Relations among passive electrical properties of lumbar alpha‐motoneurones of the cat.

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Abstract
The relations among passive membrane properties were examined in cat motoneurons using exclusively electrophysiological techniques. A significant relation existed between the input resistance and the membrane time constant. The estimated electrotonic length showed no evident tendency to vary with input resistance but did show a tendency to decrease with increasing time constant. Detailed analysis of this trend suggests that a variation in dendritic geometry is likely to exist among cat motoneurons, such that the dendritic trees of motoneurons projecting to fast-twitch muscle units are relatively more expansive than those of motoneurons projecting to slow-twitch units. Both membrane time constant (and thus likely specific membrane resistivity) and electrotonic length showed little tendency to vary with surface area. After-hyperpolarization (a.h.p.) duration showed some tendency to vary such that cells with brief a.h.p. duration were, on average, larger than those with longer a.h.p. durations. Apart from motoneurons with the lowest values, axonal conduction velocity was only weakly related to variations in estimated surface area. Input resistance and membrane time constant varied systematically with the a.h.p. duration. The major part of the increase in input resistance with a.h.p. duration was related to an increase in membrane resistivity and a variation in dendritic geometry rather than to differences in surface area among the motoneurons. The voltage response of motoneurons to a constant-current step reaches a peak and decays thereafter to a lower steady-state value. The extent of this decay varied systematically with a.h.p. duration such that cells possessing brief a.h.p. durations exhibited the largest effect. Apparently, the motoneuron pool overall is not organized such that the input resistance variation among motoneurons projecting to different types of muscle units is determined by motoneuron size or size-related factors. Apparently, the input resistance variation is determined by specific membrane resistivity and resistivity-related factors. Within a motoneuron type, size may be a major factor determining the variation in input resistance.