Plantar cutaneous input modulates differently spinal reflexes in subjects with intact and injured spinal cord

Abstract

Study design: Spinal reflex excitability study in sensory-motor incomplete spinal cord-injured (SCI) and spinal intact subjects.

Objectives: To investigate the effects of plantar cutaneous afferent excitation on the soleus H-reflex and flexion reflex in both subject groups while seated.

Setting: Rehabilitation Institute of Chicago and City University of New York, USA.

Methods: The flexion reflex in SCI subjects was elicited by non-nociceptive stimulation of the sural nerve. In normal subjects, it was also elicited via innocuous medial arch foot stimulation. In both cases, reflex responses were recorded from the ipsilateral tibialis anterior muscle. Soleus H-reflexes were elicited and recorded via conventional methods. Both reflexes were conditioned by plantar cutaneous afferent stimulation at conditioning test intervals ranging from 3 to 90 ms.

Results: Excitation of plantar cutaneous afferents resulted in facilitation of the soleus H-reflex and late flexion reflex in SCI subjects. In normal subjects, the soleus H-reflex was depressed while the late flexion reflex was absent. The early flexion reflex was irregularly observed in SCI patients, while in normal subjects a bimodal reflex modulation pattern was observed.

Conclusion: The effects of plantar cutaneous afferents change following a lesion to the spinal cord leading to exaggerated activity in both flexors and extensors. This suggests impaired modulation of the spinal inhibitory mechanisms involved in the reflex modulation. Our findings should be considered in programs aimed to restore sensorimotor function and promote recovery in these patients.

Sponsorship: NIH, NICHD, Grant no. 1R03 HD 043951-01 and PSC CUNY Research Award no. 67051-0036.

Figures

Figure 1

Effects of plantar cutaneous afferent excitation on the soleus H-reflex in normal and SCI subjects. (a) The average soleus H-reflex (n = 20) under control conditions (solid lines) and during conditioning of the reflex (dashed lines) at 15 and 60 ms of conditioning test (C-T) intervals for one normal and one SCI subject is indicated. (b) Pool data showing the effects of plantar cutaneous afferents excitation on the soleus H-reflex in normal and SCI subjects. For each C-T interval tested, the overall average (mean±SEM) size of the conditioned H-reflexes for both subject groups is indicated. Asterisks indicate cases of statistically significant differences between the control and the conditioned reflex sizes (P < 0.05)

Figure 2

Effects of plantar cutaneous afferent excitation on the TA flexion reflex in SCI subjects. (a) The average (n = 20) flexion reflex recorded under control conditions and following conditioning stimulation delivered at 3 and at 90 ms conditioning intervals for two subjects (S1 and S9) is presented. See text for further description. (b) Pool data (all SCI subjects tested) showing the effects of plantar cutaneous afferent excitation on the late flexion reflex. For each conditioning test interval, the average size of the conditioned late flexion reflexes (as a percentage of the control late flexion reflex) was calculated for all subjects tested. Asterisks indicate cases of statistically significant differences between the control and the conditioned reflex sizes (P < 0.05). Error bars indicate the SEM

Figure 3

Full-wave rectified early flexion reflex elicited via medial arch stimulation in two normal subjects under control conditions (a) and following excitation of plantar cutaneous afferents at conditioning test (C-T) interval of 3 and 90 ms (b, c). Each waveform constitutes the average of 20 reflex responses elicited every 10 s. For both subjects, the early flexion reflex was facilitated at the C-T interval of 3 ms, while at the longest C-T interval tested (90 ms) the reflex was significantly depressed compared to control reflex values

Figure 4

Histogram showing the effects of plantar cutaneous afferents excitation at three times perceptual threshold on the early TA flexion reflex elicited by medial arch stimulation in normal subjects. The average (all subjects tested) amplitude (mean±SEM) of the conditioned reflex is plotted for each conditioning test interval tested. Asterisks identify the conditioning test intervals in which the reflex modulation was significant at 95% confidence level