Development and characteristics of airstepping in chronic spinal cats

Abstract

Airstepping, walking-like movements of the hindlimbs, is a commonly observed behavior in chronic spinal animals when they are held vertically. The purpose of this study was to: describe the development of airstepping after spinalization and compare it to the onset of segmental reflexes, characterize the EMG pattern of muscle activity during spontaneous airstepping, and examine the effects of sensory perturbation on the characteristic pattern. Airstepping was analyzed during three conditions of tonic sensory perturbation which included: tail pinching, tape applied to one hindpaw, and immobilization of the ankle and knee in a plaster cast. Seven adult cats were spinalized at T-12, and bipolar electrode wires were surgically implanted in selected hindlimb muscles at the hip, knee, and ankle. Testing began within 48 hr of transection. Segmental reflexes and pawshake responses were present in the first week; however, the earliest observed airstepping occurred during tail pinching at 2 weeks after surgery, and the average onset of spontaneous airstepping (without exteroceptive stimuli) was at 33 days. The average cycle period of spontaneous airstepping (691 msec) was comparable to the shortest periods reported for fictive rhythms and to treadmill walking between 1 and 2 m/sec. Intralimb coordination was characterized by flexor and extensor synergies typical of locomotion, while interlimb coordination was characterized by alternating cycles similar to that reported for treadmill walking and fictive locomotion. Neither intralimb nor interlimb patterns of coordination were altered by conditions of sensory perturbation, although cycle period and EMG recruitment level were variable. Many characteristics of airstepping are similar to those of treadmill and fictive locomotion. Since airstepping is unencumbered by the effects of contact forces and belt speed present during treadmill locomotion and is uncomplicated by the effects of L-DOPA (needed to elicit fictive locomotion), it may provide a more reliable model with which to investigate the neural networks responsible for coordinating hindlimb locomotion.