Rostrocaudal Distribution of the C-Fos-Immunopositive Spinal Network Defined by Muscle Activity during Locomotion

Abstract

The optimization of multisystem neurorehabilitation protocols including electrical spinal cord stimulation and multi-directional tasks training require understanding of underlying circuits mechanisms and distribution of the neuronal network over the spinal cord. In this study we compared the locomotor activity during forward and backward stepping in eighteen adult decerebrated cats. Interneuronal spinal networks responsible for forward and backward stepping were visualized using the C-Fos technique. A bi-modal rostrocaudal distribution of C-Fos-immunopositive neurons over the lumbosacral spinal cord (peaks in the L4/L5 and L6/S1 segments) was revealed. These patterns were compared with motoneuronal pools using Vanderhorst and Holstege scheme; the location of the first peak was correspondent to the motoneurons of the hip flexors and knee extensors, an inter-peak drop was presumably attributed to the motoneurons controlling the adductor muscles. Both were better expressed in cats stepping forward and in parallel, electromyographic (EMG) activity of the hip flexor and knee extensors was higher, while EMG activity of the adductor was lower, during this locomotor mode. On the basis of the present data, which showed greater activity of the adductor muscles and the attributed interneuronal spinal network during backward stepping and according with data about greater demands on postural control systems during backward locomotion, we suppose that the locomotor networks for movements in opposite directions are at least partially different.

Keywords: C-Fos technique; backward and forward stepping; decerebrated cat; locomotor networks; spinal cord.

Figure 1

Rostrocaudal distribution of FOS+ neurons in cats stepping forward (FW) and backward (BW). (A) A scheme for the experiment. ES—epidural stimulation. Red boxes show examples of FOS+ neurons. (B) Individual data for individual FW (Fw1–Fw3) and BW (Bw1–Bw3) cats. OX axis, spinal segments, OY axis, absolute number of FOS+ neurons. Red arrows mark ES sites. (C) Difference between the averaged percent curves for the distribution of FOS+ neurons, for FW and BW cats; red and grey rectangles mark peak in the number of FOS+ neurons and inter-peak drop. (D) A rostrocaudal distribution of the motoneuronal pools assessed from the [17] data. (E) Pearson correlation coefficient between the angle range of the hip movements (allowing assessment of the general hip activity) and an expression of the first peak in FOS+ nuclei. HF—hip flexors; HE—hip extensors; KF—knee flexors; KE—knee extensors; AF—ankle flexors; AE—ankle extensors; Add—adductors. 0-100—rostrocaudal (R-C) levels of the spinal cord defined in Reference [17]. ** p < 0.01.

Figure 2

Muscle activity during FW and BW stepping. (A) Representative example of the EMG activity of the m. iliopsoas (IP), m. rectus femoris (RF), m. vastus medialis (VM) and m. adductor magnus (ADD) during the same registration. A, P—anterior and posterior directions of the hindlimb. SW—swing, ST—stance. HL L and HL R—left and right hindlimbs. ES—stimulation; light gray arrows mark hindlimb movements. (B) Averaged percent EMG activity during step cycle, for FW (dark gray) and BW (light gray) stepping. OX axis, step cycle. (C) Individual percent ratio in the muscle activity during FW and BW stepping, for ADD and IP (top) and RF and VM (bottom). OX axis, individual cats. (D) Averaged percent ratio in the muscle activity during FW and BW stepping. OX axis, muscles. * p < 0.05; *** p < 0.001; **** p < 0.0001.

Figure 3

Abduction test and step characteristics during FW and BW stepping. (A) Representative example of the EMG activity during normal FW stepping (FW) and stepping with abduction (FW_abd). (B) The ratio in the muscle activity during FW and FW_abd conditions. OX axis, muscles. (C) A percent ratio of the step width between FW and BW stepping. (D) A percent ratio of the stability index between FW and BW stepping. Left, right—left and right hindlimb. *** p < 0.001.