The present study was designed to investigate the effects and action mechanism of electrical stimulation on functional recovery following spinal cord injury in Sprague-Dawley rats. Electrical stimulation with 0.2 ms, 20 Hz, 1-3 V was applied to the sciatic nerve for 4 hours/day during 6 days following dorsal hemisection of the T10 spinal cord. After 7 days of spinal cord injury, mechanical properties of muscle contraction including contraction time, half relaxation time, maximum twitch tension, maximum tetanic tension, and fatigue index were measured in the soleus and medial gastrocnemius muscles, and the number of c-fos immunoreactive cells was counted in the upper lumbar cord. In mechanical properties of muscle contraction of normal rats, contraction time and half relaxation time of the soleus muscle were 1.5 times and 2 times as long as those of the medial gastrocnemius muscle, respectively. And fatigue index of the soleus muscle was 0.19⁑0.4 and the medial gastrocnemius muscle was 0.82⁑0.03. According to the above characteristics, the soleus muscle was mainly composed of slow muscle fibers and the medial gastrocnemius muscle was composed of fast muscle fibers. Maximum twitch tension, maximum tetanic tension, and fatigue index of both muscles following spinal cord injury were decreased significantly compared to the control group (p<0.01). In electrically stimulated rats following spinal cord injury, maximum twitch tension, maximum tetanic tension, and fatigue index were significantly increased compared to spinal cord injured rats. The number of c-Fos immunoreactive cells was increased markedly at the upper lumbar cord in electrically stimulated rats.

These results may suggest that electrical stimulation not only prevents from muscle atrophy in slow and fast muscles through efferent nerve fibers, but also promotes functional plasticity through afferent nerve fibers by activating silent synapse and regulation of receptors for neurotransmitters.