Spinal motoneuron activity during gait

We have compiled a movie depicting activation of cat lumbosacral spinal motoneurons (MNs) during gait. Locations of MNs of 23 hindlimb and lower back muscles were digitized from 32 transverse sections of cord spanning ~30mm from L4 to S1 (Vanderhorst & Holstege, J. Comp. Neurol. 382, 46-76, 1997). The sections were combined into a transparent 3-D representation of the lumbosacral cord. Next, a series of such images was generated, each representing the MNs active at a given moment in the step cycle. Activation was determined from a set of electromyogram (EMG) profiles of the corresponding muscles in the cat step cycle (Prochazka et al., Prog Brain Res 80: 61-74, 1989). The step cycle was divided into 76 intervals of 10ms. For a given frame, each muscle's EMG level was used to scale the number of dots representing that muscle's active MNs. The 76 frames were then played as a movie.

The movie shows a clear rostro-caudal oscillation of MN activity during the step cycle. To quantify this, we plotted the locus of the centre of mass of the dots in the 76 consecutive frames. The locus comprised a thin rostro-caudal loop. One might have expected this trajectory because hip flexor MN pools are the most rostral and their activation during swing would therefore tend to shift the centre of activation rostrally. However, knee flexor MN pools are very caudal, so this was not a foregone conclusion. The rostro-caudal oscillation was consistent with the idea that the locomotor pattern could be generated by connections between MN pools, the time course of activity in one pool influencing activity in the next. The surge of activity rostrally during swing is also consistent with the idea of a flexor burst generator, which might attract the centre of activation rostrally from its caudal location during stance. To download the GIF animation on the right, right-click on it and save (GIF movie, 58Kbyte).

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