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

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

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

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

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.

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.

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.

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