Comparison of spontaneous motor pattern generation in non-hemisected and hemisected mouse spinal cord

Spontaneous electromyogram (EMG) patterns in the gastrocnemius (G) and tibialis anterior (TA) muscles of spinal cord-hindlimb explants from neonatal mice were investigated. Compared to non-hemisected explants, neither longitudinal hemisection of the spinal cord nor hemisection plus transection at L1 significantly altered the incidence of spontaneous motor rhythm. Therefore, not only does each half of the neonatal spinal cord contain sufficient circuitry to generate motor rhythm, but the more reduced preparations were just as likely to produce such activity. Hemisected preparations, however, exhibited slower rhythm, perhaps due to the loss of excitatory commissural connections. No correlation was found between the number of cycles in a rhythmic sequence and cycle period. In hemisected as well as non-hemisected explants, sequences of spontaneous EMG rhythm occurred in either the G or TA muscle, but not in both muscles simultaneously. Consequently, cycle-to-cycle alternation between rhythmic bursting in the G and TA muscles was not observed. The excitability in such preparations was apparently insufficient for maintained activations of both muscles (either for cycle-to-cycle alternation or for co-contraction).