Abstract
Antidromic action potentials following distal stimulation of motor axons occasionally fail to invade the soma of alpha motoneurons in spinal cord, due to their passing through regions of high non-uniformity. Morphologically detailed conductance-based models of cat spinal alpha motoneurons have been developed, with the aim to reproduce and clarify some aspects of the electrophysiological behavior of the antidromic axon-somatic spike propagation. Fourteen 3D morphologically detailed somata and dendrites of cat spinal alpha motoneurons have been imported from an open-access web-based database of neuronal morphologies, NeuroMorpho.org, and instantiated in neurocomputational models. An axon hillock, an axonal initial segment and a myelinated axon are added to each model. By sweeping the diameter of the axonal initial segment (AIS) and the axon hillock, as well as the maximal conductances of sodium channels at the AIS and at the soma, the developed models are able to show the relationships between different geometric and electrophysiological configurations and the voltage attenuation of the antidromically traveling wave. In particular, a greater than usually admitted sodium conductance at AIS is necessary and sufficient to overcome the dramatic voltage attenuation occurring during antidromic spike propagation both at the myelinated axon-AIS and at the AIS-soma transitions.
Highlights
Since the early Fifties of the last century, microelectrode intracellular recordings from spinal motoneurons allowed the detailed analysis of the electrophysiological events generated at the soma by an antidromic spike evoked at the distal part of a motor fiber (Brock et al, 1953; Eccles, 1955)
GEOMETRIC PROPERTIES OF THE MODELS The morphologically detailed models are made up of three parts: (a) the soma and a highly detailed three-dimensional dendritic tree, (b) a frustum-shaped axon hillock with the proximal enlarged end attached to the soma, and the opposite end attached to the unmyelinated axonal initial segment (AIS), (c) a myelinated axon (23 mm long) attached to the distal end of the AIS
As the reconstructed 3D morphologies are limited to the somatodendritic part of the spinal motoneurons, we modeled the efferent pathway based on average measures of cat spinal motoneurons (Conradi, 1969; Burke et al, 1977; Cullheim and Kellerth, 1978; Ulfhake and Kellerth, 1982, 1984; Cullheim et al, 1987)
Summary
Since the early Fifties of the last century, microelectrode intracellular recordings from spinal motoneurons allowed the detailed analysis of the electrophysiological events generated at the soma by an antidromic spike evoked at the distal part of a motor fiber (Brock et al, 1953; Eccles, 1955). This technique, compared to the extracellular recordings which usually collected potentials from a population of neurons, provides much of the most direct information on antidromic invasion of single motoneurons. The magnitude of the delay at the AIS to soma transition, or the presence of a complete block at this point depends on morphologic and electrophysiological (mainly the Na+ conductance) properties at the transition region (Goldstein and Rall, 1974)
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