Figure 1.

Multi-site stimulation that utilizes spatio-temporal independent monopolar stimulation strategies at L₂ and S₁ reveal unique effects of change in frequency of stimulation and relative timing between stimulation pulses.

(A–D) During a testing session rats are suspended using a body weight support system over a moving treadmill and stimulated at the L₂ and S₁ spinal segments to elicit a stepping response. In this demonstration, a single rat is stimulated (monopolar) at the L₂ spinal segment with a frequency of 40 Hz (L₂ only) and S₁ stimulation is altered between four different frequencies. Note that with increasing frequency of stimulation, the number of spinal evoked responses from the tibialis anterior (TA) and medial gastrocnemius (MG) muscles also increases. Trials C and D resulted in the best stepping pattern and suggests interaction of frequencies in a way that engages a wider flexor-extensor neuronal pool for robust locomotor output. Note that a clear interaction response is evoked only in the presence of S₁ stimulation pulse. E–F) Keeping the frequency of stimulaiton at S₁ constant, our data also reveal that the time at which the L₂ or S₁ pulse is initiated with respect to each other strongly shapes locomotor success. Two distinct relative times that enhance step quality were identified. Red and blue traces in F indicate responses to individual S₁ or L₂ pulses respctively. Traces in black are a resultant evoked response consequent to the added pulse from L₂ or S₁. Note that a L₂ pulse initiated 3–19 ms after onset of the S₁ pulse results in a polysynaptic response (condition 1); while a S₁ pulse initiated after the L₂ pulse drastically amplifies the interaction evoked response.