Changes in motoneurone electrical properties following axotomy.

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Abstract
Passive electrical properties, after potential properties and the pattern of repetitive discharge induced by constant current injection were studied in axotomized lumbar motorneurons. Following axotomy, the motoneurons showed a larger input resistance and membrane time constant, but had a normal electrotonic length. Duration and peak amplitude of the afterhyperpolarization AHP were on average unchanged following axotomy. There was a significant reduction in the conductance underlying the AHP. The distribution of values for AHP duration was also narrower following axotomy, with an absence of long and short values. As in normal motoneurons, the AHP conductance, calculated from the voltage, decayed in an approximately exponential manner with a phase of slower decay corresponding to the hyperpolarizing phase of the AHP. The phase of slower decay was less accentuated and several axotomized motoneurons showed an exponential decay of the AHP conductance. The frequency-current (f-I) curves for the 1st interspike intervals were, as in normal motoneurons, non-linear, deviating upwards at higher frequencies. The steady-state f-I relations were linear in most of the axotomized neurons. The slopes of the f-I curves were steeper following axotomy. These steeper slopes were well correlated with the decreased AHP conductance. The interspike voltage trajectories were similar to those in normal motoneurons i.e., concave at low current strength and changing to a convex shape with increasing current injection. The changes in the trajectory shape were not correlated with the changes in the slope of the f-I curves. The after hyperpolarization conductance is the major factor in the regulation of repetitive firing in axotomized motoneurons.