Effects of axotomy on the distribution of passive electrical properties of cat motoneurones.

Previously obtained experimental results concerning the effect of axotomy on motoneurone passive electrical properties have been re-analysed. As shown earlier, axotomy causes an average increase of motoneurone input resistance, membrane time constant and after-hyperpolarization duration. The present analysis suggests that the increased input resistance is related to a higher specific membrane resistivity, a decreased cell size and an altered dendritic geometry. The results also suggest that the change takes place only in neurones projecting to fast-twitch muscle units and produces in them passive electrical properties normally exhibited only by motoneurones projecting to slow-twitch units. Based on the notion that axotomy causes a 'dedifferentiation' of motoneurone properties, the present results might be taken to indicate that undifferentiated motoneurones are slow in character. A possible scheme in which a post-natal differentiation of motoneurone properties may lead to muscle differentiation is discussed.