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

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.

Effects of electrically induced muscle contraction on flexion reflex in human spinal cord injury.

STUDY DESIGN: Flexion reflex study in motor complete human spinal cord injury (SCI). OBJECTIVES: To examine changes in the magnitude of the flexion reflex following functional electrical stimulation (FES) of the rectus femoris (RF) muscle. SETTING: Bioengineering Unit, University of Strathclyde, Glasgow, Scotland, UK. METHODS: The flexion reflex was evoked by electrical stimulation of the sural nerve, and was recorded in the tibialis anterior (TA) muscle. RF muscle conditioning stimulation was performed at 0.7, 1, and 2 times motor threshold ( x MT) over a range of conditioning test intervals. RESULTS: The incidence of the early component of the flexion reflex (<100 ms) was low, suggesting that this reflex component might be suppressed in SCI. The long latency flexion reflex component (>120 ms) was observed in all subjects during control conditions and following sensorimotor conditioning. FES applied to the RF muscle (above and below MT) in the main induced a significant early and long lasting depression of the long latency flexion reflex. CONCLUSION: The depression of the flexion reflex was a result of multisensory actions on flexion reflex pathways resulting from the direct and indirect (mechanical) consequences of electrically induced muscle contraction on cutaneous and muscle afferents. Our findings emphasize the importance of sensory feedback mechanisms in modulating flexion reflex excitability, and highlight the need for rehabilitation professionals to consider the central actions of FES-induced afferent feedback when incorporating FES into a rehabilitation program. SPONSORSHIP: State Scholarships Foundation (IKY) of Hellas.

Modulation of soleus H-reflex following ipsilateral mechanical loading of the sole of the foot in normal and complete spinal cord injured humans.

The modulation of the soleus H-reflex in response to tonic mechanical loading applied to the plantar aspect of the foot sole was examined in nine normal subjects and five patients with a clinically defined complete spinal cord injury (SCI). With the subjects seated, tonic pressure applied to the metatarsal region of the ipsilateral foot sole significantly depressed soleus H-reflex excitability in all subjects. The demonstration of a decrease in H-reflex excitability in both subject groups as a result of applied pressure to the foot suggests that the change in reflex excitability is the result of a common spinal mechanism. The results highlight the modulatory effects that natural stimulation of cutaneous afferents can have on reflex excitability and may have practical application in gait rehabilitation and in the management of disorders of muscle tone following SCI.