The "fastness" of rat motoneurones: time-course of afterhyperpolarization in relation to axonal conduction velocity and muscle unit contractile speed.

In normal adult rats, intracellular recordings were obtained from motoneurones of the medial gastrocnemius (MG) and other tibial-nerve muscle branches. Among the whole population of tibial motoneurones, there was a significant negative correlation between the duration of afterhyperpolarization (AHP) and axonal conduction velocity (CV). However, this AHP vs CV relationship differed from that previously demonstrated in cats, in that for a given axonal CV, the AHPs were briefer in the rat than in the cat. For MG cells, twitches were also recorded from their muscle units. As has previously been observed in cats, there was a significant correlation between the time-course of the AHP of a motoneurone and the time-course of its muscle unit twitch. This relationship was, in principle, similar between the two species, but the AHPs and the twitches were briefer in the rat. The present results provide the first demonstration, for the rat model, of the presence of a functionally relevant "speed-match" between the intrinsic properties of alpha-motoneurones and those of their muscle units.