with a 60X, 1.4 NA objective. Due to the reduced field of view at this magnification, each neuron had to be imaged in several separate volumes. The relative position of the volumes was later determined using the VIAS software. To obtain a detailed morphological model we used the semiautomatic skeletonization tool described in Evers et al. (J Neurophysiol 93: 2331-2342, 2005); the diameter of the neurites was sampled every 0.5 µm, resulting in about 17,000 compartments. This model was then reduced to one with 300 compartments, more suitable for computational simulations. Our initial simulations, carried out using the GENESIS neural simulation system, show that a purely passive propagation can account for most, but not all, of the observed attenuation of the action potentials. We are now analyzing how addition of different conductances contributes to the experimentally observed attenuation.