The Stumbling Correction Reaction during Fictive Locomotion in the Cat a

The present experiments were designed to address two questions: Can the stumbling corrective reaction be evoked during fictive locomotion and can it be evoked by cutaneous stimulation alone? Observations were obtained during fictive locomotion produced by stimulation of the midbrain in decerebrate cats following neuromuscular blockade (see reference 7 for details). The superficial peroneal (SP) nerve containing the cutaneous innervation of the dorsum of the foot was stimulated using a train of 20-40 shocks (200Hz) at twice threshold. Rectified, integrated, electroneurogram (ENG) activity was used as a monitor of fictive locomotion and using a window discriminator, to trigger the delivery of the stimulus train during a particular part of the fictive locomotor step cycle. FIGURE 1 shows an average of 14 trials in which the SP nerve was stimulated about 200 ms after the onset of activity in the tibialis anterior (ankle flexor) nerve. The traces are aligned at the onset of stimulus delivery (vertical dashed line). Shortly after the beginning of the stimulus train there is upward deflection of the TA ENG indicating an excitation of TA motoneurons which is quickly followed by a reduction in TA activity to below pre-stimulus levels. Posterior biceps (PB) and semitendinosis (ST) are activated soon after stimulus onset and a few milliseconds later, activity in lateral gastrocnemius (LG) appears. The sequence of a sustained excitation of knee (PB and ST) but not ankle (TA) flexors and the delayed excitation of ankle extensors (LG), and the latencies of these responses are remarkably similar to those reported during treadmill locomotion in intact cats (e. g. Figure 3 in reference 3). Although intracellular records and accurate determination of the arrival of the volley at the spinal cord are needed to determine the minimum intra-spinal latency of TA, PB and St excitation, the 7.3 ms latency from the onset of stimulation illustrated in Fig 1. includes conduction time to and from the spinal cord and the delay in motoneuron recruitment as well as the delay through the neuronal pathway. Thus the central pathway for initiation of the stumbling corrective response can consist of only a few interneurons. These results show that the full stumbling corrective response can be evoked in the fictive locomotor preparation and with cutaneous stimulation alone. For unknown reasons, not all preparations in the present and previous (7) experimental series displayed the full stumbling response. It should, however, be possible to identify the neuronal circuitry underlying the stumbling corrective response using intracellular recordings from motoneurons and interneurons in selected decerebrate fictive locomotion preparations. Supported by the Canadian MRC and Rick Hansen Legacy Fund.